ENMA 310 Microstructural Analysis Laboratory Fall 2001

Microstructure plays an important role in a wide variety of behaviors (mechanical, thermal, electrical, optical, magnetic). In general when we refer to microstructure we are referring to the size, shape and distribution of each phase in a polycrystalline and/or multiphase material. Microstructure is largely developed during processing. In materials formed from a melt (most metals) the casting conditions and subsequent treatments (cold work and annealing, hot working, annealing, etc.) determine the microstructure. In materials made via powder processing, powder characteristics (size, shape and surface impurities) along with the time-temperature history tend to determine microstructure. In polycrystalline thin films the deposition parameters (pressure, substrate temperature, etc.) and annealing conditions tend to determine the microstructure. Application at high temperatures (relative to Tmp or Tg) may alter the microstructure since grain growth, evaporation of volatile components, oxidation and other processes may occur.

In the lab you will look at metal and ceramic samples using optical and scanning electron microscopy and learn some basic microstructural analysis techniques.

1. To familiarize students with microstructural features and their characterization.

a. Estimated size of the features of interest.

b. Sample preparation techniques.

c. The information the technique can provide.

I. Scanning Electron Microscopy (SEM)

You will look at five samples in the SEM. We will use the ESEM (environmental scanning electron microscope) so that we do not have to coat the samples to prevent charging from the build-up of electrons from the beam on the surfaces of the insulating samples. You will observe two or three of these samples again under an optical microscope.

1. Polished and etched Al (from ENMA 362).

a. How many phases do you think are present? Why?

c. Does the grain size look relatively uniform?

2. Polished and etched ZnO samples.

a. How many phases do you think are present? Why?

3. AlN-TiB2 high thermal conductivity, controlled dielectric loss composite.

c. What is the typical shape of each phase?

4. Fracture Surface Analysis

Examine the fracture surface of the ZnO or AlN-TiB2 sample.

1. Polished and etched ZnO. Observe at 100X.

2. Polished and etched Al (from ENMA 362). Observe at 100X.

b. Do all the grains look the same? Why or why not?

1. Refractory Brick

Look at the mag-chrome (MgCr₂O₄ spinel plus other phases) refractory brick (they are used to melt steel) with your eyes (do not use the microscope yet!) and then put it under the stereomicroscope. Observe the brick at 10X. Then slowly zoom the magnification up to 40X.

2. Naval Brass Plate

Look at the cast brass plate with your eyes. Sketch the microstructure of the plate.

Look at the columnar grains under the microscope. Sketch these grains.

Using what you learned in ENES 230 or elsewhere can you explain why the brass plate has the microstructure it does including the columnar grains?

If your whole sample consisted of columnar grains like those you observed in the microscope, would it affect the mechanical behavior of the plate? If so, in what way? Why?

1. Number of phases present

2. Relative amounts of each phase present

- areal analysis (surface area analysis): Usually done using image analysis. The area of a given phase is compared to the overall area. Again, you must consider statistics and analysis a sufficiently large area. Also, you need to have enough phase contrast for you system to be able to differentiate phases. You may need to go back and "help" the computer to get a good analysis.

3. Grain size

HINT: Always include a size marker (nm, µm, mm, etc.) on photomicrographs that you put in reports, papers or presentations. A size marker helps you "calibrate" your eye and put things in perspective. Listing the magnification at which the micrograph was taken does not provide this calibration. Also, when the photograph is processed, i.e. printed, Xeroxed, scanned into a computer, made into a transparency, etc. size may be changed, this will change the magnification!

• ASTM Grain Size Number, G

G is defined such that n = 2 G-1

where n is the number of grains per square inch at 100X. This is most commonly used for metals and is more often an estimate based on "chart ratings".

See Buehler Tech-Notes

• Lineal intercept method

After correction, this gives you the average grain diameter if you have equi-axed grains. If you have platy grains you can use it to determine the platelet diameter and thickness. You must "correct" your size since you are taking a 2-D sample of a 3-D structure and thus, your analysis plane will frequently not actually intercept the diameter of the grains.

mean intercept length = total line length / number of grain intersections -1

Remember you must take the magnification into account when you determine the length of the line. Some standard corrections are:

mean intercept length = 2r where r is the rod diameter for uniform rods

mean intercept length = 2 t where t is the thickness of a platy phase

Also see Introduction to Ceramics

4. Grain shape (morphology)

This is qualitative not quantitative. The shapes encountered in metals and ceramics include equi-axed grains, platy grains (platelets), columnar grains, and very high aspect ratio grains (needles-accicular grains or lathes). You may encounter spherulites in polymers.

1. Calculate the ASTM grain size number for the Al sample. SHOW YOUR WORK. Compare your answer with that of the other students. How well do they compare? If there are significant differences, give some possible reasons for the differences.

