BY - Higher Education Commissionprr.hec.gov.pk/jspui/bitstream/123456789/1503/1/1224S.pdfMuhammad,...

A STUDY OF MECHANICAL PROPERTIES OF

IRRADIATED TITANIUM AND ITS ALLOYS

AT LOW TEMPERATURES

A THESIS SUBMITTED TO

THE UNIVERSITY OF THE PUNJAB

IN FULFILMENT OF THE REQUIREMENTS

FOR THE DEGREE OF

DOCTOR OF PHILOSOPHY IN PHYSICS

BY

SYED KARRAR HAIDER

DEPARTMENT OF PHYSICS,

UNIVERSITY OF THE PUNJAB, LAHORE, PAKISTAN

2009

A STUDY OF MECHANICAL PROPERTIES OF

IRRADIATED TITANIUM AND ITS ALLOYS

AT LOW TEMPERATURES

SYED KARRAR HAIDER

DEPARTMENT OF PHYSICS,

UNIVERSITY OF THE PUNJAB, LAHORE, PAKISTAN

iii

ACKNOWLEDGEMENTS

To God I am grateful for providing me the opportunity and strength, first to

undertake this gigantic task and then to accomplish it successfully.

I am extremely grateful and immensely indebted to my supervisor,

Prof. Dr. Khadim Hussain and Director CASP, who worked in tandem to provide

me the best possible support and priceless guidance in addition to ever present

encouragement during the course of this study.

I would like to thank a long list of people whose unqualified support was a

constant source of inspiration during my research work. Special thanks are due to

DG PCSIR Lahore, for allowing me to use the facilities, available at Pakistan

Council of Scientific and Industrial Research (PCSIR) at will. Thanks are also due

to Ms. Asma Haleem Khan for her invaluable help during polishing process. How

can I skip mentioning the name of Mr. Shakeel of Marghoob Traders, who

generously provided me the raw material so earnestly required at the initial

stages?

At Centre for Advanced Studies in Physics (CASP), GC University Lahore where

I carried out the experimental work, I was lucky to be assisted by a team of

selflessly dedicated young scientists, comprising Messrs Naveed Afzal, Nawaz

Muhammad, Akram Raza et al, who not only generously shared my burden, but

made invaluable suggestions that mattered a lot at the end of the day. Here I

would like to mention the monumental contribution by Mr. Naveed Afzal, who not

only proved to be a worthy co-researcher, but also a tireless willing colleague.

Mr. Shafique, a Senior Technician at CASP deserves a special mention, as

without his ingenious methods, the vacuum phase could have become a cause of

iv

concern. Dr. Adnan Latif deserves special thanks as he rendered considerable

help in microscopy of fracture surfaces and explaining fractographs. Director,

CASP, Dr. Ijaz Mujtaba Ghauri’s magnanimous patronage was phenomenal.

Mr. Arif Hussain, a long time companion contributed monumentally in ordering

the thesis and bringing it in the final form. May God reward him for his marathon

contribution. All these noble souls deserve my sincere thanks.

My brothers: Iqtadar Shah, Abrar Shah, Waqar Shah and my sisters:

Shahnaz Meenoo Cheher and Shahaeen Aftab Ejaz, all provided me the much

needed moral support. The parents of my wife admirably filled the void created by

the death of my beloved mother. My old and most trusted friend Mr. Javed Iqbal

did all the spade work at the out set and hence deserves thumbs up.

In the end I wish to express my heart felt appreciation to my beloved wife, Shaila

Batool Haider, whom I have always found by me through thick and thin, making

sure that the thesis work never gets hampered. My sons, S.M. Jarrar Haider and

S.M. Azadar Hussain remained perpetual cheer leaders all these long years.

They along with their mother, at times, showed more concern than me in my

thesis work.

May God shower His bounties on all these people?

DEDICATED

TO

THE LIVING MEMORY

OF

MY DECEASED MOTHER

PREFACE

This thesis describes research performed at the Department of Physics,

University of the Punjab, Lahore. Except where specific reference is made to the

work of others, it is the result of my own work and includes nothing which is the

outcome of work done in collaboration. No part of this work has been, or is being

submitted for any other qualification at this, or any other university.

Some of the work described in the thesis has been published as follows:

1. I.M. Ghauri, Naveed Afzal and Karrar Haider. Rate Controlling Process of

Stress Relaxation in High Purity Irradiated Titanium. J. Phys. D: Appl.

Phys. 39 (2006) 2829-2831.

2. I. M. Ghauri, Karrar Haider, Naveed Afzal and S.A. Siddiqui. A

Comparative Study of Stress Relaxation Rate in un-irradiated and

Irradiated Pure Titanium at Low Temperatures. International Journal of

Modern Physics B. 21 31(2007)5247-5255.

3. Karrar Haider, I.M. Ghauri, Khadim Hussain and Naveed Afzal.

Irradiation Energy Dependence of Tensile Properties in Polycrystalline

Titanium. Proceeding of 2nd International Conference on Frontiers of

Advanced Engineering Material (FAEM-06) 04-06 December 2008,

Lahore-Pakistan.

4. Karrar Haider, Asma Haleem, Rehmat Ali Gohar, I.M. Ghauri and

Khadim Hussain. Proceeding of 2nd International Conference on

Frontiers of Advanced Engineering Material (FAEM-06) 04-06

December 2008, Lahore-Pakistan.

5. Karrar Haider, Adnan Latif, Naveed Afzal and Khadim Hussain. Effects

of Irradiation Energy and Deformation Temperature on the Tensile

Properties and Fracture mode of Polycrystalline Titanium. PIP

International Conference 2009. February 23-26, 2009, Lahore

Pakistan.

CERTIFICATE

This is to certify that the research work described in this thesis

is the original work of the author and has been carried out under my

direct supervision. I have personally gone through all the data/results/

materials reported in the manuscript and certify their correctness/

authenticity. I further certify that the material included in this thesis

has not been used in part or full in a manuscript already submitted or

in the process of submission in partial/complete fulfillment of the

award of any other degree from any institution. I also certify that the

thesis has been prepared under my supervision according to the

prescribed format and I endorse its evaluation for the award of Ph.D.

degree through the official procedures of the University.

Signature:___________________ Name: PROF. DR. KHADIM HUSSAIN Designation: Former CHAIRMAN DEPARTMENT OF PHYSICS,

UNIVERSITY OF THE PUNJAB, LAHORE

i

ABSTRACT

The present work attempts to study the effects of irradiation and deformation

temperature on the mechanical properties of polycrystalline pure titanium

(99.994 %). The specimens were irradiated by electron beam of energies ranging

from 8-18 MeV at 300 K. The irradiated and unirradiated specimens were

deformed using Universal Testing Machine in the temperature range 300-100 K

at a strain rate of 1.2 10-3 /sec. A comparison between stress strain curves of

unirradiated/ irradiated specimens recorded through the attached machine at

room temperature, was carried out. The yield stress and ultimate tensile stress of

irradiated specimens were found to be higher than that of unirradiated ones

however the ductility was found to decrease. The changes in tensile parameters

became more pronounced with increase of irradiation energy and decrease of

test temperature.

The stress relaxation tests were also conducted to observe stress relaxation

behavior of the material at a certain fixed load. The stress relaxation rate was

found to be lower in irradiated specimens than that of unirradiated ones lowering

of test temperature and enhancing of irradiation energy. The activation

parameters of stress relaxation like activation energy and activation volume were

also analyzed from the relaxation tests. The activation energy was found to

increase with decrease of test temperature and with increase of incident

irradiation energy. The analysis of activation energy (U0) and activation volume

(Vσ) suggests that the initial plastic deformation in irradiated titanium specimens

is controlled by the interaction of dislocations with irradiation induced defects

while this advances into dislocation-dislocation intersection as the deformation

ii

proceeds which seem to control the rate process in irradiated polycrystalline

titanium. Scanning electron microscope micrographs of the fractured surfaces

show a combination of ductile and cleavage fractures, corresponding to different

values of irradiation energy and test temperature.

v

LIST OF PUBLICATIONS

This thesis describes research performed at the Department of Physics,

University of the Punjab, Lahore. Except where specific reference is made to the

work of others, it is the result of my own work and includes nothing which is the

outcome of work done in collaboration. No part of this work has been, or is being

submitted for any other qualification at this, or any other university.

Some of the work described in the thesis has been published as follows:

1. I.M. Ghauri, Naveed Afzal and Karrar Haider. Rate Controlling Process of

Stress Relaxation in High Purity Irradiated Titanium. J. Phys. D: Appl.

Phys. 39 (2006) 2829-2831.

2. I.M. Ghauri, Karrar Haider, Naveed Afzal and S.A. Siddiqui. A

Comparative Study of Stress Relaxation Rate in un-irradiated and

Irradiated Pure Titanium at Low Temperatures. International Journal of

Modern Physics B. 21 31(2007)5247-5255.

3. Karrar Haider, I.M. Ghauri, Khadim Hussain and Naveed Afzal.

Irradiation Energy Dependence of Tensile Properties in Polycrystalline

Titanium. Proceeding of 2nd International Conference on Frontiers of

Advanced Engineering Material (FAEM-06) 04-06 December 2008,

Lahore-Pakistan.

4. Karrar Haider, Asma Haleem, Rehmat Ali Gohar, I.M. Ghauri and

Khadim Hussain. Proceeding of 2nd International Conference on

Frontiers of Advanced Engineering Material (FAEM-06) 04-06

December 2008, Lahore-Pakistan.

5. Karrar Haider, Adnan Latif, Naveed Afzal and Khadim Hussain. Effects

of Irradiation Energy and Deformation Temperature on the Tensile

Properties and Fracture mode of Polycrystalline Titanium. PIP

International Conference 2009. February 23-26, 2009, Lahore

Pakistan.

6. Haider Karrar, Hussain Khadim, Latif Adnan, Afzal Naveed. Effects of

Irradiation Energy on Tensile Properties of Polycrystalline Titanium.

Canadian Journal on Mechanical Sciences & Engineering, Vol,2, No.5,

June (2011) 84-89.

vi

TABLE OF CONTENTS

Abstract i

Acknowledgements iii

List of Publications v

List of Tables ix

List of Figures x

CHAPTER # 1: INTRODUCTION 1

1.1. Historical Background 1

1.2. Occurrence and Production 2

1.3. Physical Properties 2

1.4. Metallurgy 3

1.5. Naming Titanium Alloys 3

1.6. Notable Characteristics 4

1.7. Applications of Titanium 5

1.7.1. Aerospace Industry 5

1.7.2. Food, Petroleum and Electrical Industries 5

1.7.3. The Marine Industry 6

1.7.4. Biomaterials 6

1.7.5. Chemical and General Engineering Industry 6

1.7.6. Sports 7

1.7.7 Computers 7

1.8. Miscellaneous Applications 7

1.9. Objectives 8

CHAPTER # 2: REVIEW OF LITERATURE 9

2.1. Irradiation of Metals and Alloys 9

2.2. Irradiation Hardening of FCC Metals/Alloys 10

2.3. Irradiation Hardening of BCC Metals/Alloys 11

vii

2.4. Effects of Irradiation Temperature 13

2.5. Comparison of Irradiation Induced Hardening in FCC and BCC Metals/Alloys

14

2.6. Irradiation Hardening of HCP Metals/Alloys 16

2.7. Comparison of Irradiation Induced Hardening in BCC, FCC and HCP Metals at Different Temperatures

20

2.8. Mechanical Behaviour of HCP Metals/ Alloys at Low Temperature

23

2.9. Mechanical Behaviour and Test Temperature 25

2.10.

Stress Relaxation of Metals and Alloys 27

CHAPTER # 3: MATERIAL AND METHODS 31

3.1. Specimen Preparation 32

3.1.1. Cutting 33

3.1.2. Mounting 33

3.1.3 Grinding and Polishing 34

3.1.4 Annealing 38

3.2. Irradiation of the Specimens 41

3.2.1. Energy Dependence of Irradiation Effects 42

3.2.2 Exposure Time Dependence of Irradiation Effects 42

3.3. Tensile Testing 43

3.3.1. At Room temperature 44

3.3.2. At Low Temperatures 45

3.4. Microstructural Observations 46

CHAPTER # 4: RESULTS AND DISCUSSION 47

4.1. Effects of Irradiation Energy on the Stress-Strain Curves of Titanium

47

4.2. Effects of Test Temperature on the Stress-Strain Curves of 48

viii

Titanium

4.3. Effects of Exposure Time on the Stress-Strain Curves of Titanium

49

4.4. Effects of Irradiation Energy on the Stress Relaxation Behaviour of Titanium

49

4.5. Effects of Test Temperature on the Stress Relaxation Behaviour of Titanium

51

4.6. Effects of Exposure Time on the Stress Relaxation Behaviour of Titanium

51

4.7. Calculation of Frenkel Pairs 52

4.8. SEM Fractroraph Results 52

Discussion of Results 54

CHAPTER # 5: CONCLUSIONS AND FUTURE WORK 84

CONCLUSIONS 84

FUTURE WORK 87

REFERENCES 88

ix

LIST OF TABLES

Table 3.1. Data regarding SiC Polishing 37

Table 3.2. Electrolytic Polishing 37

Table 4.1. Variations in Yield Stress and Ultimate Tensile Stress and % Elongation with incident Beam Energy

63

Table 4.2. Variation in % Reduction in Area with Energy 63

Table 4.3. Change in Yield Stress, Ultimate Tensile Stress and % Elongation with Test Temperature in Unirradiated Titanium

65

Table 4.4. Change in Yield Stress, Ultimate Tensile Stress and % Elongation with Test Temperature in Irradiated Titanium

65

Table 4.5. % Reduction in Area with Test Temperature in Irradiated Titanium

66

Table 4.6. Effect of Irradiation Exposure Time on Yield Stress and Ultimate Tensile Stress

