Zhegina_use of Fractography for Estimating

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UDC 620.184.6:669.715

USE OF FRACTOGRAPHY FOR ESTIMATING FRACTURE RESISTANCE

OF ALUMINUM ALLOYS

I. P. Zhegina1 and L. V. Morozova1

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 18 – 22, September, 2013.

Fracture surfaces of aluminum alloys VAD23, AK6, V95, 01911, 1420, 1430, 1440, 1451 are studied after dif-

ferent variants of aging. Common features of formation of fractures are determined depending on the modes of

heat treatment, conditions of deformation and presence of impurities in the aluminum alloys. Efficiency of the

use of fractography for estimating the working capacity of the aluminum alloys is demonstrated.

Key words: fractography, fracture, failure, cracks, aluminum alloys, lithium.

INTRODUCTION

Aluminum alloys remain the most frequently used struc-

tural materials possessing the properties required for the air-

craft industry [1, 2].

In the 1950s – 1960s VIAM specialists started intense re-

search of the structural strength and reliability of materials in

operation. It became clear that in addition to the results of

mechanical tests of notched and unnotched test pieces other

fracture characteristics should be allowed for in such esti-mates.

Analysis of the structure of a fracture is an effective

means for studying the behavior of the material in the frac-

ture process depending on the loading conditions and on the

condition of the material. Growth of a crack and formation of

a fracture surface is a local process accompanied by marked

localization of strain. Studying a fracture surface we obtain

data on the capacity of the material for local plastic straining

and on the kinetics of crack growth, which is especially im-

portant for analyzing operating fractures and comparing the

capacities of various materials for resisting fracture. This sci-

entific direction has been developed for aluminum alloys un-der the guidance of Ya. B. Fridlyander, T. A. Gordeeva and

with active support of I. N. Fridlyander.

The capacity of a material to resist fracture is a struc-

ture-sensitive property. It depends much on the chemical and

phase composition of the material, the presence of impuri-

ties, and the modes of heat treatment and deformation.

All the works aimed at creation of novel alloys in the

laboratory headed by I. N. Fridlyander were performed with

the use of fractography, which gave ample data on the effect

of various structural and production process factors on the

structure of fractures and allowed the researchers to work out

approaches for analyzing operating fractures and estimating

the reliability of the developed materials and constructions.

I. N. Fridlyander has created a theory of three stages of

aging of aluminum alloys, i.e., zonal, phase, and coagulation

ones, which yield different combinations of properties [3].

This scale of aging modes has been used to study aluminum

alloys including grade VAD23 of the Al – Li – Cu system.

The aim of the present work was to determine the fea-tures of fracture typical for various aging modes and their re-

lation to the structure of aluminum alloys in order to employ

the results in the creation of new materials, advancement of

technologies and analysis of emergency fractures.

METHODS OF STUDY

We studied the fracture behavior of aluminum alloys

VAD23, AK6, V95, 01911, 1420, 1430, and 1440 as a func-

tion of their heat treatment, content of impurities, presence of

stress concentrators, and conditions of deformation.

The fracture surfaces of the test pieces were studied un-

der a JEOL JSM-840 scanning electron microscope. We de-

termined the mechanical properties (short-term tensile

strength and impact toughness KCT of test pieces with a fa-

tigue crack) of the alloys using an MTS testing machine and

a Walter bai PH300-CHC impact machine.

RESULTS AND DISCUSSION

The same test pieces of alloy VAD23 were used to ana-

lyze the microstructure and the structure of fractures in order

Metal Science and Heat Treatment , Vol. 55, Nos. 9 – 10, January, 2014 (Russian Original Nos. 9 – 10, September – October, 2013)

4720026-0673/14/0910-0472 © 2014 Springer Science + Business Media New York

1 FGUP “VIAM,” Moscow, Russia (e-mail: [email protected]).



to find the correspondence between this or that surface tex-

ture and structural state.

After zonal aging (at 125 – 140°C for up to 6 h) which is

accompanied by growth of the formed Guinier – Preston

zones and occurs virtually without decomposition of the

solid solution, the alloy exhibits high local ductility. After

testing for short-term tensile strength the fracture has a domi-

nantly large-dimple texture (see Fig. 1a).

