Deformation by a kink mechanism in high temperature materials

A new double kink dislocation model has been developed to explain the temperature dependence of the yield stress in materials such as oxides and intermetallics that require high temperatures for plastic flow. The major variation in the free energy for the formation of a double kink nucleus with stress is the kink-kink interaction energy. However, there is also a stress dependence of the pre-exponential factor in the strain rate constitutive equation arising from kink diffusion. Numerical solution of the resulting equations shows that there are temperature regimes where the stress varies logarithmically with temperature. The model explains quantitatively the temperature dependence of the critical resolved shear stress on different slip systems for sapphire and spinel in terms of different activation energies for kink diffusion. The model can be modified to explain compositional softening in spinel by incorporating enhanced kink nucleation at cation vacancies. This changes both the pre-exponential term and the activation energy and explains why the critical resolved shear stress (CRSS) decreases as the inverse of the square of the vacancy concentration, as is observed.