Microstructures and MetallographyMicrostructures and Metallography

Metallography: Principles and PracticeGeorge F Vander Voort

ASM International, Materials Park, Ohio (1999).

Reading

MATERIALS SCIENCEMATERIALS SCIENCE&&

ENGINEERING ENGINEERING

Anandh Subramaniam & Kantesh Balani

Materials Science and Engineering (MSE)

Indian Institute of Technology, Kanpur- 208016

Email: anandh@iitk.ac.in, URL: home.iitk.ac.in/~anandh

AN INTRODUCTORY E-BOOKAN INTRODUCTORY E-BOOK

Part of

http://home.iitk.ac.in/~anandh/E-book.htmhttp://home.iitk.ac.in/~anandh/E-book.htm

A Learner’s GuideA Learner’s GuideA Learner’s GuideA Learner’s Guide

Smithells Metals Reference BookE.A. Brandes

Butterworths, London (1992).

We have already noted that there are structure sensitive properties (like yield strength and fracture toughness) and structure insensitive* properties (like density and Young’s modulus). The word structure in this context (usually) implies microstructure.

We have also stated that Material Engineers can also be called microstructure engineers. Processing used to create the component or the sample determines its microstructure. In this context we should keep in view the materials tetrahedron (as below).

Traditionally, the structure seen at high magnifications (of an optical microscope– say at 500 or 1000), is known as microstructure (in some sense it is the ‘micron-scale structure’).

As we would like to get an handle on the structure sensitive properties, we study microstructures → i.e. we want to do structure-property correlations.

This implies that the definition based on magnification is unsatisfactory. Additionally, if we do imaging at even higher magnification (say in an TEM) the structure we see will have to be termed as ‘nanostructure’ (‘nanometer-scale-structure’).

Keeping the above in view, there is a need for a ‘functional’ definition of a microstructure, which can give us an handle on the properties.

Microstructure

The Materials Tetrahedron

* The word is ‘sensitive’ and not ‘dependent’! (i.e there can be a dependence).

Microstructure

Thermo-mechanical Treatments

Phases

Defects

+

• Vacancies• Dislocations• Twins• Stacking Faults• Grain Boundaries• Voids• Cracks

+

Residual Stress

Processing determines shape and microstructure of a component

& their distributions

A functional definition of a microstructure can be given as:

[Phases (including morphology) + Defects + Residual Stress] and their distributions. The above also includes defects between the phases and those arising from a global view of

the sample (like misorientation between grains, crystallographic texture, etc.).

The rationale behind this definition is considered next (in the next few slides).

The most difficult part to understand is: “how is residual stress part of microstructure?”

Atom Structure

Crystal

Electro-magnetic

Microstructure Component

Thermo-mechanical Treatments

Phases Defects+

• Casting• Metal Forming• Welding• Powder Processing• Machining

• Vacancies• Dislocations• Twins• Stacking Faults• Grain Boundaries• Voids• Cracks

+ Residual Stress

Processing determines shape and microstructure of a component

& their distributions

Since terms like Phase and Microstructure are key to understanding phase diagrams, let us have a re-look at the figure we considered before

Click here to know more about microstructuresClick here to know more about microstructures

The distribution of phases is an important factor which determines the properties of a material. For a fixed volume fraction (or weight fraction) of phases present; the shape, connectivity and distribution of the phases will play a decisive role in determining the properties of the material.

To understand this let us consider 4 different distributions of a brittle phase (B) in a tough ‘matrix’ phase (A)– keeping the volume fraction of B constant. B is ‘incoherent’ with matrix. (a) B forms a continuous network→ leads to poor fracture toughness due to continuous path for crack propagation along B.(b) B is needle shaped with ‘sharp’ features → sharp features lead to stress concentration leading to crack initiation (which then connect via the brittle phase B and lead to low fracture toughness). (c) B is in the form of large spheres with more interparticle spacing (d) B is in the form of small spheres with less interparticle spacing as compared to (c)→ this material has more strength as compared to (c) dislocations have to bow around closer spaced obstacles.

Importance of the distribution of phases

(a) (b)

(c) (d)

Four distributions of phase-B (a brittle but hard phase) in phase-A (the tough phase) is considered.

Phase B can even be nanosized

Touch phase ATough phase A

Brittle phase B

Some of these concepts are covered in other chapters

Let us further consider two examples of real microstructures. The configuration (a below) is similar to (a) in the previous slide. Here, Cementite a brittle phase is

present as a continuous phase along the ‘prior’ austenite grain boundaries (highlighted in yellow), making the material brittle in impact→ cracks propagate along this continuous path of brittle phase.It is to be noted that, there is cementite also present within the prior austenite grains– but its effect is not as deleterious in impact, as the continuous layer of cementite along prior asutenite grain boundaries.

In (b) there is an amorphous phase along the grain boundaries of Si3N4 ceramic, giving rise to a similar

situation (i.e. a weakened material in impact).

