Modelling precipitate amorphization in Zircaloys

Abstract

The second phase particles (SPPs) that occur in Zircaloys can undergo amorphization and loss of iron in-reactor, which can have a profound effect on corrosion resistance and irradiation growth. In this paper, a classical model is presented that predicts the interaction between these processes, based on a previously proposed physical mechanism that considers iron release and transport from the SPP as the controlling factor. It is assumed that iron is released into a mobile state by a homogeneous and continuous amorphization and recrystallization process. As iron is released, it diffuses out of the SPP, causing the relative stability of the amorphous phase to increase, until a persistent amorphous layer forms. The model predicts the growth rate of this layer and the rate at which iron is lost from the SPP. It has been used to explore the effect of flux, temperature, and initial SPP iron concentration on the amorphous layer growth and iron loss kinetics. It is predicted that whilst iron loss is correlated with the thickness of the amorphous layer, the relationship is not a simple scaling, and depends on conditions. The model is also able to explain the observed occurrence of an amorphous layer only in an intermediate temperature range as a natural consequence of the competition between the same fundamental mechanisms.