Abstract
Transmission electron microscopy of single-cryatal non-stoichiometric spinel (MgO. 3–5 A12O3) deformed in compression at 1500°C have revealed dislocation networks in which the ½〈110〉 dislocations are extended by climb into ¼〈110〉 collinear partials. Dislocation nodes of two types are found, both of the form ½[110] + ½[101] + ½[110] = 0. One configuration (type I) forms by climb dissociation and contains two partial modes of the form ¼[110] + ¼[101] + ¼[101] = 0, while the second (type II) forms initially by glide dissociation and contains four nodes of this form. The fault planes of the dislocations have been determined by trace analysis. In the deformed sample only low-index fault planes were found, with {100} apparently being preferred. Annealing the deformed sample (also at 1500°C) resulted in fault planes on {112} and {113} in addition to {100}, but the faults are frequently curved and twisted. The {100} faults conserve stoichiometry but many faults are present which do not and need to be compensated for charge. These latter faults are thought to be electrostatically neutralized by the segregation of charge-compensating point defects during climb-controlled recovery of the dislocation network, the point defects originating from non-conservative deformation and from the structural requirements of non-stoichiometry. The width of the dissociated dislocations suggests an equilibrium stacking fault energy of ∼ 30 mJ/m2, although such widely separated partials can only form when climb is occurring.
