Ceramic dislocation dissociations

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 12.14 Dislocations in ceramics with low SFE. (a-c) Dissociation in graphite (d and e) dissociation... 

Figure 17.4b Veyssifere, P. and CBC. See also Veyssi re, P. and Carter, C.B. (1988) Dissociation of dislocations in MgAl204 spinel deformed at low temperatures, Phil. Mag. Lett. 57, 211. http //www.tandf.co.uk/journals Figure 17.10 Data from Kingery, W.D., Bowen, H.K., and Uhlmaim, D.R. (1976) Introduction to Ceramics, 2° Ed., Wiley, New York p. 740.

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... 

Wu and Wang [55] analyzed the dissociation of perfect dislocations into partials in BaTiOa by TEM. Figure 3.68 illustrates the results for this hexagonally-structured ceramic, in which the basal plane is commonly involved in the deformation. 

Sillimanite and mullite are sUicates with Si-Al tetrahedral chains. They occur in high-temperature metamorphic rocks and muUite is an important ceramic material. TEM analyses identify [001] as Burgers vector in sillimanite [350,351]. Dislocations are generally of screw type in the [0 01] direction [Fig. 14(b)] and sometimes are dissociated [352]. Dislocation-assisted high-temperature deformation has been documented in mullite [353]. The influence of dislocations and strain on the aluminosilicate phase transformations were investigated for kyanite [354]. 

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