Frank impurity mechanism

What causes dblocations There are a number of mechanisms that can produce dislocations, most of which have to do with internal stresses. Such stresses can be generated from differential expansion as a material is cooled from the melt or by cold-working the material. Frank and Reed proposed a mechanbm whereby a dblocation can be piimed at two pointe, either by impurity atoms or other dislocations. As stress is applied, the dislocation line becomes increasingly bowed, imtil at last it folds back on itself and pinches off to form a dislocation loop, which continues to expand under the applied stress. The remainder of the dislocation loop after it pinches off now repeate the process as seen in Figure 8.7. This mechanism is called a Frank-Reed source. 

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