Dissociation of Dislocations and Plasticity of Ionic Crystals

The dissociation of dislocations in the NaCl structure is discussed in terms of Fontaine's model. There are no well-defined partials in the dissociation of the Burgers vector a/2 [I101 on the (1x0) slip plane. The stacking fault on this plane is connected with a dilatation normal to (170). This leads to an unique effect of hydrostatic pressure on the frequency of cross slip. Cross slip in stage I11 occurs via the { 11 1 } and { 001 } planes. On the { 001 } plane dislocations are not dissociated while dissociation on { 111 ) probably is intermediate. On { 001 ) and { 111 ) planes edge dislocations interact strongly with multivalent impurities while screw dislocations move relatively freely. This strong electrostatic interaction is thought to be responsible for the choice of slip plane and for the unusually strong latent hardening observed in the NaCl structure. The dissociation of dislocations on the other hand appears to influence dislocation climb and creep determined by climb. 1. The width of dissociation. For a long time dislocations of Burgers vector b =a12 [1 101 in the NaCl structure were thought not to be dissociated. Fontaine in his thesis [I] has, however, shown that the electrostatic interaction between the ions forming this structure in fact favors the formation of stacking faults : Figure 1 shows that by a partical shear of a/4 [110], ions of opposite sign approach each other across a (110) slip plane. The stacking fault energy yo associated with this shear would be negative if the exchange energy between the ions did not oppose their close approach. The latter interaction leads to a positive yo and to a dilatation cow 0.3 of the lattice normal to the (170) stacking fault in the alkali halides, described by the reaction 0 e e e FIG. 1. Dissociated edge dislocation in the NaCl structure according to Fontaine. A closer look shows, however, that the a14 [I101 shear actually corresponds to metastable atom positions and that there is no energetically stable partial shear at all on the (170) plane. Instead Fontaine describes the X stacking fault energy due to a[I101 shear by a J 2 Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1974718