In the initial stage of phase aging (125 – 140°C for 24 h,

160°C for up to 6 h, 200°C for up to 2 h) the fracture has a

mixed pattern of large and small dimples (Fig. 1b), which in-

dicates lowering of the capacity of the matrix for local plastic

straining due to the formation of small dimples on the parti-

cles of the hardening phase. The fuller the decomposition of

the solid solution, the smaller the size of the dimples and the

lower the local plastic strain upon fracture.

After the aging of alloy VAD23 for maximum strength

(125 – 140°C for up to 100 h, 160°C for up to 12 h, 200°C

for up to 6 h, 220°C for up to 2 h) about 50% of the fracture

surface is taken by a zone of brittle detachment facets formed

on subgrain boundaries. The rest of the area is taken by small

dimples that are less ductile than in the case of incomplete

phase aging (Fig. 1c).

In the stage of coagulation aging the local ductility upon

fracture is somewhat elevated. The proportion of facets of

brittle detachment over the lamellar phase in the fracture of

alloy VAD23 decreases and the content of large dimples in-

creases (after the aging at 200°C and especially at 220°C for

24 h) (Fig. 1d ) [4].

Alloys AK6, V95 and 01911 have been studied in [5] for

determining microfractographic features of early aging

stages. All the alloys exhibited similar variation of local duc-

tility upon fracture. However, alloys V95 and 01911 did not

undergo subgrain fracture. The ductility decreased due to the

growth in the proportion of small dimples on the fracture

surface.

Further studies of the microstructure of fractures of alu-

minum alloys showed that their local ductility upon fracture

was influenced considerably by impurities. The effect of iron

impurities on the fracture behavior of alloy V95 has been

studied in detail. It turned out that the purity of the material

with respect to iron and silicon affects positively the mechan-

ical properties of this alloy [6]. This was ascribed to the de-

crease in the content of the excess insoluble Al6FeNi or Al –

Fe – Mn – Si phases. However, according to the results of

studies of fracture surfaces, iron and silicon present in the

majority of aluminum alloys in the form of alloying additives

or impurities can be bonded chemically to the other elements

of the alloy forming intermetallic phases, enter the composi-

tion of the solid solution thus causing an additional harden-

ing or softening, shift the solubility of the elements in the

solid solution, affect the recrystallization process, and so on.

At a low content of iron and silicon (0.26 – 0.14% Fe,

0.09 – 0.06% Si as compared to 0.4% Fe and 0.32% Si) local

ductility is increased considerably and the texture formed

upon fracture is characterized as a more ductile small-dimple

one. Thus, local ductility upon fracture of alloy V95 with

elevated purity grows not only due to the absence of a brittle

excess phase but also due to the elevated ductility of the solid

solution. This may be connected with the effect of iron and

silicon on the process of decomposition of the solid solution.

It should be noted that in the test pieces where fracture

developed over the height, particles of excess phases, which

serve stress concentrators, were accumulated. Therefore, the

advantage of the higher capacity of the matrix for local plas-

tic straining upon decrease in the content of impurities is not

Use of Fractography for Estimating Fracture Resistance of Aluminum Alloys 473

a b

c d

Fig. 1. Microfractograms of alloy

VAD-23 after aging by various modes:

a) 125°C, 3 h (zonal aging, 7000);

b ) 60°C, 1 h (incomplete decomposi-

tion of solid solution, 7000);

c) 140°C, 100 h (aging for maximumstrength, 5500); d ) 200°C, 24 h (co-

agulation aging, 7000).



so obvious in this case as in the test pieces cut in other di-

rections.

As a rule, high-strength alloys are highly sensitive to

stress concentrators and when they are used for designing

construction components, the places of stress concentration

of design (transition radii, combinations of components with

different thickness, etc.) and process (cuts, hairlines, nicks,

etc.) origins require special control. The effect of stress con-

centrators on the structural strength of a material with a high

content of impurities (0.4% Fe, 0.32% Si) may be illustrated

by the following example. In a static test of an aircraft wing

jacketed with a pressed panel from alloy V95 early failure

has occurred at a load of 80% of the design value. The frac-

ture started not in the zone of the highest design stresses near

an opening but at the place of intersection of a stringer with a

rib, where transition radius was virtually absent and traces of

rough conditioning with deep grooves were observable.

