Dislocations climb dissociation

Doukhan and Doukhan (1986) suggested that the climb dissociation is due to the precipitation of point defects on the prismatic loops when the specimens are cooled. The equilibrium concentration and the mobility of the point defects are both expected to be very high at 1,400°C, thus favoring deformation by dislocation climb. 

Figure 9.37. Schematic diagrams showing (a) a perfect edge dislocation in a structure consisting of alternating layers A and B, and (b) the climb dissociation of such a dislocation into two partial dislocations. As shown, the dissociation is due to the preferential precipitation of vacancies at A layers, or of interstitials at B layers. Burgers circuits are shown for the perfect dislocation (a) and the two par-tials (b). In BaTiOs, the A and B layers could correspond to BaO and Ti02 layers...

Both types of microstructure found in olivine are indicative of a significant component of dislocation climb during deformation. The dissociated dislocations present in the low-temperature microstructure have not been reproduced in any experiments nor have they been found in other naturally deformed olivines. The climb-dissociation may affect the type... 

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. 

Climb dissociation occurs when the stacking fault does not lie parallel to the glide plane of the partial dislocations. The phenomenon has not been seen in pure fee metals, but it can occur in intermetallics. It is found in both covalent and ionic ceramics. We can make two comments here ... 

Climb dissociation may be more important in a ceramic than in an fee metal because in fee metals the glide plane is also the plane with by far the lowest SFE. A point to remember is that the word dissociation refers to the final configuration it does not tell you that the perfect dislocation ever had a compact (undissociated) core. 

FIGURE 12.15 Climb dissociation of a dislocation in spinel (a) schematic (b) HRTEM image. 

Consider a climb-dissociated sdge dislocation in spinel. Draw a model for the atomistic structure. 

Figure 14.12 shows the climb dissociation of edge dislocations in a spinel tilt boundary. (This is just an extension of Figure 12.15.) In Figure 14.13 we can see a so-called extrinsic dislocation (along [100]) interacting with the... 

FIGURE 14.12 A low-angle tilt GB in spinel showing an array of climb-dissociated edge dislocations, (a) The GB viewed at an angle (b) at higher magnification, the dislocations viewed end-on. 

Figure 9.16 Formation of a faulted dipole in sapphire, (a) Original perfect dislocation dipole (b) Dipole with each partial undergoing conservative climb dissociation (c) Faulted dipole formed by the annihilation of the inner partials.

The inverse BDT for SrTiOs deformed along (100) or (110) is thought to be due to transformation of the dislocation core structure ]121] Gumbsch et al. [121] suggested a possible climb dissociation (more properly a decomposition) via the reaction... 

The Homstra dissociation into collinear half-partials [Eq. (6)] is commonly observed, most often by climb. However, evidence has been found for combinations of glide and climb dissociation on both rational and irrational planes. The 100 fault plane predominates, regardless of stoichiometry, followed by 110, 112, 113, and 111 [26]. Occasionally, pure glide dissociation is observed, for example, of screw dislocations on the 110 plane of stoichiometric crystals [26]. Although faults in spinel exhibit a dear preference for the 100 plane and other low-index planes, they are also observed to be wavy and to lie on irrational planes (see Figure 9.20). 

Hydration-induced climb dissociation of dislocations in naturally deformed mantle olivine has been reported by Drury [171]. Here, the dissociation is pro-... 

Fig. 3.71 Dissociated dislocation node in nonstoichiometric MgO-3.5 AI2O3 spinel. Node of three 1/2 (110) dislocations has dissociated by climb to produce two partial nodes each consisting of three 1/2 (110) dislocations [38]. With kind permission of John Wiley and Sons...

Thus, the main loop (A, D) is a prismatic edge-dislocation loop (the Burgers vector is normal to the plane of the loop) and can expand in its plane only by climb. Segments of the loop have dissociated into pairs of partial dislocations (B, C and E, F), presumably by the reaction... 

Mitchell states that dislocation dissociation is rare in oxides. The only two cases in which it has been clearly observed (AI2O3 and MgAl204,) involve dissociation by climb, rather than glide, in situations where the point defects are probably helping the dissociation process. Therefore, it is of interest to study such cases as additional examples in which climb is involved, as one of the mechanisms in the recovery process. Figure 3.71 illustrates dislocation dissociation by climb in MgO-3.5 AI2O3 spinel. 

By using the serial sectioning technique, the tracer diffusion of l Au into single crystals was studied. Some effect of the dislocation density was detected. This was explained in terms of a model in which vacancy generation occurred at climbing grown-in dislocations. The values of the 2 effective diffusion coefficients for the dissociative mechanism were determined from the present, and literature, results. One coefficient took a value of about 3 x lO cm /s at 900 to llOOC, while the other could be described by ... 

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