note for microstructure

Metallography

Examination of materials using:

-visible light to provide a magnified image of the macro-and microstructure:optical microscopy

Eletrons bombarding the surface to provide information to produce an image: scanning electron microscopy

A beam of electrons passing through a very specimen. The ananlysis of the transmitted beam provides structural information . transmission electrons microscopy

Macrosegregation channels, porosity and flow pattern
macrostructure of as-cast aluminum ingot

Coarse columnar grains growing from the outer surface

Grain refinement in Al casting

With no grain refiner with grain refiner

Arc but weld joining 2 steel plates

Materials chareacterization

1. analytical chemistry
2. macrostructure
3. microstructure

-optical metallography

-quantitative metallography

-image analysis

Microstructure-seen with an optical (light) microscope(<2000x)

Microstructures are very complex:

Characterize only what is necessary

What is the importance of microsctructural characterization

Microsturcture dictates properties

Why is microstructure important?

Microsturcture influences properties and behavior-

Develop structure/property date

Composition and processing influence

Microstructure-monitor process conformance

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Undesired microstructures do cause product failures

-diagnose failures (prevent future failures)

Microstructue characterization tools

Microstructure

Characterized by phases present

Characterized by

Fraction composition orientation distribution location size morphology

Measured by bulk x-ray x-rays electron diffraction optical microscopy

Sem or tem

What is the importance of microstructural characterization by optical microscopy?

What is necessary?

A good specimen preparation

Specimen preparation

-sampling

-sectioning

-mounting(if needed)

-grinding

-polishing

-etching(if needed)

Sampling

Specimens selected must be representative of the material to be examined

-average sampling: represents the average features of the larger sample

-defined sampling: contains all of the information necessary for observation of a specific detail of interest

It depends o the type of investigation(R&D, quality control, failure anlysis….)

Examples of smapling

Reandon sampling method infinitesimal samoling

Rolled product

Transvers section:1

Longitudinal section:

-// to the rolled surface:2

-perpendicular to the rooled surface:3

Important: the position and orientation of the specimen should always be retraceable

Sectioning

-must be performed carefully to avoid altering or destroying the structure of interest

-band –saws, abrasive cutoff machines, precision saws

-the damage introduced to the specimen depends on the cutting tool, the cutting speed and the hardness of the material

The damage must be removed by grinding

Mounting

-often necessary in the preparation of specimens for metallographic study

-to facilitate handling of the specimen during preparation and examination

-proper mounting of specimens also ais edge retention(i.e. surface coatings)

-uniformity of shape and size for automation identification

Mounting media should be compatible with specimen regarding hardnesss and abrasion resistance

Grinding and polishing

goals of the initial grinding step

-remove the damage resulting from sectioning

-establish a planar surface

-reach a specific plane close to a desired area/feature

Gringing and polish on a variety of avrasives leading to a mirror finish

-wet grinding on SiC papers from 60 grit to 600 grit

-polishing with alumina powders, or diamond(coarse and fine)

Time for grinding and polishing

-each step must remove the deformation from the previous step

-increase time:increase material removal

-Increases I specimen surface area may require longer times

Material removal

The total depth removed by material depends on the hardness of the samples

Etching

Techniques

-Swab

-Immersion

-Electrolytic

To reveal microstructure

-grain boundaries, dendritic patterns

-Phases,constituents

-homogeneity, deformation

-banding , segregation, heat affected zones

Select the right etchant

Etching: safety

-read MSDS before using chemicals

-personal protection equipment

-Work under a hood

-label all containers

-clean up any spill, no matter how small

-dispose according to local regulations

Microstructural anlalysis

-microscopic examination of the sample

-capture photos/ electronic images

-description of the microstructure

-qualitative

-quatitative (comparison charts, depth/thickness measurements, stereology)

-image analysis

Optical microscope components

-illumination system

-condemser

-light filters

-the objective lens

-the eye piece

-stage

Optical microscopy

-use of visible to provide an image of the structure

-on the as-polished sample, the light is uniformly reflected. Frequently, the microstructure is not revealed as the eye cannot discern small differences in reflectivity

-only features that eshibit at least 6 to 8% difference in reflectivity can be viewed without etching

Image contrast

-image contrast is produced with etching and topographic scattering of reflected light is obtained and the structure can be observed

Image contrast

-image contrast is produced with etching and topographic scattering of reflected light is obtained and the structure can be observed

Artifacts

-specimen preparation quality is the determining factor in the value of the examination

-artifacts of sectioning

-artifacts of polishing

-artifacts of etching

Artifacts from sectioning

-the depth of damage will vary with mateirla and sectioning method

-materials with low melting point materials can be altered by overheating during sectioning

-effects could include

-recovery(dislocations)

-recrystallization (grain size and shape)

-tempering (tetrgonality/dislocations/precipitates)

-precipitates coarsening

Identified by materials properties not related to the structure

Artifacts

Sractches(observed befor etching

Thin gouges resulting fro the action of avrasive during the material removal process. Their width and depth are in direct relationship to the size of the size of the avrasive and other factors

