Star-shaped unstable growth of grain-boundary cavities

Abstract

Circular cavities growing by vacancy agglomeration during creep are shown to become unstable when they reach a critical size. The size depends on the sum of the applied stress and internal gas pressure in the cavity. The instability is in the form of regular deviations from a circular geometry which lead to star-shaped and finger-like growth. The ratio of surface-diffusion to grain-boundary-diffusion coefficients is shown to substantially affect the conditions for instability. Instabilities are unlikely to be observed during typical cavity growth conditions in metals if this ratio of diffusion coefficients is greater than 100.