The fracture surfaces of the KCT test pieces cut from the

wing panel in question had a mixed pattern, bore small and

large dimples without traces of shear strain, which indicated

a low local ductility of the solid solution upon fracture. On

the contrary, fracture in a panel from alloy V95pch (with ele-

vated purity) developed with considerable local ductility,

which reflected a higher capacity of the latter to resist frac-

ture (see Fig. 2).

Thus, the early failure of the panel was a result of a com-

bination of factors, namely, rough treatment of the surface

and use of material with reduced resistance to stress concen-

tration. The results of the study have been used to redesign

the wing and to pass to making it form alloy V95pch with

higher purity.

Fracture characteristics are influenced much by the con-

ditions of deformation in the production of the semiproduct,

especially for the alloys with lithium additives. The local

ductility upon fracture estimated from the appearance of the

fracture surface characterizes the capacity to decelerate frac-

ture, which determines the reliability of operation of the alloy.

An analysis of the fracture behavior of lithium-alloyed

aluminum alloys starting with 1420 has shown that their

fracture surfaces are considerably inhomogeneous, which de-

pends on the microstructure. This explains the elevated sus-

ceptibility of the alloys with lithium to layering. Layering

connected with fracture over grain boundaries or fiber inter-

faces was observed in [7] when deformation modes were

worked out for bars from alloy 1420. Fine segregations of a

hardening phase were detected on the surface of the bound-

aries. This allows us to expect lithium (and magnesium) dif-

fusion to the places of stress concentration, for example, be-

tween layers of flow of the metal, during deformation of al-

loys containing such diffusion-mobile elements as lithium

and magnesium (Al – Li – Mg) or lithium (Al – Li – Cu).

This causes considerable local supersaturation and more in-

tense decomposition of the solid solution in local zones [8, 9].

An analysis of fractures of alloys of the Al – Li – Mg and

Al – Li – Cu systems after testing for impact bending has

shown that the region where layering occurs has a low-duc-

tility cellular texture, wheres between the regions of layering

the fracture is ductile and large-dimple (Fig. 3).

Thus, inhomogeneity of the microstructure yields a metal

with strength and ductility varying over cross section. Frac-

tures of the Al – Li – Mg alloys are more homogeneous than

those of the Al – Li – Cu alloys, because in the former case

the only diffusion-mobile element is lithium.

The susceptibility of aluminum alloys to layering de-

pends considerably on the conditions of their heat treatment.

For example, transverse test pieces of a plate from alloy 1440

474 I. P. Zhegina and L. V. Morozova

a b

c d

Fig. 2. Microfractograms of aluminum

alloy V95 (a, b ) and V95pch (c, d ):

a) large dimples; b ) cellular texture;

c) large dimples with traces of plasticshear; d ) small dimples, a, b ) 7000;

c, d ) 3000.



after aging at 100°C for 3 h exhibit a homogeneous dimple

fracture; layering and zones of brittle “cellular” texture near

them appear already after 5-h aging at 120°C. When the ag-

ing temperature or the time of the hold is increased, these

zones become more developed and brittle. The fracture pat-

tern of plates from alloy 1451 is influenced much by the tem-

perature of their heating for quenching. After water quench-

ing from 480°C and 20-h aging at 150°C the fracture has a

small-dimple structure and exhibits clusters of primary phases ( KCT = 0.11 MJm2 ). When the temperature is raised

to 570°C, the proportion of particles of the primary phase de-

creases and the dimples become larger, which indicates a

higher capacity of the matrix for local plastic straining

( KCT = 0.19 MJm2 ). The process parameters and the

method of deformation of the alloy are very important. For

example, the use of lateral-longitudinal forging instead of the

conventional forging scheme with double upsetting for mak-

ing plates from alloy 1430 has improved the properties of the

plates over the height due to refinement of lengthy zones

with fragmented texture. For example, the time of the hold

under a single load equal to 0.9 has increased from 744 to

4500 h, i.e., the operating capacity of the alloy has grown

considerably [9].

