Lecture 18 Grain Boundaries+Dislocations

# Lecture 18 Grain Boundaries+Dislocations - Generation of a...

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Generation of a Σ 5 (100) twist boundary

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Grain Boundaries and Dislocations Note: A nice source for further discussion of some of the details of grain boundary structure is Defects in Crystals, Helmut Föll, University of Kiel http://www.tf.uni-kiel.de/matwis/amat/def_en/ Grain Boundary dislocations In a bulk crystal, we can define a full dislocation as a linear translational defect which preserves the global lattice periodicity in 3D. We can extend this concept to consider a grain boundary dislocation as one which preserves the global lattice periodicity but only in the 2D plane of the boundary. (You can extend this to any other type of boundary.) To illustrate this, take a simple case of a boundary/interface step. Let “a 1 ” be the repeat in material A normal to the boundary, “a 2 ” in material B where the boundary is between A & B. If we have a step the additional shift (or material so the interface can be glued together) is going to be |b|=|a 1 -a 2 |. This is not a dislocation of either bulk. Note that the energy will correspond to this Burger’s vector – elasticity does not care. This is not a dislocation which can readily move the same way one can in the bulk. CSL boundary dislocations Another case where one can have dislocations which cannot exist in the bulk is for a CSL boundary. Consider the drawing on the right where green is one lattice (e.g. below), red the other (e.g. up) and green is where the atoms coincide. Suppose we take any two atoms for the
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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 18 Grain Boundaries+Dislocations - Generation of a...

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