0.66 nm

1.4 nm

IGF

Grain-1

Grain-2

0.66 nm

1.4 nm

IGF

Grain-1

Grain-2

Cementite in steel along prior austenite grain boundaries Intergranular glassy film in Lu-Mg doped Si3N4 sample

(a)

(b)

Layer of cementiteCementite as a part of the pearlitic mixture

Residual stresses are those which arise in a body in the absence of external loads or constraints. Residual stress can be beneficial or deleterious, depending on the context. Processing used to make the material/component can give rise to residual stresses. Vacancies and dislocations give rise to stress fields at the atomic scale, while residual thermal stresses

could pervade the entire component. Some points in this regard are:

a) the stress fields associated with GP zones in Al-Cu alloys is of the same scale as dislocation stress fieldsb) large cracks in the material can lead to macroscopic stress fields, while micro-cracks may have much small effective region of stress fieldsc) in micron sized components, the scale of thermal residual stress may is expected to be smaller than that in their large scale counterparts.

In the example of GP zones in Al-Cu alloys, the stress/strain fields are intricately associated with the distribution of GP zones (~ the phases); which further highlights the importance of adopting a definition of microstructure as done in here.

Importance of residual stress Click here to know about the definition and origins of residual stressClick here to know about the definition and origins of residual stress

[Note. Cracks themselves are not sources of stresses; they merely amplify a far field stress. As they can amplify a small far field stress (which could be residual in nature), they have been included in this section].

Plot of x stress contours

Edge dislocation stress field

+ 2.44

+ 1.00

+ 0.67

+ 0.33

0.00

− 0.33

− 0.67

− 1.00

− 1.16x

y

z

All values are in GPa

Simulated σy contours

Stress state (plot of y) due to a coherent -Fe precipitate in a Cu–2 wt.%Fe alloy aged at 700 C for (a) 30 min.

+ 2.44

+ 1.00

+ 0.67

+ 0.33

0.00

− 0.33

− 0.67

− 1.00

− 1.16

+ 2.44

+ 1.00

+ 0.67

+ 0.33

0.00

− 0.33

− 0.67

− 1.00

+ 2.44

+ 1.00

+ 0.67

+ 0.33

0.00

− 0.33

− 0.67

− 1.00

− 1.16x

y

z x

y

z

y

zz

All values are in GPa

Simulated σy contours

Stress state (plot of y) due to a coherent -Fe precipitate in a Cu–2 wt.%Fe alloy aged at 700 C for (a) 30 min.

Residual stresses due to an coherent precipitate

Often one gets a feeling that residual stress is only harmful for a material, as it can cause warpage of the component- this is far from true.

Residual stress can both be beneficial and deleterious to a material, depending on the context.

Stress corrosion cracking leading to an accelerated corrosion in the presence of internal stresses in the component, is an example of the negative effect of residual stresses.

But, there are good numbers of examples as well to illustrate the beneficial effect of residual stress; such as in transformation toughened zirconia (TTZ). In this system the crack tip stresses (which are amplified over and above the far field mean applied stress) lead to the transformation of cubic zirconia to tetragonal zirconia. The increase in volume associated with this transformation imposes a compressive stress on the crack which retards its propagation. This dynamic effect leads to an increased toughness in the material.

Another example would be the surface compressive stress introduced in glass to toughen it (Surface of molten glass solidified by cold air, followed by solidification of the bulk → the contraction of the bulk while solidification, introduces residual compressive stresses on the surface → fracture strength can be increased 2-3 times).

The reason for elevating residual stress to be an integral part of the definition of microstructure will become further clear in the next two examples considered.

It is well known that nucleation is preferred at any of the high-energy sites in a material (heterogeneous nucleation). Typical examples of heterogeneous nucleation sites are: surfaces, internal interfaces (grain boundaries, interphase boundaries, stacking faults etc.), crystallographic defects (dislocations), cracks, voids etc. Stressed (and hence strained) regions also can act like heterogeneous nucleation sites.

If a 3 stage process of the growth of epitaxial islands is followed: (i) growth of islands, (ii) capping the islands with a layer, (iii) growing a layer of islands above the capped layer → It is observed that the second layer of islands nucleate in the highly strained region of the capping layer (usually right above the layer below). Strained regions acts like a preferred nucleation site.

3

21

In corrosion formation of a galvanic cell leads to corrosion of the anode. At the level of the individual phases, one phase may be more anodic as compared to another (figure (a) below), thus forming a 'micro'- galvanic cell. Similarly, tensile regions in the sample may behave anodically with respect to compressive regions in the sample (figure (b) below), which can behave cathodically.

strained regions can play a role of the phases in galvanic corrosion.

(a)

(b)

BainiteBainitePearlitePearlite

AlCoFeNiAlCoFeNi

Let us look at some ‘microstructures’ before we proceed

‘Surface grain’ in SEM

Lamellae in AFM

Pearlite Lamellae in SEM

AlCoFeNiAlCoFeNi

Surface of canine tooth in SEMSurface of canine tooth in SEM

Traditionally metallography is the process by which a sample is prepared so as to reveal the microstructure under a optical microscope.

