Mobile dislocation walls

It should be noted in passing that a [ 1121] twin is nothing but a special case of a KB, where a basal plane dislocation is nucleated every c-lattice parameter [136]. The fundamental difference between a KB and a twin is in the shear angle for the latter, it is crystallographic, but for the former it is not. What determines the angle of a kink boundary is the number of mobile dislocation walls that end up in that boundary. 

To explain many of our observations (see below), it was necessary to invoke the idea of an IKB - a KB that does not dissociate into mobile dislocation walls (MDWs) (Figure 7.13b). Because of its shape, the IKB shrinks when the load is removed and is thus, by definition, fully and spontaneously reversible. An IKB consists of multiple... 

Figure 7.19b) the mobile dislocation walls (Figure 7.8a) generated at high temperature are swept into the kink boundaries, immobilizing them. On subsequent loading (blue cycle in Figure 7.19b), only IKBs are activated such that the loop is fully reversible and reproducible. 

In the Frank and Stroh analysis, it was assumed that once a KB was nucleated, it would immediately extend to a free surface, thus eliminating the mutual attraction and resulting in two parallel, mobile noninteracting dislocations walls (Figure 7.11 f). It is the repetition of this process that results in the generation of new dislocation walls, the coalescence of which forms the kink boundaries (Figure 7.11h). 

More recently Mackwell, Kohlstedt, and Paterson (1985) studied the deformation of single crystals of San Carlos (Arizona) olivine deformed under hydrous conditions at 1,300 C, 300 MPa confining pressure, and 10 s strain-rate and found they were a factor of 1.5-2 weaker than those deformed in an anhydrous environment. TEM observations showed that specimens deformed under dry conditions, in an orientation such that the slip systems (001)[100] and (100)[001] would be activated, were characterized by a microstructure of generally curved dislocations and dislocation loops, but no organization into walls. The dislocation density was 10 -10 cm compared with an initial value of < 10 cm . Most of the dislocations and the loops lie approximately in the (010) plane because they are in contrast for g = 004, they probably have b = [001] dislocations with b = [010] and [100] would be out-of-contrast for this reflection. However, the slip system (010) [001] is not expected to be active. It is not clear, therefore, if these dislocations are actually involved in the deformation. The general geometry of the dislocation microstructure is not inconsistent with some climb mobility in fact, on the basis of the observations of Phakey et al. (1972), climb is certainly expected at 1,300°C. 

Completeness of a crystal the permeability is reduced by the imperfection of the crystal, that is, a defect, dislocation, etc. because it can lower the mobility of domain wall or increase the induced magnetic anisotropy. 

In solid-state physics there are many examples of domain walls which can move Bloch walls in ferro-magnets, dislocations in crystals, etc. Phase-slip cent in charge-density waves can also move. It could also be conjectured that the conjugational defects of Figures 1.13 and 1.14 are also mobile and perhaps move as solitary waves (the step in the alternation parameter... 

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