67

Table 4.7. Increase in Activation Energy with Incident Energy 71

Table 4.8. No. of Frenkel Pairs corresponding to Irradiation Energy 79

x

LIST OF FIGURES

Fig.3.1. Rectifier and other facilities (PCSIR) 36 Fig. 3.2. A photograph of the used vacuum system 39 Fig. 3.3. Annealing Furnace 40 Fig.3.4. Mevatron Linear Accelerator 41 Fig.3.5. Universal Testing Machine of 50 kN (UTM) 43 Fig.3.6. UTM with Cryogenic Chamber 45 Fig.4.1. Comparison between the Stress-Strain Curves of

Unirradiated and 8 MeV irradiated Pure Titanium 61

Fig.4.2. Comparison of Stress-Strain Curves of 8 and 10 MeV Irradiated Ti

61


62

Fig.4.4. Overall Comparison of Stress-Strain Curves (8-15 MeV)Error = + 0.5%

62

Fig.4.5(a). Comparison of Stress Strain Curves of Unirradiated Titanium at Low Temperature

64

Fig.4.5(b). Comparison of Stress Strain Curves of Irradiated Titanium at Low Temperature

64

Fig.4.6. Effect of Exposure Time on the Stress-Strain Curves of 12 MeV Electrons Irradiated Titanium

66

Fig.4.7. Stress Relaxation Curves of Un-Irradiated Titanium annealed at 977K for 30 minutes

67

Fig.4.8. Stress Relaxation Curves of Irradiated Titanium annealed at 977K for 30 minutes

68

Fig.4.9. Stress Relaxation Rate(s) as a function of initial stress 68 Fig.4.10. Stress Relaxation Curves of Titanium Irradiated with 8

MeV Electrons 69

Fig.4.11 Stress Relaxation Curves of Titanium Irradiated with 12 MeV Electrons

69

Fig.4.12. Stress Relaxation Curves of Titanium Irradiated with 18 MeV Electrons

70

Fig.4.13. Variation of Stress Relaxation Rate(s) with initial stress levels σ0

70

Fig.4.14. Variation of Activation Volume Vσ with initial stress levels σ0

71

Fig.4.15(a) Stress Relaxation Curves of Un-irradiated Titanium obtained at 300 K

72

Fig.4.15(b) Stress Relaxation Curves of 12 MeV Electron Beam Irradiated Titanium obtained at 300 K

72


73


73

Fig.4.17(a) Stress Relaxation Curves of Unirradiated Titanium obtained at 100 K

74

xi


74

Fig.4.18. Stress Relaxation Rate(s) as a function of initial stress levels σ0 for Un-irradiated Titanium Specimens in the Temperature Range 300 to 100 K

75

Fig.4.19. Stress Relaxation Rate(s) as a function of initial stress levels σ0 for 12 MeV Electron Beam Irradiated Titanium Specimens in the Temperature Range 300 to 100

75

Fig.4.20. Variation of Intrinsic Height of Energy Barrier U0 with decrease of temperature in both Un-irradiated and Irradiated Titanium

76

Fig.4.21. Stress Relaxation Curves of Titanium Irradiated with 12 MeV Electrons for 5 mins

76


77


77


78

Fig.4.25. Variation of Activation Volume Vσ with initial stress levels σ0 for 5-25 minutes

78

Fig.4.26(a) 8 MeV (250 K) 80 Fig.4.26(b) 8 MeV (100 K) 80 Fig.4.26(c) 15 MeV (250 K) 81 Fig.4.26(d) 15 MeV (100 K) 81 Fig.4.27(a) 15 MeV (250 K) 82 Fig.4.27(b) 15 MeV (250 K) [Enlarged] 82 Fig.4.27(c) 15 MeV (100 K) 83 Fig.4.27(d) 15 MeV (100 K) [Enlarged] 83

Chapter 1: Introduction

1

CHAPTER 1

INTRODUCTION

Titanium is a metal of immense potential. Its attractive physical and

mechanical properties makes it a metal of choice in many a structural material.

Its light weight, excellent corrosion resistance, and high strength to weight ratio

have led to its widespread applications in the chemical, aerospace, marine and

medical fields. Application of titanium alloys in the petrochemical industry and in

the manufacture of sports equipment is among recent developments. Where

other metals fail titanium is the solution.

1.1 Historical Background

Titanium is a transition metal, one of the elements found in Rows 4, 5, and

6 of the periodic table. It has an atomic number of 22, an atomic mass of 47.88,

and a chemical symbol of Ti. Titanium (Latin Titans, Earth or the first sons of

Gaia) was discovered in England by Reverend William Gregor in 1791. He

recognized the presence of a new element in ilmenite (FeO.TiO2) and named it

menachite. At around the same time, Franz Joseph Muller also produced a

similar substance but could not identify it. The element was independently

rediscovered several years later by German chemist Martin Heinrich Klaproth in

rutile (TiO 2). Klaproth confirmed it as a new element and in 1795 he named it for

the Latin word for Earth (also the name for the Titans of Greek mythology)

[Gregor W. 1922]. The metal has always been difficult to extract from its various

ores. Pure metallic titanium (99.9%) was first prepared in 1910 by Mathew A.

Hunter by heating TiCl4 with sodium in a steel bomb at 700-800°C in the Hunter

process. Titanium metal remained a laboratory curiosity until 1946 when William


2

Justin Kroll from Luxembourg proved that titanium could be commercially

produced by reducing titanium tetra chloride with magnesium in the Kroll process

which is the method still used today [Kroll W. J. 1940].

1.2. Occurrence and Production

Titanium is not found unbound to other metals, but the element is the fourth most

abundant in the earth’s crust (0.63%by mass) after aluminum, iron and

magnesium. These are 20 times more in quantity than chromium, 30 times more

than nickel, 69 times more than copper, 100 times more than tungsten and 600

times more than molybdenum.

It is widely distributed and occurs primarily in the minerals anatase,

brookite, ilmenite, perorskite, rutile, titanite (sphene) as in many iron cores.

Significant titanium ore deposits are in Australia, Scandinavia, North America and

Malaysia. The metal is found in meteorites and has been detected in the sun as

well. Rocks brought back from the moon during the Apollo 17 mission are

composing of 12.1% TiO2. Titanium is also found in coal ash, plants and even the

human body. [Hanaor, D. Sorrell, C. 2011].

1.3. Physical Properties

As a pure metal, titanium has a melting point higher than that of steel,

3040oF (1671oC), a density 4.5g/cm3, a coefficient of thermal expansion of 5x 10-6

in./in.F(10.8x10-6 m/m K), a thermal conductivity of 11.5 Btu/h ft2 F/ft (20 W/m K),

and a tensile modulus of elasticity as high as 18x 106 psi (12.7x 104 MPa).The

physical properties of prime importance are its density and modulus. Titanium

weighs only about half as much as steel, 0.16 versus 0.28 lb/in2 (4.5 versus 7.87

g/cm3). Its mechanical properties can be better than those of many alloy steels,

and thus has a very high specific strength. The same thing is true about stiffness.


3

It has a much higher modulus than the other light metals, magnesium and

aluminum.

1.4. Metallurgy

The physical metallurgy of titanium is dominated by four factors:

1. Titanium is allotropic. The low temperature phase is HCP, called α. Above

882oC; it is BCC, called β. This provides three alloy families- α, β and α/ β.

The microstructure can be manipulated by heat treatment [Askeland D.R.

1996].

2. Titanium is a transition metal and forms solid solutions with many of the

transition metals. It also forms an interstitial solid solution with oxygen.

[Collings E.W, 1994]

3. It is highly reactive and forms compounds with oxygen, nitrogen and

hydrogen.

4. The β form transforms martensitically to α on quenching.

1.5. Naming Titanium Alloys

There are α alloys, near α alloys, α/ β alloys and β alloys.

Specific titanium alloys are referred to by the numbers of the concentration of the

major alloying elements. Thus:

Ti-5Al-2.5Sn is referred to as Ti-5-2.5(α)

Ti-8Al-1Mo-1V is referred to as Ti-8-1-1(near α)

Ti-6Al-2Sn-4Zr-2Mo is referred to as Ti-6-2-4-2(α/ β)

Ti-6Al-4V is referred to as Ti-6-4(α/ β)

Ti-10V-2Fe-3Al is referred to as Ti-10-2-3(β) [Avener 1974]


4

1.6. Notable Characteristics

Titanium is a multifunctional material, securing its future in steadily

growing market. It has been rightly called a wonder metal owing to attributes

enlisted below:

Elevated strength to density ratio, highest among all metals.

Excellent corrosion resistance

Excellent elevated temperature properties (up to 600 oC(1100 oF) )

High fatigue strength in air and chloride environments

High fracture toughness in air and chlorinated environments

Essentially nonmagnetic

High intrinsic shock resistance

High ballistic resistance-to-density ratio

Non toxic, non allergenic and fully biocompatible

Very short radioactive half life

Excellent cryogenic properties

All this gives titanium, its established uses and ever emerging applications,

great prospects beyond the New Millennium.

1.7. Applications of Titanium

1.7.1. Aerospace Industry

The major market for Ti is the aerospace industry. Aeronautical design

engineers find in titanium and its alloys a metal whose light weight and high

strength, particularly at elevated temperatures, render it a highly desirable

material in air craft construction. Titanium is finding increasingly greater

preference over aluminum and stainless steel in aircraft utilization. Aluminum


5

loses its strength rapidly at elevated temperatures. Titanium on the other hand,

has a distinct high temperature strength advantage at temperatures up to 800oF

(426oC), such elevated temperatures occur at high speeds due to aerodynamic

heating. The advantage of titanium substitution for steel in aircraft stems from its

accompanying weight reduction with no loss in strength. The overall reduction of

weight and the increased elevated temperature performance allowed by the

utilization of titanium permit increased payloads, as well as an increase in range

and maneuverability [Moiseyev, V. N. 2006].

Typical applications include jet engines and fire extinguisher bottles

replacing steel, landing gear beams substituting steel and aluminum to name few.

1.7.2. Food, Petroleum and Electrical Industries

Food processing tables as well as steam tables, where titanium has been

substituted for stainless steel due to superior and potential utilization. Titanium is

also used extensively in off-shore rigs.

The Electrical Industry equally desirous taking advantage of the metals’

high strength-to-light-weight ratio and, in addition, its high electrical resistance

and non-magnetic properties for utilization as cable armour materials.

1.7.3. The Marine Industry

The corrosion resistance of titanium and its alloys makes this metal a

prime consideration for use in marine environments. Of almost equal importance

in this regard is the high strength-to weight ratio. Naval applications comprise wet

exhaust mufflers, for submarine diesel engines, meter disks, and thin wall

condenser and heat exchanging tubes. In the case of the exhaust mufflers,

titanium may offer greater service life than offered by most materials.


6

1.7.4. Biomaterials

It is the combination of mechanical properties, tissue tolerance and

corrosion resistance to body fluids that makes titanium attractive in prosthetic

devices. An additional benefit is that Ti does not induce the growth of a fibrous

tissue barrier when placed in contact with healthy bone. Its non-toxicity also

makes it useful for surgical implants such as pacemakers, artificial joints and

bone pins. Titanium is used in prosthetic devices for dentistry and orthopaedics

and for heart valves.

1.7.5. Chemical and General Engineering Industry

It is the corrosion resistance, particularly in chloride containing

environments that is the attractive property. The offshore oil rigs provide a

classical condition for the use of Ti. Titanium vessels, heat-exchangers, tanks,

agitators, coolers and piping systems are utilized in the processing of aggressive

compounds, like nitric acid, organic acids inhibited reducing acids and hydrogen

sulfide. [Handbook of corrosion engineering By Pierre R. Roberge P;755-756].

1.7.6. Sports

Titanium is used for its high specific strength and stiffness. Here the cost

can even be an advantage. The major applications for titanium in sports are golf

clubs, bicycles, tennis rackets, and wheelchairs designed for those who want to

participate in sports [Forrest, A. L. 1981].

1.7.7. Computers

Since titanium does not become magnetized, it is used in the structural

parts surrounding computer components such as disk drives and microchips,

which can be ruined by stray magnetism.


7

1.8. Miscellaneous Applications

Other common applications of titanium include shape memory eyeglass

frames, watches, paper, plastic, rubber, roofs, surgical instruments and jewelry.

The automotive industry uses it in automotive components including connecting

rods, valves and suspension springs. It has applications in defense industry in

battle tanks, artillery guns and missile frames. Titanium is also used to

manufacture chlorine. As titanium dioxide, it is used in paints (replacing the use

of lead), lacquers, paper, plastic, ink, rubber, textiles, cosmetics, smokescreens,

leather, food colouring, and ceramics. Finally, as a compound known as titanium

tetrachloride, it is used for smoke screens and sky writing [Science

Encyclopaedia; Bomberger H.B. et al., 1985; Ogdom H.R. et al., 1956; Hunter

M.A. 1910; McQuillan A.D. et al., 1956; Froes F.H. 1992].


8

1.9. Objectives

Titanium has been irradiated with high energy electrons and mechanically

tested at low temperatures down to 100 K with the objectives:

1. To analyze the effects of high energy electron irradiation and low testing

temperature on the strength of irradiated material, which has scarcely

been done in the past?

2. To assess the response of titanium, an essential structural material of the

space craft, to the hostile space environments where space crafts are

confronted with high energy electrons and low temperatures.

3. To ascertain the viability of irradiation strengthened titanium as a material

for cryogenic vessels and components.

4. To gauge the efficacy of electron irradiated titanium in food processing and

packaging.

5. To investigate the relaxation resistance of structural material like titanium

before and after irradiation with high energy particles.

6. To highlight the credibility of high energy electron beam irradiation, being

an economical and useful means, for the development of materials with

the improved properties.

Chapter 2: Review of the Literature

9

CHAPTER 2

REVIEW OF THE LITERATURE

Metals have an immense importance in human life. In fact metals and

material progress work in tandem with each other; one complements the other.

This is precisely the reason why researchers in general and metallurgists in

particular show keen interest in un-earthling their hidden properties. Another field

of interest sprang up when metal surfaces were exposed to various kinds of

irradiation.

