TIME TEMPERATURE TRANSFORMATION DIAGRAMS

The objectives of this laboratory are:

1. To demonstrate the effect of temperature on the kinetics of microstructural evolution during solid to solid phase transformations.
2. To familiarise the student with metallographic preparation techniques.
3. To introduce communication skills and record keeping in the context of scientific experimentation.

Part 1

The Time-Temperature-Transformation (TTT) diagram for Cu-12.0 wt% Al is to be determined. The samples are to be solution treated at 900°C to produce a coarse equi-axed structure of b -Cu₃Al. The samples should be quickly transferred, one at a time, to the salt baths and allowed to transform for periods ranging from 1 s to 24 hrs. Isothermal Transformations should be carried out at 525°C, 475°C, 425°C, and 375°C. Do not forget to label all the specimens. Since any one group will only be carrying out a part of the experiment, everyone should use their notebooks to record everything that was done, what was found, thoughts, comments, and any other relevant information.

Part 2

A specimen should be cut from each sample, mounted in bakelite, labeled on the back, ground and polished until a mirror-like finish is produced. After each stage of the grinding process, the samples and your hands should be thoroughly washed to prevent large particles of abrasive being carried over to the next, finer, stage of preparation. At each step the sample should be rotated through 90° to allow you to judge the extent of grinding needed to remove the previous coarser set of scratches. The specimens should be completely dry before starting to polish on the 6mm diamond wheels as these use an oil-based lubricant. Again completely remove all traces of oil and diamond wastes from the sample and your hands before the final polishing step. Final polishing should be carried out using a suspension of 0.05µm alumina in water.

Clean the samples and etch in 1% FeNO₃ in methanol until you can see, with your eye, the grain structure appear. Wash and dry the samples thoroughly before examining them under the microscope. If there is insufficient contrast between the parent and product phases, repeat the etching process as necessary.

SAFETY: You must wear safety glasses and rubber gloves while handling all etchants. All acids and alkalis can cause serious damage to your skin - some, particularly those containing HF can damage your nervous system through the blood stream.

Part 3

Photomicrographs should be prepared from each of the samples and mounted on a graph of Transformation Temperature (y-axis) vs. LOG (Transformation Time) (x-axis). Evaluate where the transformation starts and where the transformation ends.

Part 4

As you were carrying out the experiment, you should have been noting down everything you did. Based on these and your colleagues' notes, write down everything you did in the experiment, thoughts, etc. Read through what you wrote and underline only those pieces of information which should go into the lab write-up. Write a concise paragraph detailing the experimental procedure. This should be given in prior to completing the lab write-up.

Notes on Metallography

Sample Preparation In general, sample preparation of opaque materials such as metals involves obtaining a flat mirror-like finish which can be examined by reflected light.

The sequence of operations is typically as follows:

1. If the sample is too small or has an awkward shape, it should be mounted in hot or cold setting plastic for easy handling.
2. By means of a belt sander or wet abrasive wheel, grind a flat surface on the sample and bevel the corners and edges. Avoid heating or deforming the sample.
3. Using wet silicon carbide papers of successively finer grades, obtain a finely ground surface covered with even parallel scratches from the smoothest paper. Excessive pressure is not required. It is important to wash the sample and your fingers before moving on to a finer paper in order to avoid carrying over the coarser abrasive particles. The common rule is to grind on each paper for twice as long as it took to remove the scratches from the previous grade, turning the specimen through approx. 90° at each change.
4. Having thoroughly washed away all traces of grinding material, a final surface is prepared by polishing the surfaces on a polishing wheel which is covered with a fine abrasive paste or slurry. A variety of polishing cloths and polishes are available commercially.

The polishing wheels provided are to be used with a suspension of 6µm diamond paste in kerosene followed by 0.05µm Al₂O₃ in water. The final polish will give a satisfactory mirror surface, but again it is important to clean the sample and one's fingers to avoid carrying over coarse particles to the final wheel. Again, excessive pressure is not required.

Etching Sources of Contrast

Following the preparation of a near-perfect mirror surface, it is usual to etch the specimen lightly to reveal details of the microstructure. Metals are opaque to light of visible wave lengths and the contrast across the surface is obtained by reflection. There are three principal sources of contrast:

1. Chemical attack may be sensitive to the crystal orientation, leaving exposed small facets of planes of certain {HKL} indices. The result is that the light is reflected from adjacent crystals in different directions and consequently reveals the grain structure as areas of variable darkness.
2. Parts of the structure of high energy are the grain boundary interfaces or two phase interfaces. Depending on the nature of the chemical attack, these regions may be preferentially dissolved so that their intersections with the polished surface are delineated as fine grooves or line traces.
3. Variation in chemical composition within an alloy inevitably means that the etchant attacks some regions more rapidly than others, either removing one phase preferentially, or staining parts of the structure to give color contrast. A mixture of two phases, a and b, might then etch to give a stepped profile.
4. Note that in addition to the above, surface contrast may also be obtained by optical methods such as the use of polarized light or interference techniques.

Materials which have non-cubic crystal structures rotate the plane of polarization of light in transmission or reflection depending on the orientation of the crystal with respect to the optical axis. In reflection a sample does not need to be etched to produce contrast.

The incidental light is plane polarized by insertion of a "polarizer" slide into the optical path while a second slide is inserted in the optical path behind the reflector and can be rotated axially; this is called the "analyser." If the planes of polarization of the polarizer and analyzer are crossed at right angles, no light will pass through to the eye unless light from the samples has been rotated in transmission or reflection.