Particular dislocation-dissociations

As we saw in Section 12.1, dislocations would always like to dissociate since this reduces the strain energy. Whether a particular dislocation will dissociate or not thus depends on the magnitude of the energy associated with the stacking fault. If the dislocation core spreads on the glide plane, it is glide dissociation otherwise it is at least partly climb dissociation. 

The central role of imperfections in mechanistic interpretations of decompositions of solids needs emphasizing. Apart from melting (which requires redistribution of all crystal-bonding forces, by a mechanism which has not yet been fully established) the decompositions of most solids involve the participation of atypical lattice constituents, structural distortions and/or surfaces. Such participants have, in particular instances, been identified with some certainty (e.g. excitons are important in the decompositions of some azides, dislocations are sites of nucleation in dissociations of a number of hydrates and carbonates). However, the... 

As pointed out in Section 9.1, Kronberg [7] and Hornstra [8] produced seminal reports on the dissociation of dislocations in ceramics, particularly sapphire and spinel. Kronberg suggested that basal dislocations in sapphire should dissociate according to... 

To summarize this brief history of dislocations in silicon, it appears that some points are well established. In particular, at high and medium temperatures, which are the best documented domains, dislocations lie in the glide set where they are dissociated both at rest and when they move. They have to overcome high Peierls potentials of the first and second kinds. However, several questions remain a matter of debate what is the mean free path of kinks What is the density of localized obstacles along the dislocation lines What is the role of point defects on dislocation mobility ... 

Since it appears that two types of dislocations are nucleated in two very different temperature and stress regimes, it is of interest to derive the transition stress at which the nature of the nucleated dislocations change from dissociated to perfect. For that purpose, it is necessary to compare deformation tests performed in very different experimental conditions. In particular, the deformation conditions that produce deformation microstructures containing undissociated dislocations are such... 

Spatially resolved photoluminescence could help understanding the shuffle-glide transition since it was shown that perfect dislocations have different signatures than dissociated ghde dislocations. It would be interesting to follow locally the change in signature of dislocations and, in particular, to check whether some transient core structures or premonitory events are associated to the transition. 

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