2.1. Irradiation of Metals and Alloys

The phenomenon of radiation induced defects in metals and alloys and

their correlation with the change of material’s mechanical properties has been an

interesting subject of research for more than 40 years [Zhecheva, Ani, Wei Sha,

Savko Malinov and Adrian Long 2005]. A great deal of effort has been made to

explain [Byun, T. and Farrell, K. 2004] radiation induced microstructural changes

in pure metals and alloys and its effects on the hardening of materials under

different conditions.

He found out that irradiated microstructures of FCC and BCC metals and alloys

are dominated by high density of defect clusters or loops of vacancies [Seeger A.

1958]. These clusters/loops are assumed to act as obstacles to dislocation

motion. This causes hardening in metals and alloys which results in the [Singh,

B.N.1995] reduction of ductility [Moteff J. et al.,1965; Ono K. and Mesni, M. 1967;

Waddington, J.S. and Lofthouse, K. 1967]. Woo, C.H. observed that irradiation

damage in crystalline solids due to the impingement of energetic particles occurs


10

in the form of atomic displacements, the initial morphology of which depends on

the energy transfer during the impact [Woo, C.H. 2000]. For example, MeV

electrons produce initial displacement damage in the form of Frenkel pairs, i.e.,

isolated vacancies and interstitials. Fast neutrons and heavy ions, on the other

hand, produce damage in the form of cascades and sub-cascades. The highly

localised deposition of the impact energy in the cascade volume in this case

produces a large amount of displacements, resulting in a structure that can be

described as a high concentration of vacancies and interstitials. The effects of

displacement damage on the physical and mechanical properties of metals and

alloys, caused by the bombardment of energetic particles, have been under

active investigation for many years. Besides the obvious technical and industrial

implications, an important motive of such investigations is to understand the

factors that differentiate the response of different metals under different radiation

conditions.

2.2. Irradiation Hardening of FCC Metals/Alloys

The authors observed that the proton and neutron irradiation of FCC

metals e.g. Ni, Cu, Al and Au causes the formation of defects like Stacking Fault

Tetrahedral. They further observed that a high density of small defect clusters,

similar to those observed in irradiated or quenched metals were there in the

deformed FCC metals. They analysed that rate of migration of defects

responsible for radiation hardening and those inducing radiation embrittlement.

They also studied the role of crystalline structure. They concluded that the

preliminary results show that the defect clusters are predominantly stacking fault

tetrahedral (SFT), dislocation loops and voids etc. They found that the interaction

of radiation induced defects, with moving dislocations, results in an increase of


11

yield stress and decrease in ductility [Dai Y. and Victoria M. 1997; Hashimoto N.

et al., 2006 Mohamed H.G. and Malta M.K. 2001].

Ghoniem et al. investigated the interaction of dislocations with radiation

induced defects clusters in FCC metals. They found that dislocations sources are

activated in spatial region of low SFT density, where their destruction by glide

dislocations leads to subsequent growth of localized plasticity in dislocation

channels [Ghoniem et al., 2002 and Ghoniem et al., 2001].

. Yao et al. studied microstructure and tensile behaviour of pure single

crystalline nickel with 590 MeV protons at room temperature in the dose ranging

from 0.002 dpa to 0.13 dpa [Yao et al., 2003]. It was observed that defect density

increases with increasing dose which in turn causes a wide serrated yield region.

Furthermore, the work hardening rate was found to decrease with increase of

dose. In the later study, [Bloom, E.E. et al., 1967] reported that irradiation

hardening in 590 MeV proton irradiated pure nickel crystal is present at lower

dose of 3.5×10-3 dpa. In another study regarding the irradiation hardening of FCC

metals, [Yao, Z. et al., 2004] investigated the deformation processes in proton

irradiated copper single crystals in the temperature range 293 to 77 K. Their

analysis of activation energies, inferred from relaxation tests, indicated that

multiple deformation processes are involved within the investigated temperature

range.

2.3. Irradiation Hardening of BCC Metals/Alloys

In case of BCC metals, [Bacon D.J. and Ostesky Yu. N. 2005] observed

the most common defects as dislocations loops, voids, precipitates etc. The

authors studied the radiation induced hardening of Fe and Fe-Cr alloys [Klueh,


12

R.L. 1991] using tensile tests. They found that irradiation hardening of the alloys

of Fe was due to the presence of small dislocation loops formed at the sites of

cascade damage collapse, while that of Fe-Cr alloys was on account of due to

Cr-rich precipitates formed at the same sites. According to the authors, this

hardening mechanism explains that there is a linear increase with Cr content in

the athermal component of the irradiation hardening of Fe-Cr alloys [Suganuma

K. and Kayano, H. 1983].

It is well known fact that the irradiation temperature can have a profound

impact on the microstructure that develops in materials. Kirtani found that in

qualitative terms, nucleation of defect clusters is maximized at low temperature

where as growth and coarsening of clusters is maximized at higher temperatures

[Kirtani, M.1988]. The authors studied the results of several previous ion

irradiation and low dose neutron irradiation and indicated that these temperature

changes may cause a significant influence on the microstructural evolution

[Kirtani M., et al., 1990; Kirtani M., et al., 1991; Kirtani M., et al.,1994; Yoshida N.

et al., 1994; Xu Q., et al., 1994; Matsui H., et al., 1994; Xu Q., et al., 1996;

Ohnuki T.S., et al., 1998; Ochiai K., et al., 1998; Kurtz, R.J. et al.,2004]. Voids

formation was generally enhanced and loop formation suppressed for the cyclic

temperature irradiation or pre-irradiation at low temperature.

Rice and Zinkle performed Transmission electron microscopy (TEM) on a

V±4Cr±4Ti alloy irradiated to damage levels of 0.1± 0.5 displacements per atom

(dpa) at 110oC [Rice, P.M. and Zinkle S.J. 1998]. They observed a high density of

small faulted dislocation loops when the irradiation temperature was kept below

275ºC. They also found that the dislocation loops became un-faulted when the

irradiation temperature was raised about 270ºC. At 300ºC, the dislocation loops


13

were found to be un-faulted and a high density of small Ti rich defect clusters

lying on [001] planes were observed to appear along with these un-faulted loops

at this temperature. Result of TEM tensile measurements provided an information

about the dislocation barrier strengths of faulted dislocation loops and [001]

defect clusters which were found to be 0.4 ± 0.5 and 0.25, respectively

[Fukumoto, K.i. et al., 2008]. They also observed cleared dislocation channels

when a specimen was irradiated at 268ºC and later put to tensile test. They found

this as an indication of the fact that both these defects can be easily sheared by

dislocations during deformation.

2.4. Effects of Irradiation Temperature

Bloom et al. studied the effect of irradiation at temperatures between 93

and 454°C upon the room temperature mechanical properties and electron

microstructures of AISI type 304 stainless steel have been determined

[Bloom, E.E. et al., 1967]. They found that the specimens, irradiated at

temperatures between 93 and 300°C, there were found defect clusters of the

order of 100 A in dia that led to an increase in yield strength. They also studied

the effect of instant and varying test temperatures on the microstructure of

irradiated stainless steel, vanadium and vanadium alloys extensively. However,

when irradiated temperatures was kept at 371ºC, no defect clusters appeared

and the yield stress decreased by a factor of too. When the temperature

increased beyond 371ºC precipitates within the grains [Kirtani M., Yoshiie T. and

Iseki M. 1994; Xu Q., et al., 1994; Xu Q., et al., 1996 and Kurtz R.J. et al., 2004].

In some cases, rather spectacular differences were observed. For example,

[Yoshida and co-workers 1994] observed that varying temperature (either

200/400°C or 300/500°C) neutron irradiation of Fe-16Cr-17Ni-0.25Ti austenitic


14

stainless steel produced dramatically higher void swelling levels compared to

constant temperature irradiation at 400°C.

The authors have studied the irradiation temperature effects on the

mechanical properties of precipitation-hardened copper alloys They determined

the tensile properties of specimens of four precipitation-hardened copper alloys

namely: Cu-Be, Cu-Cr, Cu-Cr-Zr and Cu-Cr-Zr-Mg that were irradiated in SM-2

reactor between 100 and 500°C. They observed that mechanical properties of the

alloys changed by changing the irradiation temperature. They further observed

that some strengthening occurred at low irradiation temperature between 100 and

200ºC. They also noticed that the yield strength of copper alloys was found to be

more temperature dependant in the test temperature range of 200 – 400ºC

[Osetsky, Y. 2000; Fabritsiev S.A. et al., 1992].

Hamilton et al. performed tensile tests and shear punch tests on a variety

of vanadium alloys that were irradiated in the advanced Test Reactor (ATR). The

irradiation temperature was kept between 200 - 300ºC, while the irradiation dose

was between 3 – 5 dpa. They carried out the tensile tests and the punched tests

first at room temperature and then at the irradiation temperature. They concluded

that following low temperature irradiation, the yield strength (YS) increased by a

factor of 3±4 while the ultimate strength increased by a factor of approximately 3

[Hamilton, M.L. et al., 2000].

2.5. Comparison of Irradiation Induced Hardening in FCC and BCC Metals /Alloys

Victoria et al. investigated the differences and similarities of behaviour

between FCC and BCC metals after irradiation. For this purpose, they irradiated

FCC Cu, Pd and 304 stainless steel and BCC Fe, Mo and Mo-5% with either


15

neutrons or 590 MeV protons at temperatures below recovery stage V. They

showed that a dense population of defect clusters develops. They also showed

that the type of cluster formed depends apparently on the stacking fault energy.

They found that in the case of stacking fault tetrahedral formed in Cu, the size is

independent of dose, while interstitial loops in stainless steel grow at neutron

doses higher than 1 dpa. They further found that the defected microstructure is

independent of the recoil energy spectra in this temperature region, but showed a

very strong dependence on the type of crystalline structure.

The authors studied the effect of irradiation on hardening both in FCC and

BCC metals, treated in terms of the so-called depleted zone (i.e. cluster or loop of

vacancies) formed in the cascade core. They observed that these clusters/loops

are assumed to act as obstacles to dislocation motion. They interpreted the

swelling behaviour of both FCC and BCC metals and alloys in terms of a

preferential absorption of Self-Interstitial Atoms (SIA) to dislocations(segments

and loops), commonly known as dislocation bias. They inferred that with regard

to the impact of irradiation conditions on the microstructural evolution and the

physical and mechanical properties, very little distinction could be made between

FCC and BCC crystals. They concluded that the reason for this was the lack of

information on fundamental features of the damage production in the form of

multi displacement cascade (e.g. intracascade clustering of SIA).

In recent years, computer simulations as well as experimental

investigations have demonstrated, however, that production as well as

accumulation of defects under cascade damage conditions in FCC and BCC

metals are likely to be substantially different. Further more, a closer examination

of available experimental evidence on defect production and accumulation


16

reveals the facts that under neutron irradiation FCC metals behave very

differently from BCC metals [Victoria, M. et al., 2000 Singh, B.N. et al., 2001 and

Singh, B.N. et al., 1995].

Caturla et al. made a comparative study of radiation damage accumulation

in Cu and Fe. They found significant differences in behaviour of BCC and FCC

metals, when these were exposed to neutron or heavy ion irradiation. They used

TEM to observe the extent of irradiation damage. They found that Stacking Fault

Tetrahedra (SFT) is noticeable in Cu irradiated to low doses, but no such damage

is visible in iron irradiated at the same total dose [Caturla, M.J. et al., 2000].

2.6. Irradiation Hardening of HCP Metals/Alloys

Radiation effects on the mechanical behaviour of hexagonal close-packed,

(HCP) materials have received considerable attention during the past few years,

partly due to their increasing technological interest for nuclear and aerospace

applications. The authors found that because of crystallographic anisotropy, the

diffusion of the lattice defects in HCP metals and alloys is also likely to be

anisotropic. In addition, in these metals and alloys, dislocations with different

Burgers vectors are non-equivalent in their motion and their reactions with the

point defects and their clusters. They reached a conclusion that damage

accumulation in HCP produces effects that are different from those in cubic

metals, (FCC and BCC). They further concluded that in addition to void swelling

and irradiation creep, the anisotropy of the evolving dislocation structure

produces a deviatoric straining even in the absence of an external stress. Indeed,

irradiation growth is the name given to the volume-conserved shape deformation

that occurs in non cubic crystalline materials under irradiation in the absence of


17

an applied stress. According to the authors the best known examples of

irradiation growth are found in graphite, uranium, zirconium and its alloys. The

authors generalized the rate theory in order to apply it to HCP metals like

zirconium (Zr), using the reaction kinetic theory of an isotropically diffusing

reactants [Woo, C.H. et al., 2003].

The authors carried out the irradiation of pure titanium with high energy

protons at 300 K. They found that defect clusters cause the hardening of the

material. They also found that the plastic deformation of irradiated titanium occurs

via propagation of dislocations through a cloud of defects produced during

irradiation, leading to their annihilation and the formation of a cellular dislocation

structure together with twins . The authors concluded that twins play an important

role in the plastic deformation of titanium [Leguey, T. et al., 2000; Leguey, T. et

al., 2005. Nemat Nasser et al. observed that enhanced strain rates increase the

density of twins [Nemat Nasser et al., 1999] which results in further hardening.

Lavrentiev et al. studied the effect of higher temperature and doses on the

hardening mechanisms. They saw that hardening becomes more pronounced

due to the increased density of radiation induced defects [Lavrentiev, V. et al.,

1999].

Leguey et al. investigated that the micro structural modifications due to

irradiation of pure titanium by 590 MeV protons and their consequences on the

mechanical properties of the irradiated metal. They irradiated the samples to a

low dose range at two different temperatures, room temperature and 523 K. They

found that the dose dependence of the irradiation hardening depended strongly

on the investigated temperatures. They observed that mean defect size increases

with temperature and dose, the dependence on the latter being stronger at 523 K


18

than at 300 K. They also studied the effects of temperature on the mechanical

properties of the samples. They showed that these effects were more significant

for the samples irradiated and tested at high temperature, namely at 523 K

[Leguey, T. et al., 2005].

