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Plastic Deformation and Static Recovery in OCP

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10. A beautiful FOAM TEXTURE in OCP. This area is entirely "strain-free". The maximum interference colors are more varied than in the last sample because this sample is thicker. Most of the grain boundaries are very steeply dipping with respect to the screen. Where they are not so steep, the boundaries appear doubled, or marked by a light-colored band. The two red ink spots mark material points that will be shown again in a later image, at the end of the deformation. The deformation will be a pure shear, approximately, with the direction of maximum extension about vertical. The strain rate will be about 2% per minute. The temperature is room temperature through most of the deformation, but it started out a little warmer than this.

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11. By this stage, there has only been a few percent shortening, but several changes in microstructure are already evident. Many grain boundaries are becoming wavy; some of the grains show non-uniform colors (indicating lattice bending); and many grains have changed color somewhat. The color changes, here and in subsequent images, reflect increase in sample thickness normal to the screen and lattice rotations with a component of rotation about axes parallel to the screen. There are at least six examples of dilation of grain boundaries, leaving gaps between grains with nothing in them but OCP vapor. Can you find them? (ans)

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12. The green grain at (30,40) now has pronounced non-uniform extinction, with diffuse subgrain boundaries about parallel to the shortening direction. This is a common orientation of subgrain boundaries in pure-sheared OCP.

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13. Much has changed since the last picture, but with care you can still identify many of the grains in the last picture. (Start, e.g., with the black grain at (5,27) in the last picture and work your way out from there). The grain boundaries are now very irregular and most grains contain subgrains. The shortening strain at this stage can be estimated by measuring the distance between the black grain at (15,25) and the green grain at (58,26), and comparing this distance with the distance between the same two grains in image 10. Can you think of any reason why the extinguished grain at (15,25) has remained black throughout the history of deformation to this point? (ans)

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14. End of deformation. The ink spots identify the same material positions as in image 10. Details of the structure along the boundaries of the original grains are impossible to see because the small grains and subgrains in these regions are smaller than the thickness of the sample.

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