By J. Corish (auth.), Alan V. Chadwick, Mario Terenzi (eds.)
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The faults are surrounded by partial dislocations. Indeed dislocations may seek to r educe their energies during slip by dissociating into two partial dislocations. These dislocations will then produce a sheet of stacking fault between them the energy of which prevents the dislocations from becoming too separated (Seeger; 1957). g. in hexagonal close-packed metals the defect structure may 32 stabilize by nucleating a partial dislocation which sweeps through the fault and changes the stacking sequence in the layers above it.
In summary, there is now a good understanding of the major features of the point defects in lightly doped materials at moderate to low temperatures. The dominant defects have been identified, the thermodynamic parameters for defect formation and migration are accurately known and the sites occupied by many impurities have been established. There are still a number of points that require further testing by experiment. For example, it is usually assumed that alkali metal cations dissolve only substitutionally although there is evidence for a partitioning between substitutional and interstitial sites in SrCl 2 (Gervais et al, 1976).
25 Jogs on an edge dislocation lie. Figure 12. In this case ~ is perpendicular to the dislocation line, for a screw dis- location it is parallel to the dislocation line and in the more general case of a mixed dislocation they enclose an arbitrary angle. Dislocation lines may end at grain boundaries or at surfaces but never inside a crystal. This implies that dislocations either branch into a number of other dislocations at a node or form closed loops (Read; 1953). The mechanism through which a dislocation greatly increases the ease with which slip occurs in a crystal is most readily understood by considering the edge dislocation EF in Figure lO(a).
Defects in Solids: Modern Techniques by J. Corish (auth.), Alan V. Chadwick, Mario Terenzi (eds.)