The authors carried out an analysis of damage structure in annealed and

cold-worked Marz grade titanium exposed to following high dose irradiation of

between 1025 and 1026 n/m2 (E > 0.1 MeV) over a temperature range of 589 to

778 K. The authors found that the damage consists mainly of (i) (a) type

dislocation loops aligned in bands parallel with (0001) and (ii) network

dislocations. They also found that in most cases the majority of loops have

vacancy character, especially at the lower temperatures. They explained this in

terms of the larger interstitial loops forming a network in annealed samples and

interstitials being absorbed at network dislocations in cold-worked samples

(Griffiths, M. et al., 1983 and Griffiths, M. et al.,1991].

Bradley et al. irradiated the specimens of vanadium and titanium at 300K

with T (d, n) neutrons. The aim was to study the flow properties and

microstructures of the irradiated metals. The authors observed the hardening

effect to be similar in both the irradiated materials. None of the materials showed

the low fluence hardening plateau, regular pattern observed in BCC metal which

is a result of interstitial impurity atom [Bradley, E.R. and Jones, R.H. 1981].

Titanium alloys are used to make the flexible connectors installed as

attachment of the first wall modules of the ITER FEAT fusion reactor. The authors

carried out an assessment of the tensile and fatigue performance of two

candidate alloys, a classical two phase Ti6Al4V alloy and a monophase α alloy,


19

Ti5Al2.5Sn using 590 MeV protons for the simulation of the fusion neutrons. The

dose deposited was up to 0.3 dpa and the irradiation temperature was between

40°C and 350°C. They found that similarity existed in tensile behaviour of both

the alloys in un-irradiated condition. However, the tensile properties of both the

alloys showed appreciable difference after irradiation. They further discovered

that radiation hardening and the ductility showed different trends in α and β

alloys. They also found that radiation hardening was more prominent in the α

alloy than in the β alloy. They also noticed that ductility reduced considerably in β

alloy as compared with the α alloy. They carried out the TEM analysis of the

samples and observed that precipitation in the primary and secondary α grains of

the dual phase alloy seems to be the cause of the intense radiation hardening

[Marmy P. and Leguey T., 2001; Marmy P. and Luppo M. 2003]. The authors

studied the tensile and fracture toughness properties of unirradiated and neutron

irradiated titanium alloys. They observed that in the unirradiated condition the

Ti6Al4V (α+β) alloy has slightly higher tensile strength and noticeably lower

ductility compared to that of the Ti5Al2.5Sn (α) alloy both at 50 and 350°C. They

further observed that the fracture toughness behaviour of both alloys is similar at

ambient temperature. At 350°C, on the other hand, the fracture toughness of the

(α) alloy is lower compared to that of the (α+β) alloy. The authors also found that

neutron irradiation at 50°C to a dose level of 0.3 dpa caused hardening, plastic

instability and a substantial reduction in fracture toughness of both alloys. They

noticed that irradiation at 350°C resulted in a substantial hardening and a

significant decrease in the fracture toughness in the (α+β) alloy due to irradiation

induced precipitation whereas only minor changes in the tensile and fracture

toughness behaviour were observed in the (α) alloy. They concluded that the

tensile and fracture toughness properties of the (α+β) alloy are more strongly


20

affected by neutron irradiation compared to that of the (α) alloy [Tahitinen, S. et

al., 2002; Tahitinen, S. et al., 2007].

Gelles et al. investigated the microstructural evolution in response to 1

MeV electron irradiation for three sample ferrite alloys, pure beryllium, pure

vanadium, and two sample vanadium alloys over a range of temperatures and

doses. They found that microstructural evolution in Fe-3Cr Fe-9Cr and Fe-18Cr

ferrite alloys consists of crenulated, faulted a (100) loops and circular unfaulted ~

a (111) loops at low temperatures, but with only unfaulted loops of both types at

high temperatures. They attributed this complex dislocation evolution to sigma

phase precipitants arising from chromium segregation to point defect sinks. They

observed that Beryllium is found to be resistant to electron damage; the only

effect was enhanced dislocation mobility. They further investigated that pure

vanadium, V-5Fe, and V-Ni microstructural response was complicated by

precipitation on heating to 400°C and above, but dislocation evolution was in the

range of room temperature to 300°C and at 600°C. They observed similarity in

behaviour of three materials, except that pure vanadium showed more rapid

dislocation evolution. According to the authors, this difference does not explain

the enhanced swelling observed in vanadium alloys [Gelles, D.S. et al., 1992].

2.7. Comparison of Irradiation Induced Hardening in BCC, FCC and HCP Metals

Griffiths et al. and Singh et al. found that Radiation damage in HCP metals

is significantly different from that in FCC or BCC metals. They also found that in

the latter metals, loops that are primarily interstitial in nature simply form on the

close-packed planes, (111) for FCC and (110) for BCC. The loops may then

shear into configurations of lower energy that is unfaulted depending on the


21

stacking-fault energy, the temperature and size of the loops. As the interstitial

loops grow, the remaining vacancies eventually cluster to form voids when a

sufficiently high super saturation has been achieved. They concluded that in

contrast with the FCC and BCC cases, radiation damage in HCP metals varies

from one HCP metal to another, generally reflecting the relatively close packing

of different planes, which is dependent on the c/a ratio [Griffiths, M. et al., 1991

and Singh, B.N. et al., 1995].

Wen et al. noticed that one of the important factors that differentiate the

damage accumulation behaviour of different metals and alloys under different

irradiation conditions is the crystal lattice structure [Wen, M. et al., 2000]. The

authors found that in this regard, the usual rate theory model for cubic metals

(FCC and BCC), which assume the isotropic diffusion of point defects do not

accurately reflect the reaction kinetics of point defects in the HCP metals. They

also noticed that the diffusion anisotropy difference (DAD) between vacancies

and interstitials can produce a bias among sinks that dominates the conventional

dislocation bias. This effect adds a new dimension of complexity to the defect

accumulation behaviour of HCP metals, and is found to be a major factor in the

damage accumulation behaviour between the HCPs and the FCC and BCC

metals [Woo, C.H. 2000].

It was observed [Mott N.F. and Nabarro F.R.N. 1940] that in cubic metals it

is well known that the irradiation-induced climb of dislocations and growth of

interstitial loops cause a concomitant volume increase due to the growth of voids.

They discovered that in the HCP, the anisotropy of [Woo, C.H. 2003] both the

diffusion of the intrinsic point–defects and the evolving dislocation structure

[Woo, C.H.2000] necessarily produce a deviatoric straining in addition to


22

volumetric swelling, even in the absence of an external stress [Woo, C.H. 1996].

In pure metals, having cubic crystal structure, the plastic deformation largely

takes place through slip mode. They showed that dislocation kinetics theory

provide insight into the slip mechanism in this case. However, slip system of HCP

metals, being an isotropic in nature, are quite different from those cubic metals.

The HCP metals have fewer slip system than the usual five which are essential

for sustained plastic deformation to take place in case of cubic metals [Frank F.C.

and Nicholas J.F. 1953 and Song S.T. and Gray III G.T. 1995]

In one of the studies the authors [Byun, T.S. and Farrell, K. 2004]

characterized irradiation hardening behaviours of BCC, FCC and HCP pure

metals and alloys after neutron or proton and neutron irradiations at low

temperatures. They also compared parameters of radiation effects and concluded

that the transition from the low-dose to the high-dose regime in irradiation

hardening occurs either when the tensile specimens undergoes prompt plastic

instability at yield or when saturation of defect cluster density occurs. Leaguey et

al. investigated irradiation effects on the mechanical behaviour and

microstructure of polycrystalline pure titanium. They found that as previously

observed for FCC and BCC pure metals, proton irradiation at ambient

temperature of pure HCP titanium produces hardening of the material that was

proportional to the fourth root of the irradiation dose, behaviour similar to that of

pure BCC iron. They further found that a stronger dependence at 523K. They

concluded that the microstructure of irradiated and/or deformed specimens is

similar to that previously observed for pure FCC metals, the formation of twins

being intrinsic to deformation of HCP titanium. When they analyzed the finding in

the frame of a dispersed obstacles hardening model, the irradiation defects were


23

observed to be weak obstacles, with a lower strength than those observed for

pure FCC metals [Leaguey, T. et al., 2005].

Hashimoto et al. analyzed the behaviour of defect mechanism of pure

polycrystalline metals vanadium (BCC), Copper (FCC) and zirconium (HCP).

When these metals were irradiated with neutrons at temperature of 80ºC, they

observed the appearance of dislocation channels in all the deformed metals. All

these metals showed plastic instability at yield point. They further noticed a

dislocation pile up at the grain boundary of deformed vanadium metal. The

irradiation dose in this case was found to be 0.012 dpa. The result was a channel

deformation, giving rise to dislocation pile up, leading to stress delocalization, a

source of boundary cracking [Hashimoto, N. et al., 2004].

2.8. Mechanical Behaviour of HCP Metals/Alloys at Low Temperatures

The authors studied the mechanical behaviour of HCP metals at low

temperature, which has received considerable attention of researchers during

recent years. They found that successful use of HCP metals as structural

materials at low temperature is conditioned by their high specific strength and

high ductility [Ghauri, I.M. and Afzal Naveed, 2007]. Several methods such as

mechanical alloying and equal channel angular pressing have been used to

improve the strength and ductility of HCP metals [Valiev R.Z.,2000; Koch C.C.

and Whittenberg J.D. 1996; Suryanarayana C. 1995; Bengus V.Z., et al., 2002;

Bengus V.Z., et al., 2002; Tabachnikova E.D. 2005]. Recent investigations by the

authors have revealed that high strength and ductility can be achieved by

reducing the grain size of the materials [Csach K. et al., 2004; Tabachnikova

E.D.V.Z. et al., 2001; Kojima Y. 2000]. The authors observed that with grain size


24

refinement, strength and ductility was observed to increase [Mabuchi M.1999]

even at low temperature [Makin M.J. Sheely F.(Ed.),1967]. Irradiation by

energetic particles is another way of increasing the strength of materials.

Titanium being HCP in nature is widely considered as structural material for

cryogenic and aerospace applications. In the past, authors have investigated the

irradiation hardening of pure titanium at room temperature. It was found that the

plastic deformation in pure titanium at room temperature, [Ames, S. 1954]

irradiated with 590 MeV protons, occurs via propagation of dislocations through

cloud of defects produced during irradiation, leading to their annihilation and the

formation of a cellular dislocation structure together with twins resulting in

hardening [Diehl J. and Seidel G.P.1969; Lucas G.E. 1993; Leguey T. et al.,

2002]. In case of titanium irradiation with 800 KeV electrons, hardening is the

result of an intensive process of twinning produced by shear stress waves. The

author made an attempt to explain how uniform elongation (UE) takes place

when vanadium alloys were irradiated with neutrons in Advanced Test Reactor

(ATR) and at Experimental Breeder Reactor II (EBR-II) to the fluence of 11 dpa.

They observed that the UE was maintained about 6% after irradiation at 388°C.

However, this trend became negligible when the alloys were annealed at 1000ºC

and 1100ºC, and when irradiation temperature was kept below 300°C. On the

other hand, the specimens annealed at 900°C and 700°C showed relatively large

UE after such low-temperature irradiation [Chuto, T. et al., 2000]. The authors

compared their findings with the results in literature and proposed that oxygen

concentration should be kept below 200 ppm to maintain the UE after irradiation

below 400°C. The control of interstitial impurities, especially oxygen in solution,

was important to maintain the UE of the alloy [Satou, M. et al., 2000].


25

Walter Bauer studied the recovery of point defects produced by 1.2- MeV

electron and 40 MeV alpha particle irradiation in the temperature range 50 to

270oK. He also studied the influence of varying irradiation dose, pre-irradiated

elevated temperature, and pre-deformation. He found that the recovery below

150K is dominated by the annihilation of Frenkel pairs, with long range or

uncorrelated migration of the interstitial beginning at about 120K. He further

observed that the recovery of the annealed samples irradiated to relatively low

dose is characterized by super-recovery. He proposed a model involving the

migration of interstitial hydrogen impurities from interstitial to substitutional sites

to explain this phenomenon [Walter Bauer 1969].

2.9. Mechanical Behaviour and Test Temperature

The authors studied the effect of strain rate, temperature, grain size, and

texture on the substructure and mechanical response of high-purity

polycrystalline titanium. The compressive stress-strain response of 20 and 240

um grain size high-purity Ti was found to depend on both the applied strain rate

and the test temperature. They found that the rate of strain hardening in Ti

increases with increasing strain rate. They also found that the substructure of

high-purity Ti deformed at high strain rate or quasi-statically at 77K displayed a

higher incidence of deformation twinning than in during quasi-static deformation

at 298K [Gray III, G. T. 1996; Gray III, G. T. 1997; Kaschuer G.C. and Gray III

G.T. 2000].

Byun et al. studied the tensile properties of EC316 LN austenitic stainless

steel and 9Cr-2WVTa ferritic/martensitic steel after 800 MeV proton and

spallation neutron irradiation to doses in the range 0.54-2.53 dpa at 30-100°C.


26

Tensile testing was performed at room temperature (20°C) and 164°C. They

observed that the EC316LN stainless steel maintained notable strain-hardening

capability after irradiation, while the 9Cr-2WVTa ferritic/martensitic steel posted

negative hardening in the engineering stress-strain curves. They also observed

that in the EC316 LN stainless steel, increasing the test temperature from 20°C to

164°C decreased the strength by 13-18% and the ductility by 8-36%. They

noticed that the effect of test temperature for the 9Cr-2WVTa ferritic/martensitic

steel was less significant than for the EC316 LN stainless steel [Byun, T.S. et al.,

2002].

Bailat et al. investigated the effect of neutron irradiation of different doses

and testing temperature on tensile behaviour of stainless steels. They found two

modes of deformation, twinning and channelling, depending on the testing

temperature [Bailat, G.C. et al., 2000].

Hamilton et al. performed tensile tests and shear punch tests on a variety

of vanadium alloys that were irradiated in the advanced Test Reactor (ATR). The

irradiation temperature was kept between 200 - 300ºC, while the irradiation dose

was between 3 – 5 dpa. They carried out the tensile tests and the punched tests

first at room temperature and then at the irradiation temperature. They concluded

that following low temperature irradiation, the yield strength (YS) increased by a

factor of 3±4 while the ultimate strength increased by a factor of approximately 3.

