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Development of an OCP "Mylonite"

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The sample material is octachloropropane (OCP) C3Cl8. It is hexagonal, optically uniaxial positive, and melts at 160°C. Like quartz, it can deform by slip on the basal and prism planes.

4. OCP sample prepared by hot-pressing an OCP granule between the glass slides. Field width here, and in all other slides unless noted, is about 0.8 mm (800 µm). Sample thickness normal to screen is about 60 µm.

This sample is not entirely strain-free. There are many large, blocky subgrains, still reflecting the hot-pressing deformation by which the sample was prepared. Note, however, several features characteristic of polycrystals that have low stored energy. The grains boundaries are mostly straight or smoothly-curved; there are numerous triple junctions (where three grain boundaries meet) with approximately 120°-120°-120° angles; and large areas of many of the grains are uniformly colored (and would extinguish uniformly if the stage were rotated).

The sample is about to be deformed at room temperature (70% of its absolute melting temperature). It was supposed to be sheared dextrally with the "plane of the cards" (in a card-deck model of shearing) normal to the screen as in image 3. However, the glass/sample interfaces were inadequately lubricated and the sample was sheared unintentionally with the "plane of the cards" parallel to the screen. This made the strain-rate much higher than it was meant to be, and a nice "mylonite" was produced as follows.


5. The grain boundaries have become irregular, and subgrain boundary preferred orientation is developed.


6. New grains or subgrains are beginning to form near the old grain boundaries. Examples of new grains, formed by an unknown mechanism, are seen at (74,11) and (48,4).


7. By this stage it is hard to define the positions of many of the original grain boundaries, because of extensive subgrain development along them. Note that the two new grains noted in the last slide seem to have lost their identity already. See how the diffuse white-yellow boundary through (22,27) was previously a sharp grain boundary (refer back to image 4). This is an apparent example of AMALGAMATION of two differently oriented grains into one grain, by rotation of one or both lattices until the original grain boundary becomes only an array of subgrain boundaries. This is a grain-size coarsening process that can evidently occur even where grain- size reduction is going on nearby.


8. Some original grains, now loaded with blocky subgrains remain (e.g. at 70,25), but the original grain structure is obscured by the new generation of smaller grains and subgrains.


9. Mylonitization is nearly complete. Can you think of a reason for the weakly defined, vertical preferred orientation of grain and subgrain boundaries? (ans) The total shear strain at this stage is unknown but probably exceeds ten. The shear strain rate was around 20% per minute.


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