Glide dissociation

Motion of dislocations in C leads to an important plasticity of C even at low temperature. Notice that if at low temperature and high stress, the plasticity can be analysed in terms of glide dissociation of the dislocations, it is analysed at... 

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 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). 

Primary glide occurs on the (111) planes. Shear of a carbon layer over a metal layer (or vice versa), when the core of a dislocation moves, severely disturbs the symmetry, thereby locally dissociating the compound. Therefore, the barrier to dislocation motion is the heat of formation, AHf (Gilman, 1970). The shear work is the applied shear stress, x times the molecular (bond) volume, V or xV. Thus, the shear stress is proportional to AHf/V, and the hardness number is expected to be proportional to the shear stress. Figure 10.2 shows that this is indeed the case for the six prototype carbides. 

At least a dozen slip systems have been identified by TEM in experimentally and naturally deformed feldspars (see Gandais and Willaime 1984). In many cases, the dislocations are dissociated, though the separation of the partial dislocations is usually small (<50 nm). The dissociation of dislocations of b = [100] gliding in (010) in experimentally deformed sanidine was first observed by Kovacs and Gandais (1980), who suggested the following reactions ... 

More recently, Montardi and Mainprice (1987) made a detailed TEM study of dislocations in naturally deformed calcic plagioclases (An6 -7o)-As in the specimens studied by Olsen and Kohlstedt (1984), the microstructure was dominated by the slip system (010) [001]. The [001] dislocations dissociated according to the reaction just given, the separation being about SO nm. The pair of gliding partial dislocations left behind a fault characterized by fringes of low contrast, as previously discussed. Image... 

The pyroxenes are the most abundant minerals, after olivine, in perido-tites, which are the dominant constituents of the upper mantle. It is not surprising, therefore, that there has been considerable interest in the mechanical properties of the pyroxenes, and a review has recently been given by Doukhan et al. (1986). The orthorhombic pyroxenes deform by slip and by a shear transformation that produces monoclinic lamellae (one or a few unit cells thick) parallel to (100). Coe and Kirby (1975) and McLaren and Etheridge (1976) have shown that the shear transformation is achieved by the glide of partial dislocations of b = 0.83[001] in (100), which leave partial dislocations of b = 0.17[001] terminating the shear lamellae. The dominant slip system is (100) [001]. Recent TEM observations by van Duysen, Doukhan, and Doukhan (1985) suggest that the dislocations associated with this slip system may be dissociated into four partials and that the slip system (100) [010] may also be activated. These observations are discussed in Section 9.9.1. 

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. 

This reaction can occur by glide in the basal plane only for screw dislocations, and is thought to be the mechanism for the formation of a dislocation network in crystals undergoing deformation by prism plane slip [97]. Alternatively, the dislocation can lower its energy by dissociating into three collinear partials according to the reaction (see also Table 9.3) ... 

This dissociation has been observed to occur by climb for high-temperature deformation, but can also occur by glide. 

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