Uniform elongation (UE) and tensile reduction in areas showed that the ductility

diminishes following irradiation. They found that the correlation between uniaxial

ultimate strength and effective shear maximum strength was in excellent

agreement with previous studies on other materials. They revealed that using the

room temperature test data, the correlation between uniaxial YS and effective


27

shear YS was in excellent agreement with previous studies on other materials

[Hamilton, M.L. and Toloczko, M.B. 2000].

2.10. Stress Relaxation of Metals and Alloys

The phenomenon of stress relaxation plays a vital role in the construction

of structural materials [Ghauri, I.M., Afzal Naveed and Haider Karrar, 2006].

Efforts have been made investigate the effects of grain size to [Carp, O. et al.,

2004] annealing temperature, deformation temperature, aging and irradiation etc

on the stress relaxation rate in different metals and alloys. [Akhtar et al.,1994 and

Akhtar et al.,1995] studied the effect of grain size on the stress relaxation rate in

Ti-Nb alloy. They observed that stress relaxation rate decreases with decrease in

grain size [Conrad, H. 1981] in the range 117 to 67μm. This was attributed to the

retarding effect of un-relaxed dislocation pile-ups developed at grain boundaries

in the course of relaxation. Later on it was observed that for a given grain

diameter ‘D’ and fixed initial stress level σ0, the relaxation rate ‘s’ is sensitive to

annealing temperature; the higher is the annealing temperature the lower is the

relaxation rate and vice versa [Butt, M.Z. et al., 2000]. Feltham investigated the

stress relaxation behaviour of pure iron in the vicinity of upper yield stress

between 358 K and 77 K [Feltham, P. 1961]. It was found that stress relaxation

rate depended upon both the temperature and initial stress levels at which the

further deformation of specimens was stopped [Bashir F. and Butt, M.Z. 2007].

Butt et al. analyzed the data on iron and observed that the activation energy of

the stress relaxation process in iron polycrystal was compatible with the energy of

formation of jogs by intersection of dislocations at T>260 K and to recovery

processes such as vacancy formation, cross slip etc. below 260 K [Butt M.Z. and

Yousaf Sani M.1987]. Agarwal et al. carried out tests on titanium wire as a


28

function of grain size (5 to 42 um) at 77 to 623 K to study the rate-controlling

mechanism. Values for the activation volume were derived from the slope of the

stress relaxation curves [Agarwal, S.P. et al., 1973]. These values are in good

agreement with those obtained in the more conventional strain rate [Bailat, G.C.

2000] cycling tests and support the earlier conclusion, i.e., the rate controlling

mechanism during the low temperature deformation of titanium is thermally

activated overcoming of interstitial solute obstacles on the [Tanaka,T. and

Conrad, H. 1972] first order prism planes.

Marmy et al. carried out stress relaxation tests of Ferritic-Martensitic steel

before and after irradiation. The specimens were irradiated with 590 MeV protons

at irradiation temperatures between 360 and 700 K at doses up to 0.7 dpa. The

authors used relaxation stress experiment systematically in monotonic tensile

tests. They found that: between 290 and 460 K, the deformation is controlled by

the glide of screw dislocation in Peierls hills through a net of localized obstacles,

between 460 and 720 K, the deformation is characterized by the presence of the

Portevin-Le Chatelier effect, and between 720 and 870 K, the deformation

mechanism corresponds to a thermally activated dislocation motion through a

distribution of microscopic obstacles [Marmy, P. et al.,1994].

Yao et al. carried out the irradiation of single-crystal nickel with 590 MeV

protons to 10-1 dpa at room temperature. Irradiated and unirradiated tensile

samples were deformed and relaxation tests were performed at temperatures

between 77 and 423 K. They found that a strong temperature dependence of the

flow stress for samples irradiated to 0.1 dpa as compared to the unirradiated

case. They also observed strong irradiation hardening in single-crystal Ni


29

irradiated at room temperature and deformed in the range from 77 to 423 K

[Yao, Z. et al.,2005].

Blanter et al. studied the interaction of dissolved atoms and relaxation due

to interstitial atoms in HCP metals. For determining the cause of induced

relaxation, modelling of the short-range order and atomic displacement fields

around the solute atom clusters was carried out by the Monte-Carlo technique for

typical Ti-O-Zr alloys. The energies of strain-induced (elastic) O-O and O-Zr

interactions and displacement fields of host atoms around the solute atoms were

calculated and used in modelling. The concentration dependence of relaxation

strength due to diffusion under stress of oxygen atoms was evaluated using the

values of local displacement around the solute atom complexes. It is shown that

the developing short-range order cannot be described by the single O-O or O-Zr

pair and the associated relaxation, as simple reorientation of any specific atomic

pairs. It seems likely that in many cases the internal friction is caused by more

complicated clusters constituted by interstitial and substitutional atoms

[Blanter, M.S. et al., 2004].

Wen et al. analysed the effects of stress on the diffusion and its anisotropy

in the HCP metals bombarded by energetic particles [Wen, M. And Woo, C.H.

2000]. The diffusion anisotropy of intrinsic point defects is an important factor

governing the behaviour of the HCP metals bombarded by energetic particles.

The effects of stress on the diffusion and its anisotropy, although known to be

important, have not been well understood. They used a combination of molecular

dynamics and molecular statics methods to investigate energy states of a self-

interstitial in α-titanium, a typical HCP metal. It was found that the most stable

configuration of the self-interstitial is the basal-split dumbbell configuration on the


30

basal plane. Compression along the [0001] or the [1100] directions leads to an

insignificant change in the migration energies, while compression along the

[1120] direction leads to larger migration energy. A significant change of the

diffusion anisotropy was observed when a uni-axial compressive stress of

200 MPa is applied along the [1120] direction. Similar stress along the other two

directions does not produce substantial changes of the anisotropy. It was also

shown that an applied hydrostatic stress could significantly change the diffusion

anisotropy of HCP metals and alloys. Thus, under irradiation, a hydrostatic stress

can produce a significant creep-like deformation (i.e., with a deviatoric strain rate)

through a stress-dependent change of the growth rate.

As deduced from literature survey high energy particles and ions

bombardment on the materials can affect the microstructure which in turn may

affect the material’s properties. Hence a newly found phenomenon attracted

cluster of researchers looking out for new horizons in this field. This phenomenon

paved the way for opening new vistas of research.

The present work has been taken up to investigate the effect of irradiation

energy, test temperature, exposure time, on the yield stress, stress relaxation

and microstructural evolution of high purity titanium irradiated by high energy

electrons (8-18 MeV).

Chapter 3: Material and Methods

31

CHAPTER 3

MATERIAL AND METHODS

The material used in the present research work is high purity titanium

(99.995%) supplied by Alfa Aesar, USA, as cylindrical rods of 100 cm length and

2 mm diameter. To begin with, specimens were prepared for tensile testing and

microstructural analysis. Tensile specimens of 60 mm length and 2 mm diameter

were machined from the as received pure titanium rod, where as the

metallographic specimen was 15 mm long and 2 mm diameter, cut from the same

rod. All the specimens were sealed in a silica tube under vacuum of the order of

10-6 torr so that no oxidation or any other reaction takes place during heat

treatment. Vacuum annealing of titanium reduces hydrogen embrittlement of the

metal. Annealing was carried out at 704°C for 30 minutes to relieve internal

stresses. The metallographic specimens were annealed prior to mounting,

grinding and electrolytic polishing, to keep mounting material, bakelite from any

damage during annealing. The specimens for tensile testing, on the other hand,

were electrolytically polished in the annealed state.

A series of specimens were irradiated at 300K with 8, 10, 12, 15, and

18 MeV electron beam, keeping exposure time constant at 12 minutes. Another

series of specimens were irradiated to constant incident energy of 12 MeV but at

varying exposure time.

The un-irradiated and irradiated specimens were deformed at room

temperature and then at low temperatures, using a soft ware operated 50 kN

tensile testing machine [Ghauri, I.M., Afzal Naveed and Haider Karrar, 2006].


32

Experiments at low temperatures in the range 300 to 100K were performed in

liquid nitrogen atmosphere. Deformation experiments were performed up to

fracture. The [Leguey, T. et al., 2005] stress and strain measurements were

recorded on the computer attached with the machine. During the deformation,

straining was frequently interrupted by arresting the crosshead to observe the

stress relaxation at a fixed load. The stress relaxation curves of both irradiated

and un-irradiated specimens were obtained for different values of initial stress

(σo) and recorded by the computer.

In microstructural analysis, the influence of varying irradiation energy and

effects of test temperature were studied. In addition, fractured surfaces of the

fully deformed and failed tensile specimens were examined using a scanning

electron microscope (SEM) [Singh, B.N. 1996].

The experimental work consisted of the following steps:

3.1. Preparation of the specimens

3.2. Irradiation of the specimens

3.3. Tensile testing

3.4. Microstructural Analysis

3.1. Specimen Preparation

Titanium, being a highly ductile metal, needs careful handling during

specimen preparation and maximum care was exercised while cutting, grinding

and polishing the specimens. Overheating can lead to large burrs. Customary

polishing techniques like diamond polishing results in mechanical deformation in

the form of smearing and scratching. Therefore electrolytical polishing was used

as this polishing technique leaves no mechanical deformation on the surface of


33

the specimen, in addition to the ease of operation and speedy in getting results

[Sheldon Wiederhorn 2007].

The sequential order of specimen preparation method was:

3.1.1 Cutting

3.1.2 Mounting

3.1.3 Grinding and polishing

3 .1.4Annealing

3.1.1. Cutting

Polycrystalline pure titanium was in the form of a rod; 100 cm long and

2mm diameter. Specimens 6 cm long were cut from the as-received rods for

[Butt, M.Z. 2009] tensile testing purpose and 1.5cm long for microstructural

studies. Maximum care was exercised, as due to its high ductility titanium can

easily overheat during cutting and large burrs can occur [Boyer R., handbook,

Welsch G., Collings E. W. 1994].

3.1.2. Mounting

Small specimens generally require mounting, so that specimen is

supported in a stable medium for grinding and polishing. Specimens of irregular

shape, great fragility or very small size are best mounted in plastic. Specimens

can be mounted quickly by using some thermosetting substance, such as

bakelite or, alternatively, a transparent thermoplastic material. These substances

mould at about 150ºC, which is usually too low to cause any structural change in

the specimen.


34

In the present research work Bakelite powder was used as mounting

material. Specimens were mounted by Prestopress-3 (Model: 4415115, Struers,

Denmark) mounting machine at about 150ºC.

3.1.3 Grinding and Polishing

Grinding is the most important operation in sample preparation. It is first

necessary to obtain a reasonably flat surface on specimen. This can be done

either by using a fairly coarse file or by using a motor-driven emery belt. Which

ever method is used, care must be taken to avoid overheating the specimen

which may lead to alterations in the microstructure. .

Intermediate and fine grinding was carried out on emery papers of

progressively fine grade. Emery or silicon carbide cloths and papers are normally

used. However, pure and softer titanium, and mounted specimens should always

be ground using silicon carbide paper [Boyer R., Welsch G., Collings E. W. 1994;

Chern, G.L. 2006; Bell T. and Dong H.2000]. In the present work five different

grades (220, 320, 500, 1000 and 1500, from coarse to fine) of silicon carbide

(SiC) papers were used. All grinding should be done wet, provided water has no

adverse effects on any constituents of the microstructure. Specimen should be

cleaned thoroughly after each grinding step to avoid any carryover of abrasive

particles to the next step. Water solutions containing detergents are excellent

cleaners, and ultrasonic cleaning is an effective technique Wet grinding

minimizes loading of the abrasive with the metal removed from the specimen

being prepared beside its cooling effect which provides control of overheating

which may cause alterations of the microstructure.


35

When the specimen appears to be flat over the entire surface and grinding

has removed all surface imperfections, this operation is considered to be

completed.

Polishing is the final step in producing a surface that is flat, scratch free

and mirror like in appearance. Such a surface is necessary for subsequent

accurate metallographic interpretation both qualitative and quantitative. The

precautions for cleanliness must be strictly followed.

Contrary to the usual procedure of using finer and finer diamond for

polishing, as is the case in most of the metals, diamond polishing actually

introduces continuously mechanical deformation which leaves scratches and

smearing on the surface. Therefore diamond polishing should be avoided,

especially with polycrystalline high purity titanium. Its extreme ductility makes

titanium prone to mechanical deformation and scratching [Chern, G.L.2006],

which necessitates a chemical-mechanical polish or electrolytic polish, but the

latter gets precedence over the former. As an alternative to mechanical polishing,

electrolytic polishing can be recommended when fast results are required.

Electrolytic polishing methods are particularly appropriate for the following

reasons: speed of results, ease of operation, reproducibility. Also, electrolytic

polishing leaves no mechanical deformation on the surface. With these ends in

view, both rod specimens and embedded specimens were electrolytically

polished, making use of the arrangement shown below in Fig.3.1.


36

Fig.3.1. Rectifier and other facilities

The Electrolyte consisted of H2SO4 90% solution, density 1.84 (25 vol %),

HF 40% solution, density 1.10 (15 vol %), and glacial CH3COOH 100% solution,

density 1.05 (60 vol %), with an additional agent cetyltrimethyl ammonium

bromide (0.1-0.5 gml-1). This agent modifies, the polarization of one of the two

electrodes (alternate adsorption and desorption phenomena) in the medium and

leads to modifications in the double-layer phenomena. As a result, the quality of

the polishing is improved and less material is removed. The electrolytical

polishing procedure requires a fine ground surface to #1200 or finer. Table 3.1

shows a general procedure for grinding titanium and titanium alloys.


37

Table 3.1. Data regarding SiC Polishing

Table 3.2. Electrolytic Polishing

Equipment: Rectifier(100V)

Electrolyte: As described

Temperature: 15-18o C

Current: .015A

Voltage: 11V

Time: 15-18 sec

Step PG 1 PG 2 PG 3 FG 1 FG 2

Surface SiC paper #220

SiC paper #320

SiC paper #500

SiC paper #1000

SiC paper #1500

Lubricant Water Water Water Water Water

Rpm 300 300 300 300 300

Force[N] By hand By hand By hand By hand By hand

Time 130 sec. 100 sec. 80 sec. 60 sec. 60 sec.