Sample preparation so as to reveal the microstructure in a SEM or a TEM can also be considered to be in the realm of metallography.

In biological specimens we rely on absorption contrast, while in material specimens on reflection contrast.

A typical metallographic specimen preparation involves (there are other possibilities as well): Sampling (selecting (say by cutting) representative regions of sample) Polishing (uniform material removal to make the sample surface flat) Etching (selective electro-chemical removal of the material to create a non- uniform surface)

The reader may refer to standard texts to know more about metallographic specimen preparation.(e.g. Metallography: Principles and Practice, George F Vander Voort, ASM International, Materials Park, Ohio (1999)).

MetallographyMetallography

Make sure that the sample preparation does not lead to artifacts (overheating, loss of phase in small volume fraction, pitting, etc.).

For viewing areas close to edges (e.g. in case carburized samples) use resin mounting.

Use right kind of emery paper (wet or dry).

In porous samples, ultrasonicate between emery papers to remove debris from previous emery paper.

Use separate polishing cloth for composites with hard second phase, which can fall off during polishing. Remember that metallographic of soft materials, two phase mixtures (especially of soft and hard phases) and composites is very difficult.

Some samples may be very difficult to etch (to reveal essential features).

Temperature is a very important parameter in etching. There will be considerable difference in etching rate from cold winter day (0C) to a hot summer day (40C).

A deeper etch is preferred for SEM studies (as secondary electron imaging is not as sensitive to topography as optical metallography). A polished but unetched sample can be used in addition to a etched sample if back scattered imaging is to be done in SEM. This can avoid etching out of ‘small sized’ phases.

Important points regarding optical metallography

Why do some regions look bright and some dark in an optical micrograph?

If we look at the micrograph from a Cu (cold worked and recrystallized) sample (polished and etched), we see different shades of gray*.

In an optical micrograph from a metallic specimen the main mechanism of contrast is reflection contrast.

Regions which reflect into the optical axis (i.e. in a bright field image) look bright and which reflect away from the optical axis look dark.

A rough surface reflects in a diffuse manner while a smooth surface gives rise to ‘specular’ reflection (like a mirror).

A polished surface is flat, while an etched surface is not uniform and has a ‘topography’. The ‘electro-chemical’ action (leading to a ‘etched’ surface) depends on the crystallographic orientation

of the surface plane.

* The actual micrograph will be in colour (with different shades of ~brown) → converted to gray-scale for didactic purposes

Regions from the sample showing at least 3 shades of gray

AA’

Portion of the micrograph from Cu sample

Line scan of intensities from AA’A A’

Bright

Dark

Intermediate

Schematic of possible topography of an etched surface leading to such variations in intensity Exaggerated schematic

Bright field imaging is ‘on optic axis’ imaging, while dark field imaging is ‘off optic axis’ imaging.

In a Bright Field Image (BFI) the regions of interest looks dark in a bright background, while in a Dark Field Image (DFI) the features look bright in a dark background. Hence, the contrast is better in a DFI.

DFI in addition offers specificity with respect to the features observed (you may pick and chose the features you want to observe). DFI is a routine method in TEM.

DFI may be obtained in three ways: (i) tilt the light source, (ii) tilt the angle of viewing, (iii) tilt the specimen.

In practice not all of these methods is used commonly. Usually, the a ‘hollow cone’ light source is used for obtaining dark field images.

Bright field and dark field imaging in optical microscopy

Dark Field Images

Observed features and interpretation

All micrographs should be interpreted with care. In the micrographs below, the pearlite spacing seems to differ from region to region. Even with a single spacing of

pearlite this could occur because of angle the colony makes with the cut surface (schematics below)

Large spacing

Fine spacing

What can we interpret from circles in micrograph?

We should be careful not to make over-interpretations from micrographs. For example in the schematic figure below we seen circles of different diameters in a micrograph- what does this

mean (in terms of the size of the second phase particles; assuming them to be spheres for now)? we should not conclude without further investigation that the second phase particles are spheres of different sizes!

The micrograph could arise because of sectioning at different heights of the spheres (as shown in the schematics below).

Sample with sectioning plane

View in the micrograph

Note that though the spheres are of the same size it is seen in the micrograph as circles of different sizes

As we noted we should be careful not to make over-interpretations from micrographs. For example in the schematic figure below we seen circles of different diameters in a micrograph- what does this

mean (in terms of the morphology of the second phase particles; now we relax the condition that they should be spheres)? we should not conclude without further investigation that the second phase particles are spheres!

The micrograph could arise from various morphologies of the second phase (as shown in the schematics below).

What can we interpret from circles in micrograph?

Sample with sectioning plane

View in the micrograph

View in the micrograph

The micrograph in the figure below was created by taking optical micrographs from a specimen, after polishing (& etching) to various depths (sectioning). This gives a ‘3D’ view of the sample.

Grain

Grain boundary

Triple line