Tilt boundary migration in NaCl bicrystals

Abstract

Migrations of [100] tilt boundaries in NaCl bicrystals have been investigated as a function of driving force, misorientation angle θ and temperature utilizing a novel method wherein hyperbola-like shapes are generated by capillary forces acting on a boundary originally subtending an acute angle with a free surface. Initially, intrinsic (I) boundary motion, characterized by an activation energy Q I which increases from 1.5 to 2.2 eV as θ decreases from 30 to 5°, is sustained by the driving force. For large θ, Q I denotes the activation energy for self diffusion across the boundary. For small θ. however, Q I nearly coincides with the activation energy for bulk self diffusion of the slower ionic species (Cl −), indicating the occurrence of dislocation climb processes involving self diffusion through the bulk crystal. Moreover, an effective dislocation core radius, within which enhanced diffusion occurs, is estimated to be about 6 Å from the transition between large and small θ behavior. As the driving force decreases, extrinsic (II) boundary motion, characterized by Q II ≈3 eV for large θ, eventually takes over. Steady-state boundary impurity content associated with this motion is estimated to be only a few ppm. Casual observations indicate that surface tension is relatively isotropic and that surface pinning sometimes occurs, resulting in jerky grain boundary motions.