38

Different arrangements were used for polishing rod like specimens and the

embedded ones. Two sets of crocodile clips were used for electrolytic polishing of

rod tensile specimens and embedded ones: small size clips for holding the former

and large size for holding the latter in the bath. The rod specimens were held by

small crocodile clips for dipping in the bath and the mounted specimens were

clawed by large crocodile clips. Please be aware that the polishing time can vary

depending on the purity of titanium and the area of the specimen’s surface. The

optimum parameters were devised, at Pakistan Council of Industrial and

Scientific Research (PCSIR) to obtain a mirror finished surface. Maintaining the

bath temperature within the specified values. posed another problem. This was,

however, overcome by employing a two-pronged strategy: preparing three to four

electrolytes at one time, using one of them while immersing the remaining ones in

ice tub. Similarly, time duration rationalized for polishing two kinds of specimens,

a result of repeated and concerted efforts, turned out to be 15 and 18 seconds in

case of mounted and rod tensile specimens respectively.

3.1.4 Annealing

All the specimens, both for metallographic interpretation and for tensile

testing were annealed in vacuum at 704oC for (30 minutes): first to relieve

stresses and second to increase softness, ductility, toughness and produce a

specific microstructure. Annealing refers to a heat treatment in which a material is

exposed to a specified temperature for an extended time period and then slowly

cooled. During heat treatment, titanium can react with hydrogen, oxygen and

nitrogen. Hydrogen absorption begins at temperatures of 500ºC upwards. At

temperatures above 700ºC, oxygen and nitrogen lead increasingly to the

formation of scale layers, with oxygen also diffusing into the work piece surface


39

(diffusion zone). These chemical reactions lead to reduced toughness and

thermal stability of the titanium materials. To avoid such air contaminations during

annealing, it was necessary that annealing must be carried out in vacuum.

Vacuum annealing of Ti reduces hydrogen embrittlement of the metal. With this

end in view, specimens were sealed in silica tubes under a vacuum of the order

of 10-6 torr so that no oxidation or any other reaction can take place during

annealing. The arrangement of vacuum system, shown in Fig.3.2.had following

components:

I. Rotary pump

II. Diffusion pump

III. Liquid Nitrogen trap

IV. Silica tubes

V. Tubes sealing arrangements

Fig. 3.2. A photograph of the used vacuum system


40

For heat treatment, a micro controller based muffle furnace, Naberthertm

(Model LHT 02/18 Germany) was used (Fig.3.3). It is a high power unit which can

achieve up to 1800ºC temperature in one hundred minutes. The furnace employs

four high power immersion rods / heating elements, which work in the form of two

pairs in parallel, providing high efficiency output control.

Fig. 3.3. Annealing Furnace

Vacuum sealed specimens were annealed at 704ºC for (30 minutes). The aim

was to prepare the specimens for irradiation and tensile testing purposes at a

later stage. Besides, a batch of specimens was also annealed by varying the

annealing time. The specimens were sealed in four silicon tubes evacuated to

10-6 torr [Butt, M.Z. and Khilji, M.S. 2009]. Each tube contained two rod

specimens (6 cm length, 2 mm thickness) and two short- length specimens (1.5

cm long) for mechanical testing and microstructural observations respectively.


41

Annealing temperature was kept constant in order to focus purely on radiation

effects.

3.2. Irradiation of the Specimens

Irradiation of metals with energetic particles brings a visible change in

microstructure which in turn may produce significant changes in the mechanical

properties of the metals [Bell T. and Dong H. 2000; Avener 1974.]. Titanium

specimens were irradiated with electrons of different energies obtained from a

Mevatron Linear Accelerator, installed at Shaukat Khanum Memorial Cancer

Hospital (SKMCH), Lahore, Pakistan, shown in Fig.3.4.

Fig.3.4. Mevatron Linear Accelerator

The irradiation dose used was 0.01 dpa (displacement per atom). The

temperature of the specimens was kept constant during the process of irradiation.

The range of electron beam calculated using the energy range relation

‘R = (0.524E – 0.1337) /d’, (where R = range of beam (in cm) inside the material,


42

E= beam energy (in MeV) and d= density (in grams/cc) [Joseph D.G. and Engle

P.D. 2002] was found to be between 0.89 and 2.05cm for titanium (E = 8 to 18

MeV). The irradiation process followed the following sequence:

3.2.1. Energy Dependence of Irradiation Effects

In the first instance a rod specimen and an embedded specimen, both in

the annealed state, were irradiated at 300 K for a period of 12 minutes with an 8

MeV electron beam. Rod sample was irradiated with the intent of tensile testing,

while embedded test piece was irradiated for microstructural analysis. This test

was repeated three times to ascertain the repeatability of the results obtained.

The energy of incident beam was then increased to the values of 10, 12, 15 and

18 MeV, keeping all other parameters constantans in case of earlier exposure.

The aim was to assess the effect of energy level on the evolving microstructure

and corresponding change in the hardness and strength of the irradiated

material.

Another series of specimens were irradiated with the stated energy values

for subsequent testing at low temperatures ranging from 300 to 100 K.

3.2.2 Exposure Time Dependence of Irradiation Effects

In these tests irradiation energy was kept constant at 12 MeV and

exposure time was varied to find out the exposure time dependence of incident

energy. Two titanium specimens: one in the form of rod and the other in

embedded form, were exposed to the incoming electron beam for a period of five

minutes. Similarly four more sets of specimens (each containing a rod and a

mounted specimen) were irradiated at exposure time of 10, 15, 20, and 25


43

minutes respectively. The aim was to analyse the extent of damage by gradually

increasing the exposure time.

3.3. Tensile Testing

Tensile tests were carried out, using a Universal Testing Machine of

[Chuto, T. et al., 2000] 50 kN (UTM, Model AG-1, Shimadzu, Japan), a facility

installed at Centre for Advanced Studies in Physics(CASP), Government College

University(GCU), Lahore, Pakistan.

Fig.3.5. Universal Testing Machine of 50 kN (UTM)

UTM Shimadzu

It is a micro controlled, software-operated machine; hence before

commencing the tensile operation, the apparatus has to be calibrated

electronically. Also, various parameters pertaining to specimen’s geometry, for

instance gauge length, thickness etc; along with units of the quantities to be


44

measured and scales to be chosen, cross head speed to be maintained, are to

be fed using software programming.

The tensile tests were carried out at RT in air and at low temperatures in

pure liquid nitrogen. All tests were performed at a cross head speed of 1mm/min

[Hamilton, M. 2000]. In the course of tensile testing, the cross-head of the

machine was arrested a number of times to observe stress relaxation at constant

strain at different stress [Akhtar, M. K.1995] levels on the stress-strain curve

between yield and ultimate tensile strength. Scanning Electron Microscope (SEM)

analysis of the deformed, un-irradiated specimens and of the irradiated and

deformed ones was performed.

Series of specimens were tensile tested in the present study.

3.3.1. At Room Temperature

Tensile testing was also performed on irradiated and unirradiated Ti

specimens at room temperature (300 K). The specimens irradiated with 8, 10, 12,

15, and 18 MeV [Manickam Ravikumar et al., 2005] electron energy were chosen

initially for tensile testing at room temperature. All the test pieces were in the form

of rod of 40 mm gauge length and 2 mm diameter. Stress-Strain curves were

obtained on the computer attached with tensile testing machine. Tensile tests

were also carried out on unirradiated specimens. The un-irradiated specimens

were given the same heat treatment as the irradiated specimens before the tests

were performed, in order to create uniform testing conditions for un-irradiated and

irradiated specimens. The process was repeated a number of times to establish

the repeatability of the obtained results.


45

3.3.2. At Low Temperatures

In this section, tensile tests were conducted on specimens, both in the

unirradiated and irradiated conditions [Singh, B.N. 1996] at low temperatures

namely: 300, 250, 200, 150 and 100 K. The irradiated specimens were subjected

to 12 MeV electron beam in all the tensile tests in this section. The jaws of the

tensile testing machine were inserted in a cryogenic chamber, attached with

tensile testing machine, as shown in the Fig.3.6.

Fig.3.6. UTM with Cryogenic Chamber

All tensile tests were repeated three times to ensure repeatability of the results.

Low temperature was produced in the chamber using continuous flow of

liquid nitrogen. The temperature was controlled by digital controller attached with

the chamber. The specimens, both irradiated and unirradiated, with a gauge

length of 40mm were deformed in the chamber at a strain rate of 1.6x 10-3 s-1

down to 100 K. During deformation, straining was frequently interrupted by

arresting the crosshead to observe stress relaxation at fixed load. Stress


46

relaxation curves of both irradiated and unirradiated specimens were obtained for

different values of initial stress (σ0) and recorded by the computer. The load cell

of the machine was much sensitive for low temperature applications. A water

circulation was employed to damp the cryogenic temperature gradient with the

load cell. The stainless steel arms were also used to increase the distance

between load cell and thermostatic chamber; but it did not yield the desired

results. In order to eliminate the malfunctioning, a blower heater was used

externally to restore the normal functioning.

3.4. Microstructural Observations

The damage structure was examined using a SEM at 20 kV and

magnification of 200 X for the analysis of irradiation induced defects. Fracture

surface of the specimens after the tensile tests were also examined.

Irradiating metals with energetic particles leads to a modification in the

microstructure which changes mechanical properties of the irradiated metal.

There exists, in fact, a relationship between microstructure evolved in post-

irradiated material and the accruing mechanical changes. To analyse this

relationship, titanium was selected as the targeting material and electrons as the

bombarding particles.

In order to study how the fractographs of the specimens correlate with the

mechanical properties, the fractured surfaces of two different specimens (8 MeV)

and (15 MeV) were examined under the Scanning electron microscope (JEOL

JSM 6480 LV). This was done in order to differentiate the effects of irradiation

energy and test temperature on the mode of fractures.

Chapter 4: Results and Discussion

47

CHAPTER 4

RESULTS AND DISCUSSION

These comprise following aspects:

Effects of irradiation energy on the stress-strain curves of titanium

Effects of test temperature on the stress-strain curves of titanium

Effects of irradiation exposure time on the stress-strain curves of titanium

Effects of irradiation energy on the stress relaxation rate of titanium

Effects of test temperature on the stress relaxation behavior of titanium

Effects of irradiation exposure time on the stress relaxation behavior of

titanium

Results of Deformed Microstructure of irradiated titanium specimens

Discussion of Results

Conclusions

4.1. Effects of Irradiation Energy on the Stress-Strain Curves of Titanium

The stress-strain curves of 40 μm pure titanium obtained at 300 K, before

and after irradiation with 8 MeV electrons are shown in Fig 4.1. It is evident that

the yield [Gupta V. B. and Rana, S.K.1998] stress of irradiated titanium specimen

is higher than that [Conrad, H. 1981] of unirradiated one. Moreover the elongation

to the failure in case of irradiated specimen decreases. The breaking stress for

irradiated specimens is [Ghauri, I.M. and Afzal Naveed, 2007] less than that of

un-irradiated one. The hardening of the specimens increases with an increase in

the energy of incident beam [Fig. 4.2]. From the figure, it can be derived that the


48

yield stress of 12 MeV irradiated titanium is higher than that of 10 MeV irradiated

specimens. Moreover the ductility shows a decreasing trend. Similarly with further

increase of incident energy a corresponding increase in yield stress and

decrease in ductility becomes a regular phenomenon [Fig 4.3]. Fig [4.4] shows

the overall comparison of stress-strain curves of titanium irradiated with electron

beam in the energy range 8-15 MeV. The variations of yield stress and ultimate

tensile stress and % elongation [Chang, K.H. et al., 2009] with irradiation energy

are also shown in the following Table 4.1. The variation in reduction in area with

increase of incident energy is shown in Table 4.2.

4.2. Effects of Test Temperature on the Stress-Strain Curves of Titanium

The stress-strain curves for 12 MeV un-irradiated and [Leguey T. 2005] irradiated

titanium specimens tested in the temperature range 300 -100 K are shown in

Fig. 4.5(a) and 4.5(b) respectively. It is evident that the yield stress of irradiated

and un-irradiated titanium specimens increases with decrease in test

temperature. However this increase is more noticeable in irradiated specimens.

Moreover the elongation to the failure in case of irradiated specimens at low

temperatures is less than that of un-irradiated ones. The irradiated specimens

though showed an increased strength but fractured in brittle mode. The variations

of yield stress and ultimate tensile stress and % elongation [Chang, K.H. 2009]

with temperature is also shown in the following Table 4.3. The changes in

reduction in area with decrease of test temperature are shown in Table 4.4.


49

4.3. Effects of Exposure Time on the Stress-Strain Curves of Titanium

The hardening behavior of polycrystalline titanium is further analyzed by

varying the exposure time and keeping the incident energy constant. The stress-

strain curves of titanium irradiated with 18 MeV electrons for a period ranging

from 5 to 25 minutes are shown in Fig.4.6. From the figure, it can be deduced

that hardening in the specimens enhances with increase of exposure time Fig.

4.6. The variations of yield stress and ultimate tensile stress with exposure time is

also shown in the following Table 4.5.

4.4. Effects of Irradiation Energy on the Stress Relaxation Behaviour of Titanium

Fig. 4.7 shows the stress relaxation curves obtained for un-irradiated

titanium specimen annealed at 977K for 30 minutes. For a given initial stress

level σ0 at which further deformation of the specimen was stopped, the amount of

stress relaxed ∆σ (t) = σ0-σ(t) increases linearly with logarithm of relaxation time

t, except for few seconds in the beginning of the relaxation (not shown). However

at relatively high values of t, the ∆σ/ln (t) curves flatten. Similarly Fig. 4.8 depicts

the stress relaxation curves obtained at different initial stress levels σ0 for

irradiated specimen. A pronounced effect of irradiation on the stress relaxation

rate s[s=d∆σ/d ln(t)], determined for the linear part of the semi-logarithmic stress

relaxation curves, is illustrated in Fig.4.9. It can readily be noted that for a given

stress level σ0, the stress relaxation rate “s” has decreased in case of irradiated

specimens shown in Fig.4.9. Similarly the relaxation curves for 8, 12 and 15 MeV

irradiated specimens are shown in Figures 4.10 – 4.12 respectively. It is evident

from the figures that the relaxation of stress becomes slower with increase of

incident beam energy. The decrease in stress relaxation rate with an increase of


50

incident energy is presented in Fig.4.13 [Ghauri, I.M.; Afzal Naveed; Haider

Karrar, 2006].