Possible causes

Contaminatin

Insufficient polishing time

Poorly graded abrasives

Solutions

More careful cleaning between steps

Increase step times. Use quality products

Artifacts

Relief a condition of the polished surface of a specimen where harder phases or constituents are left raised above the surface of the softer matrix

Possible causes

Excessive polishing time

Use of napped cloths

Load too low

Solutions

Minimize polishing time

Use low or napless cloths

Use higher pressure

Artifacts

Smear a superficial but sighificant form of shallow deformation that reduces the clarity of final polished specimens, making microstructural detail less distinct

Possible causes

Inadequate lubrication

Excessive pressure

Excessive polishing times

Solutions

Control lubrification, pressure and time

Etch and rejpeat final polish

Artifacts

Voids precipitates/inclusions removed: similar size and distribution to remaining ones pits: uniformly distributed

Possible causes

Over polishing

Over etching(causes also staining, etch products)

Solution

Polish fast

Shorten etching time

Recommendations to reveal the true microstructure

-remove cutting, grinding and polishing related deformation

-maintain flatness

-keep artifacts to an absolute minimum

-avoid thermal damage

-minimize relief and smearing

-produce scratch-free surfaces

Microstructrural characterization

-all examination of microstructure should begin with use of the optical microscope

-start at low magnification, such as 100X, flowed by progressive higher magnifications to assess the basic characteristics of the microstructure

-make sure you can see all key characteristics

-helpful to anticipate what micro to expect from knowledge of compostion and processing

Limitations of optical microscopy

-resolution limit: approximately 1 micron

-limited depth of field: cannot focus on rough surfaces

-does not give direct chemical or crystallographic information about the microstructural features

Examination modes in light microscopy

-bright field illumination

-dark-field illumination

-polarized light

-phase-contrast illumination

-interference techniques

“optical” etching to have adequate contrast

Optical ‘etching’ techniques

Bright field

Normal operation

Rays reflected ‘permpendicularly’ go through the objective lens (are bright)

Dark field

Rays reflected ‘perpendicularly’

Do not go through objective lens

Polarized light

Contrast from optically anisotropic metals (i.e. non cubic crystals)

How to characterize a microstructure?

Microstructure characterized by phases present and defects

Phases present characterized by morphology, distribution, location, size.

Phases present: morphology/shape

Classically geometrical: cube, sphere, ellipse, cylinder(rod)…

Note: optical micros are 2D. Real micros are 3D

Cylinder

Equiaxed /polygonal

Lamellar/lath

Plate/lenticular

Needle

Dendrite (Christmas-tree-like in 3D)

Dot-like in 2D

Distribution of the phase

Homogeneous or inhomgeneous

Orientation and location of the phase

-orientation (visually and crystallographically)

-location

Number of phases

For matrix phase:

-‘color’ of matrix phase

-size distribution of matrix grains

-morphology of matrix phase

-visual orientation of matrix phase, if applicable

Number of phases

For subsequent phases:

-volume fraction

-“color”

-size distribution of grains

-size relative to matrix

-location

-morphology

-distribution

-visual orientation if applicable

Defects

Cracks, voids

Identified by depth of fields

Inclusions

Identified by difference fro bulk material composition and infrequency of observation

Quantitative metallography

Phase characteristics that can be quantified

-volume fraction

-sizes(average and distribution)

-shape

-distribution(e.g. mean free path?)

-orientation

-location (fraction at GBs, etc)

-morphology

For lamellae, needles, plates, acicular grains use aspect ration, i.e. length: with ratio

Morphology

For non-uniform, non-equiaxed shapes, use feret concept

Faret length, parallel lines tangential to feature

Aspect ration if maximum ration of feret lengths at 90 degree to each other

Standard chart methods

Evaluation of Sr modification in Al-Si foundry alloys

Standard grain size

Note: twins are not grain boundaries

Quoted grain size is matrix

Unless size of 2^nd phas is comparable size

Grain size will vary from zero to max

(2-D section through 3-D structure)

(therefore there is already a size distribution)

Fields of view must be chosen randomly

Grain size

1) comparison procedure

-compare structures with charts

-charts must resemble resemble microstructures being measured

-several types of grain structure ‘plates’ (i.e. micros) available;

-untwinned(100X)

-twinned flate etch(100X)

-twinned contrast etch(75X)

-austenite grains in steel(100X)

Compare at least 3 fields

Grain size quantified as ASTM number (n)

Number of grains/in^2 = 2^(n-1)

ASTM 1=1 grain/in^2 at 100X

Or 15.5 grains per 10,000 mm^2

But ASTM numbers are not grains/in^2

Note: ASTM grain size (all microstructure 100X)

Grain Size

2) plainmetric(Jeffrie’s) procedure

base on number of grains within a known area

can be converted in ASTM #

Grain Size

3) General intercept procedures

Count number of times grain boundaries intersect a line mean intercept length= ‘grain size’

Can be converted into ASTM number

Quantifying multi-phase microstructure

Total length of interceptions/total line length= volume fraction