Lecture 19 Twins - Twins See Chapter 10 of Kelly and Groves...

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Twins See Chapter 10 of Kelly and Groves for some further information about twinning in metals. The normal way we consider a twin is as a reflection plane, although this is really only a first approximation and in general there are many possibilities depending upon the crystal system. A simple example for silicon is shown below. Twins also commonly occur in small precipitates or particles, e.g. the picture below (courtesy Staf van Tenderloo, Antwerp)
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There are three main ways that a twin can occur: a) Mistakes during growth. Consider a fcc material growing along a [111] direction. The stacking sequence is …ABCABCAC B An atom wanders in and sits above the outer B (red) layer. The lowest energy location is for it to sit on an “A” site, but the energy for it to sit on a “C” site is not much higher, so there will be a probability proportional to exp(-dE/kT) that it will sit there. Along comes another atom and joins it. It wants to sit in a close-packed location connected to the first atom, which means that it has to also sit on a C site. This way we nucleate a C layer ABCABCAC B C The lowest energy site for the next layer is “A”, so we would (if we don’t have another
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This note was uploaded on 04/13/2010 for the course MAT SCI 404 taught by Professor Matsci during the Winter '10 term at Northwestern.

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Lecture 19 Twins - Twins See Chapter 10 of Kelly and Groves...

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