Dislocations Peierls valley

Glide of an edge dislocation occurs when a half-plane of atoms is moved over the atoms below the glide plane. The movement occurs by the nucleation and movement of kinks. Remember that the reason that dislocations are so important in plasticity is because it is easier to move one block of material over another (shear the crystal) one halfplane of atoms at a time. Similarly, it is easier to move a dislocation by moving a kink along it one atom at a time. In fee metals, the Peierls valleys are not deep, so the energy required to form a kink is small and dislocations bend (create kinks) quite easily. 

The high strength of most ceramics is due to the difficulty of moving dislocations through the lattice that is, most ceramics have a high Peierls stress. Dislocations move from one Peierls valley to the next by the nucleation of a kink pair, under the action of applied stress and temperature. The kinks are abrupt and their further motion is controlled by a secondary Peierls barrier. Mitchell, Peralta and Hirth [22] have adapted the standard treatment of Hirth and Lothe [23] to show that the resulting strain-rate is given by ... 

Fig. 10. Deformation microstructures containing perfect dislocations (the confining pressure is 5 GPa). (a) Deformation temperature T = 293 °C (101) foil plane, weak-beam dark field (4.1g, g = 202). The dislocations nucleated at crack edges are of 1/2[1 0 i](l 11) type. These half-loops are elongated along the [3 21] direction (after Rabier and Demenet [62]). (b) In the bulk, the same dislocations tend to be aligned along several Peierls valleys < 112 > /30°, < 12 3 > /4T, and screw orientation (after Rabier et al. [62]). (c) Deformation temperature T = 150 °C. Same Peierls valleys as at room temperature some strong pinning points are indicated by arrows (after Rabier et al. [61]).

Let ns recall that for metallic monocrystals, the creep speed in a permanent regimen is expressed as a power of the applied load function, with an exponent n = 3 - 5, characteristic of a deformation by dislocation mobihty. We will show some of the characteristics of the deformation of iono-covalent sohds. The latter are generally less plastic and the dislocations are aligned in a stable manner in potential valleys because of the existence of high Peierls forces. The propagation and multiphcation of dislocations are therefore not very easy and the density of dislocations is generally low. 

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