Small-angle neutron scattering study of creep cavity nucleation and growth in sintered alumina

The early stages of creep cavitation in sintered alumina are characterized using small-angle neutron scattering (SANS). It is found that the initial cavity density is of the order of 10¹¹ cm^–3, and that the average initial pore is approximately 60 nm in radius. The incubation time for nucleating additional pores during subsequent creep is extremely short, in agreement with the theory based on the precipitation of grain-boundary diffusing vacancies. Pore density at constant stress and temperature is a linearly increasing function of time, as predicted by classical nucleation theory. However, a local stress of 10^–2E is required to achieve the measured nucleation rate. Cavities are observed to lie primarily on two-grain junctions in linear arrays, with an average cavity radius of approximately 60 nm. It is hypothesized that the cavities nucleate at grain boundary ledges which provide the necessary local stress concentrations. Calculation of the individual cavity growth rate yields a zero or near zero value. This suggests a rapid transient growth period following nucleation which quickly decreases to a negligible growth rate.