Dislocation Dissociations

Our treatment in this section will cover three primary thrusts in modeling dislocation core phenomena. Our first calculations will consider the simplest elastic models of dislocation dissociation. This will be followed by our first foray into mixed atomistic/continuum models in the form of the Peierls-Nabarro cohesive zone model. This hybrid model divides the dislocation into two parts, one of which is treated using linear elasticity and the other of which is considered in light of a continuum model of the atomic-level forces acting across the slip plane of the dislocation. Our analysis will finish with an assessment of the gains made in direct atomistic simulation of dislocation cores. 

In this section, our aim is to compute the core geometry associated with a dissociated dislocation in an fee material. Our model will be founded upon a 

The minimization of eqn (8.71) is effected by differentiating the total interaction energy with respect to d and leads to 

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Consider the dislocation dissociation reaction (a/2)[l 10] -> (a/6)[211] + (fl/6)[121](a/2)[ll0] + (a/2)[101] ->-in an fee crystal. Assume that the energy/length of a dislocation is given by El = Gb2 and neglect any dependence of the energy on the edge versus the screw nature of the dislocation. Assume that this reaction occurs and the partial dislocations move very far apart. 

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... 

It has been suggested that edge dislocations in a-alumina should dissociate into two or four partial dislocations for basal slip. Sketch the ionic structure of a-alumina and discuss the displacement paths that could be associated with these two dislocation dissociation mechanisms. 

Dislocation Dissociation in BeO and Other Crystals with the Wurtzite Structure... 

Dislocation-Dissociation in Oxides with the Fluorite Structure... 

Dislocation-dissociation has never been reported in any of the oxides with the fluorite structure, although this is not surprising as the 1/2(110) Burgers vector joins... 

Dislocation-dissociation in quartz was first observed by McLaren ef al. [32] in a natural dry quartz deformed at 500 °C, and has been studied most recently by Cordier and Doukhan [33] in a synthetic crystal containing 100 at. ppm [H]/[Si]. By using a crystal oriented to have high Schmid factors for (0001)1/3 (1120)basal slip, 1120 [0001] prismplaneslip, and 1010 1/3 (1213) pyramidal slip, and deformed under a hydrostatic pressure of 1.0-1.1 GPa, Cordier and Doukhan found a flow stress at 500 °C of almost 3 GPa, which decreased to 2 GPa at 900 °C. These high flow stresses correspond to a significant fraction of the shear modulus and reflect the fact that the quartz is relatively dry. Slip at 500 °C was heterogeneous (in slip bands). 

Dislocation-dissociation has been an important issue in studies of basal deformation in sapphire since the seminal studies of Kronberg [7], who suggested that basal dislocations would dissociate into half-partials via the reaction ... 

Dislocation-dissociation has also been an issue in olivine since the early studies of Poirier et al. [161,172]. Evidence for such dissociation, usingTEM, was first reported by Van der Sande and Kohlstedt [173], and more recently by Smith et al. [174] and Drury [34]. The dissociation [Eqs (18) and (19)] suggested by Poirier assumes that the oxygen ions can be considered as hard spheres ... 

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. 

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