2. Calculate the average grain diameter for the two different ZnO samples. SHOW YOUR WORK.

Compare your answer with that of the other students. How well do they compare? If there are significant differences, give some possible reasons for the differences. Was there a significant difference in the average grain size of the two ZnO samples? Let Prof. Lloyd know your average grain sizes she can tell you about the processing that would produce such grain sizes.

3. Using the photomicrograph of the AlN-TiB2 composite, calculate the volume fraction of each phase using the point count method. SHOW YOUR WORK. Compare your answer with that of the other students. How well do they compare? If there are significant differences, give some possible reasons for the differences.

Note: be sure to include the answers to the questions in the laboratory exercise in your lab report as well as the analyses above.

Traditionally, one prepares samples for microstructural analysis by polishing the sample with progressively smaller abrasives until the sample is smooth, shiny and featureless. The most common abrasives are diamond, silicon carbide and alumina. One chooses abrasives on the basis of the hardness of what you are polishing. You can not grind or polish a material with an abrasive that is softer than the material. Once the sample is polished, it is usually etched with an acid solution to highlight features various features. Sometimes electrochemical or thermal etching is used instead of "plain" acid etching. Etching tends to highlight features like grain boundaries, dislocation tangles, and second phases because they have different reactivity than the grains (usually they are more reactive). This technique can be used with hard, soft, brittle and tough materials. (If you have a porous material, you will probably have to "pot" the sample in a resin that prevents grain pullout as you are polishing.) Because the samples prepared by polishing and etching are very flat, they are suitable for optical microscopy as well as SEM etc. Remember, optical microscopes have a limited depth of field.

If you have a brittle material and a SEM, then fractures surfaces can be used for microstructural analysis. Fracture surfaces are commonly prepared by wrapping the sample in a laboratory tissue and then fracturing it with a hammer. If intergranular fracture occurs, then grain size and shape can be qualitatively observed. Qualitative chemical analysis using EDS (energy dispersive analysis) can also be done.

Coating: If you are using a traditional SEM rather than an environmental SEM, you must apply a conductive coating to insulating samples. Carbon and gold are the most common coatings. In an environmental SEM, the water vapor in the chamber helps to keep charge from building up on insulating samples.

Many properties of materials are strongly influenced by microstructure. A few examples follow. You will see many more examples as you take other classes and as you work with real materials throughout your career.

Strength: In both ceramics and metals strength tends to be inversely proportional to grain size. In metals this is related to the transfer of dislocation motion from grain to grain. In ceramics this behavior is related to the tendency of the flaw size to scale with the grain size. (See also ENMA 362.)

Optical Transparency: Many ceramics neither absorb or reflect much visible light, but they appear opaque. This is because of light scattering when the grain size and/or the pore size is about the same as the wavelength of light. Polycrystalline ceramics with very small (~ nm) or very large (roughly _ 50 µm) grain sizes are often transparent or translucent. In addition, highly porous materials (ceramics or polymers) can be transparent or translucent when the pores are much smaller than the wavelength of the light.

Superparamagnetism: Materials which would normally exhibit magnetic behavior (ferromagnetic or ferrimagnetic) show paramagnetic behavior when the grain size is very small.

Hardening: Precipitation hardening and dispersion hardening both take advantage of second phase particles to inhibit dislocation motion. (See also ENMA 362.)

Viscous creep: Reaction bonded silicon carbide (SiC) and silicon nitride (Si3N4) are made by using a glassy second phase at the grain boundaries to bond them together since they are highly covalent and difficult to sinter. (See also ENMA 463.) At elevated temperatures (above Tg of the glassy bonding phase) these materials have poor creep resistance since the glassy phases undergoes viscous deformation-i.e. they exhibit viscous creep.

VII. REFERENCES

R.E. Reed-Hill, Physical Metallurgy Principles, 2nd. ed. pp. 1-5 PWS Kent Publ. Co. (19XX). Textbook for ENMA 471}

J.S. Reed, Principles of Ceramic Processing, 2nd. ed. pp 84, John Wiley and Sons, Inc. (1995). [Textbook for ENMA 463]

W.D. Kingery et al, Introduction to Ceramics, 2nd. ed. pp. 526-530, John Wiley and Sons, Inc. (1976) [Supplemental Text for ENMA 420]

Buehler Tech-Notes, Vol. I, No. 5, Introduction to Quantitative Metallography, 1997

H. Luth, Surfaces and Interfaces of Solids, p. 120, Springer-Verlag (19XX)

J.I. Goldstein et al, Scanning Electron Microscopy and X-Ray Analysis, pp 1-18, Plenum Press (19XX)