Activation volume vσ associated with stress relaxation rate is given by

Vσ = kT/s … [4.1]

Here vσ is the activation volume, k is Boltzmann constant, T is the deformation

temperature and ‘s’ is the [Ghauri, I.M. and Afzal Naveed 2007] stress relaxation

rate. The activation volumes for unirradiated and irradiated specimens are plotted

as a function of σ0 in Fig. 4.14 [Singh, B.N. 1995]. The vσ comparison of un-

irradiated and irradiated specimens, calculated using equation (1) shows that vσ

decreases with increase of σ0 in both un-irradiated and irradiated specimens,

while vσ values were found to be higher in irradiated specimens than that of un-

irradiated ones.

According to single barrier model of stress relaxation, the activation energy

U0 is correlated with the activation volume vσ, as

U0 - vσσ0 =mkT … [4.2]

where m is a constant close to 25.

From [Eq. 4.3] and [Eq. 4.4] “we get an expression for stress relaxation rate‘s’,

which is given as

s =kT σ0 / [U0 –mkT] … [4.3]

or U0= kT [(dσ0/ds) +m] … [4.4]

The U0 values calculated using [Eq.4.4), are shown in Table 4.6. The values

increase with increase in incident irradiation energy.


51

4.5. Effects of Test Temperature on the Stress Relaxation Behaviour of Titanium

The stress relaxation curves of titanium deformed in the temperature

range 300-100 K are shown in Figs. 4.15 – 4.17. The relaxation rate calculated

from the slope is plotted against initial stress levels in Figs. 4.18 and 4.19. It can

be seen that the relaxation rate becomes slower with decrease of test

temperature in case of both unirradiated and irradiated specimens. This decrease

of relaxation rate is more conspicuous in irradiated specimens than in

unirradiated ones [Ghauri, I.M.; Afzal Naveed; and Haider Karrar 2006]. The U0

values calculated using [Eq.4.4] for unirradiated and irradiated specimens in the

temperature range 300 to 100 K are [Onchi, T. et al., 1980] plotted as a function

of test temperature in Fig. 4.20, which increase linearly with decrease in test

temperature.

4.6. Effects of Exposure Time on the Stress Relaxation Behaviour of Titanium

The effect of exposure time on stress relaxation behaviour of

[Kubat, J. and Nilsson, L.A.1982] polycrystalline titanium is shown in Figs. 4.21 –

4.24. The curves show that stress relaxation increases linearly with time. The rate

of relaxation depends upon initial stress as well as on the exposure time Fig.

4.25. The decrease of relaxation rate with an increase of exposure time is again

correlated to the pinning of dislocations with irradiation induced defects. The

multiplication of defects takes place as the irradiation exposure time is increased

resulting in a further decrease of stress rate.


52

4.7. Calculation of Frenkel Pairs

The number of Frenkel pairs produced per cascade is expressed by the

[Rubia, T. Diaz de la 1996] formula:

NF =0.8Ei/ 2 <Ed> ….. [4.5]

Where

NF = Number of Frenkel pairs produced per cascade

Ei = Irradiating electron beam energy

<Ed>= Average threshold energy for Frenkel pair

The values of NF for 8-18 MeV electron beam are enlisted below in Table 4.7.

4.8. SEM Fractrograph Results

The changes in microstructure and mechanical properties after irradiation

of many pure HCP metals and alloys have been object of numerous studies and

progress in the understanding of radiation damage in these materials is

continuing. The microstructures of pure titanium specimens irradiated with high

energy electrons were examined by SEM to determine the micro structural

changes produced by high energy [Bradley, E.R. and Jones, R.H.1981] electrons.

SEM images of the specimens provide an insight into the nature of fracture.

Important information can be obtained from microscopic examination of the

fracture surfaces. This study is usually called fractography. Fractography is most

commonly done using the SEM due to its ease of operation. The large depth of

field and the good resolution make the SEM an excellent tool for research and

failure analysis [Chern, G.L. 2006].

The fractured surfaces of two different specimens (8 MeV) and (15 MeV)

were examined under the Scanning electron microscope (JEOL JSM 6480 LV) as


53

shown in Fig.4.26. Both the fractured surfaces show a mixture of cleavage and

fibrous regions.

However, there are important differences between the two fractured

surfaces that inform us regarding the plastic deformation mechanisms. There is a

fine texture of lines (slip bands) on the cleavage plates in the 8 MeV specimens.

The cleavage plates in the 15 MeV specimens show relatively few of these lines

shown in Fig. 4.26(c) and 4.26(d). The dimples formed by micro void coalescence

are relatively large in 8 Mev specimens in Fig. 4.26(a) and 4.26(b) than in

15 MeV specimens. The deformation in 8 MeV specimen shows more plastic

deformation as there were relatively lower point defects obstructing the motion of

dislocations. In 15 MeV specimens more point defects are formed due to higher

irradiation energy, this results in more obstructions in the motion of dislocations

thus the fracture is largely cleavage.


54

DISCUSSION OF RESULTS

The changes in the tensile parameters of polycrystalline titanium

depending upon incident energy changes and test temperature can be explained

as follows:

The increase of yield stress and loss of ductility after irradiation is attributed to the

interaction of dislocations with radiation induced defects [Ghauri, I.M. and Afzal

Naveed 2007]. The impact of high energy electrons of 8 MeV produce initial

displacement damage in the form of vacancies and/or interstitials that evolve into

clusters. This process takes the form of cascade and number of Frenkel pairs

produced per cascade as calculated using NF=0.8 Ei /2 <Ed> relation are of the

order of 105. Here Ei is incident energy and <Ed> is average displacement energy

(30 eV for Ti). The dislocations moving in a glide plane are locally and temporarily

pinned by irradiation induced defects which act as obstacles. Thus their

propagation becomes more difficult leading to hardening. Pitt and Brimhall have

reported that a large strength increase, 440 MPa, occurs in fission neutron

irradiated rhenium [Pitt C.H. and Brimhall J.L. 1971] without viable defect clusters

which suggests radiation hardening in metals with HCP crystal structures may be

very sensitive to extremely mild clusters of point defects Irradiation induced

strengthening of crystalline materials is caused by the increased resistance to

dislocation motion provided by various irradiation produced obstacles [Bradley,

E.R. 1981] With increase in the energy of incident beam or exposure time, the

number of defects produced also increase, resulting in strong interaction of

dislocations with defects. This causes a pronounced change in yield stress as the

incident beam energy increases shown in Fig.4. 4 or the exposure time increases

in Fig.4.6.


55

The yield stress and ultimate tensile stress also increased with decrease of test

temperature in Fig.4.5. In case of HCP metals like titanium, plastic deformation is

mainly governed by mechanical twinning that occurs during the process of

deformation. The density of these twins increases with decrease in test

temperature. The interaction of dislocations with increased number of twins

engenders an increase of yield stress and ultimate tensile stress and decrease of

% elongation and [Dai, Y. 2005] stress relaxation rate of polycrystalline titanium

at low temperature. The relaxation rate decreases with further decrease in

temperature. This is attributed to the reduction in available thermal energy

necessary for the movement of dislocations pinned at defects mainly at twin

boundaries. In case of irradiation of specimens defects in the form of Frenkel

pairs are produced. Thus in case of irradiated titanium, irradiation [Woo C. H.

1998] induced defects in addition to mechanical twins act as strong pinning points

for dislocations at all temperatures under investigations. With further decrease in

test temperature, the thermal energy available to glide dislocations also

decreases thus reducing their mobility and resulting into the pile ups at radiation

induced defects like vacancies, interstitials, clusters etc. These piled up groups

(due to their long range stress fields) will then exercise sufficient force on

dislocations (sources or pinned dislocations) in the cross slip or other cross slip

planes to attract and release dislocations from them, and thus form various types

of barriers and locks [Raza, S.M. 1982]. Thus at low temperature comparatively

more stress is required to unpin dislocations from defects which results in an

increase of yield stress, ultimate tensile stress, activation volume and decreased

ductility and stress relaxation rate.


56

The decrease of relaxation rate with increase in incident energy and

decrease in test temperature leads to an increase in activation volume

[Conrad, H. 1981]: The activation volume is the measure of volume in which a

dislocation interacts with obstacles. The activation volume can mathematically be

expressed as V= bdl, where b is the burger vector [Hsu, J.C.C. and Clifton, R.J.

1974] d is the distance between obstacles and l is the length of dislocation

segment. With increases incident energy, exposure time and decrease of test

temperature the interaction between glide dislocations with obstacles like

radiation induced defects and mechanical twins increase resulting in an increase

of activation volume. Thus energy required to unpin the dislocations from

obstacles increases with increase of incident energy, exposure and time and

decrease in test temperature. These energies values calculated at low

temperatures correspond to the activation energy for recovery processes such as

vacancy formation, cross slip, mutual destruction of edge dislocations by forced

climb in dipole configuration etc. in the course of deformation, which seem to

control the rate process of stress relaxation in polycrystalline titanium at low

temperature. The pronounced increase in U0 values with decrease in temperature

in irradiated specimens is due to the strong pinning of less mobile dislocations

with higher defect density induced by irradiation.

The analysis of activation energy U0 and activation volume vσ suggests

that the initial plastic deformation in irradiated specimens is controlled by the

interaction of dislocation with irradiation induced defects, while this progress into

dislocation-dislocation intersection as the deformation proceeds which seem to

control the rate process in irradiated polycrystalline titanium.


57

Microstructure evolution in irradiated materials results from the interaction

between point defects, defect clusters and glide dislocations. When pure titanium

is irradiated with incident beam of electrons, the ductility of the metal decreases

with increasing electron energy, due to the development of a submicroscopic

damage structure that hampers dislocations, and therefore decreases elongation

of fracture. Increased yield stress and decreased ductility often accompanied by

plastic flow localization. An analysis of the effect of decreasing deformation

temperature and varying irradiation energy on the microstructure and tensile

behavior of polycrystalline Ti rod is presented. The increasing irradiation energy

increased the tensile stress and thus altered the stress strain behaviour. This can

be related with the deformed microstructure of the specimens.

Fig.4.26(a) shows the deformed microstructure of 8 MeV electron beam

irradiated specimen deformed at 250 K. The figure shows that fractured surface

contains both ductile and brittle fractures; but the ductile mode seems to be more

revealing here. By reducing the test temperature to 100 K, the mode becomes

cleavage in nature Fig.4.26(b).

By increasing the incident energy to 15 MeV and reducing the deformation

temperature, the fracture mode assumes a more prominent cleavage nature. The

fracture surfaces of two different specimens (8 MeV) and (15 MeV) were

examined under the Scanning electron microscope (JEOL JSM 6480 LV) as

shown in Fig.4.26. Both the fractured surfaces show a mixture of cleavage and

fibrous regions. However, there are important differences between the two

fracture surfaces that inform us regarding the amount of plastic deformation.

There is a fine texture of lines (slip lines) on the cleavage facets in the 8 MeV

specimen deformed at 250 K (Fig.4.26(a). However, these cleavage lines are


58

less noticeable, when the test temperature is reduced to 100 K, keeping the

incident energy constant at 8 MeV. The cleavage facets in the 15 MeV specimen

show relatively few of these lines, and still fewer in Figs.4.26(c) and 4.26(d)

respectively. There are further noticeable differences between the two fracture

surfaces such as: the fracture of Fig.4.26(a) has a more 3-dimensional character

(ductile) than in Fig.4.26(c). The dimples formed by micro void coalescence are

relatively large in the 8 MeV specimen than in the 15 MeV specimen in

Fig.4.26(b) and Fig.4.26(c). The above observations show that the mode of

fracture of the 8 MeV specimen has more ductile character than the 15 MeV

specimens. This has occurred because of relatively lower number of point defects

obstructing the motion of dislocations in the 8 MeV specimens than in the 15 MeV

specimens. More point defects are formed in the 15 MeV specimens as a

consequence of higher irradiation energy. Similarly as the test temperature is

decreased from 250 K to 100 K, the brittle fracture mode presents the dominant

feature. This results in more obstructions in the motion of dislocations thus the

mode of fracture is largely cleavage.

Griffith explained the low fracture stress level of brittle materials relative to

the ideal fracture stress as below:

He took the approach that we need to take into account two energies: elastic

strain energy and increase in surface energy. The stored elastic strain energy

during tensile testing is released in the form of an increase in surface energy

when the material fractures. Crack propagation creates new surface areas

and formation of these surface areas result in increase in surface energy.

Crack propagates when strain energy is higher than surface creation energy.

There are lots of small cracks in brittle material and fracture occurs due to the

enlargement of these cracks thus fracture stress level of brittle materials is

lower than the ideal fracture stress level [Griffith A.A 1920].


59

. The titanium surface analysed shows primarily brittle fracture, hence we

have used Griffith’s approach to explain the observations as well. Titanium

specimens irradiated with 15 MeV and deformed at 250 K Fig.4.27(a),

compared with the one irradiated and deformed at 15 MeV and 100 K

Fig.4.27(c)shows that larger surface area was created during fracture, hence

more strain energy required to fracture the specimen. The 15 MeV (250 K)

Fig.4.27(a) specimen surface shows a more conical shape of the overall

fracture (corresponding to ductile mode). The central portion of the specimen

has brittle mode as dominant, while the edges show a fibrous region

[Anderson, J.C. Leaver, K.D., Rawlings R.D. and Alexander 1990], hence

more surface area created during this fracture than at 100 K. This micrograph

(15 MeV) enlarged view clearly shows a fibrous structure which possibly

occurs before dimples are formed (plastic deformation). The formation of this

structure resulted in more energy absorption during fracture Fig.4.27(b).