The embrittlement of the alloy near the zones of layering

is affected much by the rate of the loading. For example, in

test pieces fabricated from a plate from alloy 1440 after 12-h

aging at 150°C and testing for impact bending ( KCT =0.13 MJm2 ) the zone near the layering has a cellular tex-

ture, whereas after testing for three-point bending the texture

contained brittle facets ( KCT = 0.05 MJm2 ). After testing

the alloy at a low temperature, brittle facets can appear in

zones adjoining the layering.

A great role in fatigue fracture belongs to the state of the

material near fiber boundaries. In ordinary aluminum alloys

fatigue fracture develops through formation of microcracks

from the boundaries of grains or particles of primary phases,

whereas in the alloys with lithium it develops from fiber

boundaries. Moreover, in the former case the direction of

propagation of microcracks coincides with that of the trunk crack or deviates from it by a low angle, whereas in the latter

case microcracks often propagate normally to the trunk

crack. In other words, the first to form in the alloys with lithi-

um is a tear over fiber boundaries and the fatigue crack prop-

agates over the body of grains. Thus, the presence of zones

with reduced ductility near fiber boundaries in deformation

of a semiproduct accelerates the development of the fatigue

crack.

The heterogeneity of microstructure detectable by fracto-

graphic analysis in aluminum alloys with lithium affects ne-

gatively their endurance in a corrosive environment. A

streaky intergrain fracture develops in this case in regions adjoining fiber boundaries, which gives rise to development of

fatigue cracks [10].

Studies of fracture surfaces have contributed much into

the development of the process of welding of aluminum al-

loys with lithium. The results of these studies were used to

create atlases of fractures that reflect the effect of surface

treatment before welding, to choose appropriate fillers and

kinds of welding, and to show the role of the quality of the

welded material [11 – 17].

Thus, fractography is an effective means for creating

new aluminum alloys, developing production processes,

choosing materials for specific operating conditions, and an-

alyzing operational failures.

CONCLUSIONS

1. Characteristic fractographic features have been deter -

mined for aluminum alloys VAD23, AK6, V95, 01911, 1420,

1430, 1440, and 1451 after aging at various temperatures and

tensile and impact testing.

2. Alloy VSD23 after zonal aging has the most ductile

condition, while phase and coagulation aging causes the ap-

pearance of brittle detachment facets in the fractures.

3. Alloy V95 is sensitive to stress concentrators and to

the quality of mechanical treatment. The mechanical proper-

Use of Fractography for Estimating Fracture Resistance of Aluminum Alloys 475

à

b

c

Fig. 3. Fracture of alloy 1420 after testing for impact bending:

a) layering; b, c) regions close to and between layered zones, respec-tively; a) 200; b ) 3000; c) 1500.



ties of the alloy improve when its purity with respect to iron

and silicon is raised; then the sensitivity to stress concentra-

tors decreases.

4. Deformation conditions affect aluminum alloys with

lithium. The use of lateral-longitudinal forging in the produc-

tion of plates from alloy 1430 has improved its propertiesover the height due to refinement of extended zones with

fragmented texture.

5. Embrittlement of Al – Li – Mg alloys in the zone ad-

joining layered structure is influenced considerably by the

loading rate. The higher this rate the more probable is the ap-

pearance of brittle facets on the fracture surface.

6. The determined dependences of the structure of frac-

ture surfaces on the modes of heat treatment and deformation

and on the presence of impurities have been used for design-

ing new aluminum alloys and emergency research.

7. Studies of fracture surfaces have shown that the oper-

ating capacity of aluminum alloys can be improved with the

help of factors improving the homogeneity of their structure

in forming and in subsequent heat treatment.

REFERENCES

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2. V. V. Antipov, “Strategy of development of titanium, magne-

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15. Ibid , pp. 744 – 755.16. Ibid , pp. 762 – 763.

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476 I. P. Zhegina and L. V. Morozova

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