New double kink model for deformation in high temperature materials

A new double kink dislocation model is described. It explains 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 activation 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 either with temperature or with reciprocal temperature. The model explains quantitatively the temperature dependence of the critical resolved shear stress (CRSS) on different slip systems for sapphire (α-Al₂O₃) and spinel (MgO·nAl₂O₃, n≥1) in terms of different activation energies for kink diffusion on partial dislocations. The model can be modified to explain the rapid reduction in the CRSS of spinel with increasing n by incorporating enhanced kink nucleation and diffusion due to cation vacancies. This changes the activation energy and the strain-rate term and explains why the CRSS decreases as the inverse of the square of the cation vacancy concentration, as is observed.