Electron micrograph (15 MeV, 100K) Fig.4.27(c) shows a number of flat facets

throughout the fracture. This has happened as fracture has occurred by

separation of grains along cleavage planes (Book). The figure shows a

enlarged image of a smooth feature, of the same specimen. This seems like a

cleavage plane of a relatively large grain (brittle fracture) Fig.4.27(d). Relating

our observations with Griffith’s approach, it shows that 15 MeV (250K)

specimen required more strain energy than 15 Mev (100K) to fracture. This

extra strain energy was used to form new surfaces.

During the fracture of 15 Mev (250 K) specimen initially there was some

plastic deformation due to the movement of line defects (dislocations) as shown

by fibrous regions, however due to large number of irradiation induced point

defects, these line defects were pinned down and further application of strain

resulted in cleavage of planes. In the fracture of 15 MeV (100K) specimens due

to the low temperature, movement of line defects (dislocations) is limited and the

grains of material separate along cleavage planes rather than showing much

plastic deformation.


60

0 2 4 6 8 10 12 14 16 18 20

100

200

300

400

500

600

Stres

s (M

Pa)

Strain (%)

Unirradiated Ti 8 MeV Irradiated Ti

Fig.4.1.Comparison between the Stress-Strain Curves of Unirradiated and 8 MeV Irradiated Pure Titanium

0 2 4 6 8 10 12 14 16 18 20

100

200

300

400

500

600

Str

ess

(MP

a)

Strain (%)

10 MeV 12 MeV



61

0 2 4 6 8 10 12 14 16 18 20

100

200

300

400

500

600

Str

ess

(M

Pa

)

Strain (%)

12 MeV 15 MeV


0 2 4 6 8 10 12 14 16 18 20

100

200

300

400

500

600

Str

ess

(MP

a)

Strain (%)

8 MeV 10 MeV 12 MeV 15 MeV

Fig.4.4. Overall Comparison of Stress-Strain Curves (8-15 MeV)Error = + 0.5%


62

Table 4.1. Variations in Yield Stress and Ultimate Tensile Stress and % Elongation with incident Beam Energy

Table 4.2. Variation in % Reduction in Area with Energy

Energy (MeV) Reduction in Area (%)

8 11.7

10 10.8

12 10.19

15 8.2

Electron Beam Energy (MeV)

Yield Stress (MPa)

Ultimate Tensile Stress (MPa)

Elongation (%)

8

10

12

15

407

462

476

493

509

546

558

569

13

12

11

09


63

0 4 8 12 16 20 240

200

400

600

Stres

s (M

Pa)

Strain (%)

Unirradaited Ti 250K 200K 150K 100K

Fig.4 5(a) Comparison of Stress Strain Curves of Unirradiated Titanium at Low Temperature

0 4 8 12 16 20 240

200

400

600

Stres

s (M

Pa)

Strain (%)

Irradiated Ti 250K 200K 150K 100K

Fig.4.5(b). Comparison of Stress Strain Curves of Irradiated Titanium at Low Temperature


64

Table 4.3. Change in Yield Stress, Ultimate Tensile Stress and % Elongation with Test Temperature in Unirradiated Titanium

Test Temperature (K)

Yield Stress(MPa)

Ultimate Tensile Stress

(MPa)

% Elongation

250 370 480 16

200 415 515 15

150 460 580 13

100 520 605 10

Table 4.4. Change in Yield Stress, Ultimate Tensile Stress and % Elongation with Test Temperature in Irradiated Titanium

Deformation Temperature

(K)

Yield Stress (MPa)


% Elongation

250 390 495 15

200 480 559 14

150 540 590 11

100 600 630 8


65

Table 4.5. % Reduction in Area with Test Temperature in Irradiated Titanium

Temperature (K) Reduction (%)

300 14.01

250 12.42

200 11

150 9.55

100 7.64

0 2 4 6 8 10 12 14 16 180

100

200

300

400

500

600

Str

ess

(M

Pa

)

Strain (%)

5 mins 10 mins 15 mins 25 mins

Fig.4.6. Effect of Exposure Time on the Stress-Strain Curves of 12 MeV Electrons Irradiated Titanium


66

Table 4.6. Effect of Irradiation Exposure Time on Yield Stress and Ultimate Tensile Stress

Irradiation Exposure Time (Minutes)

Yield Stress (MPa)


5

10

15

20

25

425

440

455

468

478

502

526

540

554

576

105

10

15

20

25

30

35

40

45

50

55

Str

ess

Rel

axat

ion

(MP

a)

Time (sec)

Unirradiated Titanium 471 MPa 512 MPa 538 MPa 556 MPa

Fig.4.7 Stress Relaxation Curves of Un-Irradiated Titanium annealed at 977K for 30 minutes


67

10

5

10

15

20

25

30

35

Stress

Rela

xatio

n (M

Pa)

Time (sec)

Irradiated Titanium 488 MPa 523 MPa 545 MPa 569 MPa

Fig.4.8. Stress Relaxation Curves of Irradiated Titanium annealed at 977K for 30 minutes

460 480 500 520 540 560 5804

6

8

10

12

14

s (M

Pa)

Un-Irradiated Ti Irradiated Ti

Fig 4.9. Stress Relaxation Rate(s) as a function of initial stress


68

1 10 1000

10

20

30

40

50

t (sec)

Irradiated Ti (8 MeV) 450 MPa 488 MPa 524 MPa 560 MPa


1 10 1000

10

20

30

40

50

t (sec)




69

1 10 1000

10

20

30

40

50

t (sec)



440 480 520 560 600 640 680

4

6

8

10

12

14

16

s (M

Pa)

Un-irradiated 8 MeV 12 MeV 18 MeV

Fig.4.13. Variation of Stress Relaxation Rate(s) with initial stress levels σ0


70

450 500 550 600 650

400

600

800

1000

Act

ivatio

n V

olu

me (

b^3

)

Initial Stress (MPa)

Un-irradiated 8 MeV 12 MeV 18 MeV

Fig.4.14. Variation of Activation Volume Vσ with initial stress levels σ0

Table 4.7. Increase in Activation Energy with Incident Energy

Irradiation Energy

(MeV)

Activation Energy U0

(eV)

8 1.35

10 1.38

12 1.44

15 1.48

18 1.53


71

1 10 1000

10

20

30

40

50

t (sec)

Un-irradiated Ti (300 K) 440 MPa 480 MPa 520 MPa 560 MPa


1 10 1000

10

20

30

40

50

t (sec)

Irradiated Ti (300 K) 452 MPa 487 MPa 522 MPa 557 MPa



72

1 10 1000

10

20

30

40

50

t (sec)


Fig.4.16(a). Stress Relaxation Curves of Un-irradiated Titanium obtained at 200 K

1 10 1000

10

20

30

40

50

t (sec)


Fig.4.16(b). Stress Relaxation Curves of 12 MeV Electron Beam Irradiated Titanium obtained at 200 K


73

1 10 1000

10

20

30

40

50

t (sec)


Fig.4.17(a). Stress Relaxation Curves of Unirradiated Titanium obtained at 100 K

1 10 1000

10

20

30

40

50

t (sec)


Fig.4.17(b). Stress Relaxation Curves of 12 MeV Electron Beam Irradiated Titanium obtained at 100 K


74

400 440 480 520 560 600 6402

4

6

8

10

12

14

16

s (M

Pa)

Un-irradiated Ti 300 K 200 K 100 K

Fig.4.18. Stress Relaxation Rate(s) as a function of initial stress levels σ0 for Un-irradiated Titanium Specimens in the Temperature Range 300 to 100 K

400 440 480 520 560 600 640 6802

4

6

8

10

12

s (M

Pa)

Irradiated Ti 300 K 200 K 100 K

Fig.4.19. Stress Relaxation Rate(s) as a function of initial stress levels σ0 for 12 MeV Electron Beam Irradiated Titanium Specimens in the Temperature

Range 300 to 100 K


75

50 100 150 200 250 300 3501.0

1.2

1.4

1.6

T (K)

Irradiated Ti Un-Irradiated Ti

Fig.4.20. Variation of Intrinsic Height of Energy Barrier U0 with decrease of temperature in both Un-irradiated and Irradiated Titanium

10

0

10

20

30

40

50

60

70

Exposure Time 5 MinsEnergy 12 MeVInitial Stress (MPa)

306 350 396 434

Str

ess

(MP

a)

Time (sec)



76

100

10

20

30

40

50

60

Exposure Time 10 minsEnergy 12 MeV

297 343 396 448

Str

ess

(MP

a)

Time (sec)


100

10

20

30

40

50

60

Exposure Tim e 15 m insEnergy 12 MeV

321 377 425 465

stre

ss (

MP

a)

time (sec)



77

100

10

20

30

40

50

60

Exposure Time 25 MinEnergy 12 MeV

309 366 427 481

Str

ess

(MP

a)

T ime (sec)


250 300 350 400 450 5005

10

15

20

25

30

s (M

Pa

)

In itia l S tress (M Pa)

5 m ins 10 m ins 15 m ins 25 m ins

Fig.4.25. Variation of Activation Volume Vσ with initial stress levels σ0 for 5-25 minutes


78

Table 4.8.No. of Frenkel Pairs corresponding to Irradiation Energy

Error = + 0.10×104

Irradiation energy Ei (MeV)

Frenkel pairs NF

8

10

12

15

18

9×104

1×105

1.3×105

1.7×105

2×105


79

Fig. 4.26(a) 8 MeV (250 K)

Fig.4.26(b) 8 MeV (100 K)


80

Fig.4.26(c) 15 MeV (250 K)

Fig.4.26(d) 15 MeV (100 K)


81

Fig.4.27(a) 15 MeV (250 K)

Fig.4.27(b) 15 MeV (250 K) [Enlarged]


82

Fig.4.27(c) 15 MeV (100 K)

Fig.4.27(d) 15 MeV (100 K) [Enlarged]

Chapter 5: Conclusions and Future Work

84

CHAPTER 5

CONCLUSIONS AND FUTURE WORK

CONCLUSIONS

The present work has been taken up to study the effects of irradiation

energy, exposure time, and test temperature on the mechanical behaviour of

polycrystalline titanium (99.994%). The specimens were annealed at 977 K for 30

minutes. Titanium specimens of grain size 40 µm were irradiated with 18MeV

electrons for 12 min at 300 K to a dose of 0.01 dpa. The 30 minutes annealed

specimens were irradiated with 18 MeV electron beam at 300 K. Mechanical

behaviour of un-irradiated and irradiated specimens was determined from the

autographic records of the tensile specimens carried out using Universal Testing

Machine (UTM) in the temperature range 300 to 100 K. Stress relaxation

phenomena in irradiated specimens was studied and compared with that of un-

irradiated specimens in the given temperature range. The results obtained have

been analyzed in terms of single barrier model of stress relaxation. The effects of

incident irradiation energy and exposure time were investigated at room

temperature.

Irradiation hardening in the specimens became more pronounced with

increase of incident energy and decrease of test temperature.

The stress relaxation rate ‘s’ was observed to decrease in the

irradiated specimens with decrease in test temperature. Furthermore,

the relaxation rate in un-irradiated specimens was higher compared to

the irradiated specimens.


85

The activation volume (Vσ) decreased with increase in stress levels (σ0)

both in the un-irradiated and irradiated specimens; however, Vσ values

were higher in irradiated specimens than that of un-irradiated.

The intrinsic height of energy barrier (U0) to the movement of

dislocations increased with increase of irradiation energy.

The analysis of U0 and Vσ suggests that the initial plastic deformation in

irradiated titanium specimens is controlled by the interactions of

dislocations with irradiation induced defects while this advances into

dislocation-dislocation intersection as the deformation proceeds.

Microstructure evolution in irradiated materials results from the

interaction between point defects, defect clusters and glide dislocations

altering the stress strain behavior. This can be related with the

deformed microstructure of the specimens.

With an increase in energy, the fracture shows a shift towards cleavage

fracture. The increasing irradiation energy increased the tensile stress

and thus altered the stress strain behavior.

The increased number of point defects formed due to increase in

irradiation energy results in impeding the motion of dislocations and the

mode of fracture shifts towards cleavage fracture.

Similarly brittle mode seems prominent, when the test temperature

reduces to 100K. The reduced percentage elongation gives credence

to this finding. This finding is further supported by the enhanced

hardness value at this temperature.


86

Coalescence of micro voids in the specimens under the tensile loading

occurs as the incident energy decreases, resulting in ductile mode of

fracture. As the incident energy increases the mode of fracture

becomes brittle in nature. Similarly the modes of fracture also show

temperature dependence: at higher values of test temperature, the

ductile mode is dominant, while at lower values of test temperature, the

brittle mode seems more prominent. Also the number of secondary

cracks shows an increase as the test temperature decreases, causing

brittleness in fracture mode.

It is, however, regretted that owing to the non-availability of titanium

alloys, the research work had to be confined to pure metal only. It would not be

out of place to mention here that despite concerted efforts, the alloys could not be

procured from the local market.


87

FUTURE WORK

Future guide lines for prospective researchers can comprise:

Irradiating Pure Ti with protons and neutrons, to assess the mechanical

behaviour of the irradiated metal.

Increasing the test temperature, to study the failure analysis, as compared

with those in hand.

To analyze the effect of grain size on the irradiated Ti, specimens of

various grain sizes to be prepared, by changing the annealing

temperature/time, before exposing the specimens to irradiation.

Irradiating alloys of Ti, such as workhorse Ti 6Al 4V and Ti 5AL 2.5Sn with

high energy electrons.

Irradiating the above alloys with protons, neutrons and alpha particles, for

drawing a comparison between the mechanical behaviour of pure metal

and its alloys under identical conditions: keeping irradiation dose, and

exposure time constant, while changing the test temperature.

Keeping incident energy and exposure time constant and varying the

irradiation dose for pure Ti and its alloys.

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88

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