Dislocations walls

In fig.4 (lower panel) it is schematically presented the structure of the SC vortex in the SDW/CDW + SC state. Since arising of the CDW results in the lattice modulation so that wave of dislocation walls is formed (fig.4 (middle panel)). As known, such dislocation walls are effective centers for pinning of SC vortices. Note that in such a structure every fifth wall is equivalent to first one ( cn+4 - c ). In this a model a vortex core has AF SDW structure which is also outside a core too. Because of equivalence of c and c +4 dislocation walls vortex core becames to be two part in form fluctuating in space (cf. with [14]). 

A typical application is the detection of sub-boundaries in Al-Zn solid solutions with enough Zn to allow precipitation-hardening at room temperature after quenching. During such aging, Zn atoms concentrate at the sub-boundary dislocation walls to a sufficient extent to cause a color variation of the... 

Mackwell et al. (1985) found that when specimens that had been deformed under anhydrous conditions were subsequently further deformed under wet conditions, there was a significant change in microstructure. TEM observations revealed enhanced formation of dislocation walls, despite the reduced stress levels. This observation was interpreted as due to enhanced dislocation climb under wet conditions. However, the two walls illustrated by Mackwell et al. (1985) could be interpreted as healed or partly healed fractures. One wall consists of a very irregular network of dislocations with many bubbles, particularly at dislocation intersections. 

In the preheated specimens, TEM reveals many well-organized dislocation walls parallel to (100), with fewer parallel to (010) and (001), in which the dislocations are spaced at about 50 nm. Within the cells defined by these walls, the free dislocation density is about 10 cm . Small bubbles (< 300 nm in diameter) are observed in some of the walls, suggesting that the walls may be healed cracks. 

TEM observations reveal the phase structure of NiCoCrAlY alloy more clearly. Fig.6a is the morphology of y(Ni) phase. It could be found that a lot of dislocations exist in y(Ni) phase, which twine each other and form net structure inside the grain, and squeeze to form dislocation walls at the houndary(shown by the arrow). 

Figure 2.7(a) shows the SEM image of a low-doped 6H-SiC sample anodized for 5 min. A characteristic etch pit pattern appears in the anodized sample, since in this case the pore nucleation occurs mostly at dislocations and dislocation walls. Similar to the other two types of substrates, the pore openings on the surface could not be observed neither under the optical microscope nor under the SEM without the skin layer removal. The SEM image of the cross-section of the cleaved sample demonstrates that pores increase in diameter with depth and might reach a size of several micrometers [Figure 2.7(b)]. Moreover, the pore density for the low-doped samples is relatively low compared with those in the high and... 

Fig. 28. Repeated wedge microtwins. In the classical model each wedge microtwin is equivalent to a superdislocation, so that the repeated wedge microtwins act as a dislocation wall of edge dislocations, creating a rotation between C and c regions.

Figure 7.9 (a) Schematic representation ofthe 0001 projection showing the dislocation line, and projections of observed and possible Burgers vectors. Possible arrangements of mixed dislocations and by in a dislocation wall (b) Screw components are all in the same... 

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. 

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). 

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. 

Fig. 4.7 a Dislocation wall dislocations are parallel and positioned in different basal planes one under another, b Same area as white square in (a), but at higher magnification (imaged in g (3360)). c Weak beam image of the same area as in (b), but tilted and imaged in g of (3300). Dislocations that become invisible in (c) are perfect edge dislocation those that remain visible are mixed dislocations [14]. With kind permission of Springer and Professor Barsoum... 

The self-diffusion of O in undoped alumina monocrystals was investigated by using the gas-solid isotope exchange technique. After diffusion annealing, the profiles of O were determined by means of secondary ion mass spectrometry. This revealed 2 parts, in which the behavior close to the initial surface was attributed to bulk self-diffusion while the diffusion tails were attributed to migration in dislocation walls. At 1500 to 1720C, the bulk self-diffusivity of O could be described by ... 

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