Growth of voids in metals during diffusion and creep

Abstract

A study has been made of the thermodynamic conditions under which voids may grow from suitable nuclei in a metal by a mechanism of vacancy aggregation under the influence of stress and vacancy supersaturation. The results are applied to the intergranular void formation observed during creep experiments and also void formation during Kirkendall diffusion. In the case of creep it is postulated that nuclei may grow into voids by receiving stress-motivated vacancy currents from nearby grain boundary sources. In the case of Kirkendall diffusion it is first shown that the development of stress in the diffusion zone depends directly upon the existence of a vacancy supersaturation. The roles of vacancy supersaturation and stress in producing voids are then discussed individually. It is concluded that there is sufficient vacancy supersaturation present to produce voids within grains if nuclei of reasonable size are present. The effect of stress in the absence of large stress concentrations is probably to increase the equilibrium vacancy concentrations at sinks within the grains thereby diverting extra vacancies into voids. Void formation at grain boundaries, where the vacancies are maintained near equilibrium, is explained as a result of stress.