The mechanism of cavitation in magnesium during creep

Abstract

During the creep of magnesium, cavities form on some grain boundaries, limiting the ductility of the material and causing its density to decrease. It is shown that cavitation is eliminated if a hydrostatic pressure is superimposed which is equal to the creep stress. If the hydrostatic pressure is applied after cavities are already present in the material, continuing creep results in negligible density change and existing cavities do not continue to grow. When hydrostatic pressure is applied throughout the early stages of creep and is then removed, the subsequent cavitation develops at a rate which is unaffected by the prior deformation. The fractional change in density during creep is approximately proportional to the duration of creep to the power of 2·5 increasing to about 4 immediately prior to fracture. This rate of change of density is greater than can be accounted for by the growth of a constant number of cavities by vacancy condensation and suggests a progressive creation of nuclei. The number of cavities is observed to increase with strain and the change in density during creep is interpreted in terms of a continuous formation of cavities which grow exclusively by condensation of vacancies which diffuse from grain boundaries approximately perpendicular to the applied stress.