The stability of homogeneous microstructure development under cascade-damage conditions

The development of microstructure, under cascade-damage conditions, in regions far away from any major sink is considered. Within the mean-field theory, a homogeneous distribution of point defects and their clusters is a pre-imposed artificial constraint on the kinetic system. The resulting excessive recombination of the vacancies and interstitials at a high density of accumulated point-defect clusters dictates a low rate of void growth. Considerations beyond the mean-field theory, by taking into account the concentration fluctuations of both the point defects and their clusters, relax the restriction of the homogeneous distribution. In this paper, we consider a system without pre-existing sinks, except the void nuclei, in which vacancies, interstitials and their clusters are continuously produced. Taking into account the mobility of small clusters and the stochastic fluctuations of the point-defect fluxes, a kinetic theory is formulated from first principles. It is rigorously shown that through the stochastic fluctuations, and the positive-feedback action of the mobility of the small clusters on the interstitial concentration, the homogeneous interstitial distribution is unstable at temperatures above stage V, leading to the formation of a spatially heterogeneous microstructure in pure metals at low irradiation doses. The characteristics of the microstructure evolution and void swelling, predicted from the theory, are found to be in good agreement with the experimental results. Received: 17 March 2000 / Accepted: 17 October 2000 / Published online: 25 July 2001