A ballistic mixing model for the amorphization of precipitates in Zircaloy under neutron irradiation

A model is proposed for the crystalline-to-amorphous transformation (amorphization) of Zr(Cr, Fe)₂ precipitates in Zircaloy under neutron irradiation. The model is based on the observations that a "duplex" structure forms upon neutron irradiation: an amorphous layer starts at the precipitate-matrix interface that moves into the precipitate until the precipitate is completely amorphous. A depletion of Fe from the amorphous layer is observed, and the thickness of the amorphous layer is directly proportional to fluence. This last feature cannot be accounted for by models in which the rate controlling step for amorphization is diffusion-controlled. The rate-controlling step for amorphization is a departure from stoichiometry induced by ballistic mixing across the crystallineamorphous interface. This explains the fact that amorphization starts at the interface and gives the correct linear dependence of amorphous layer thickness with fluence. It is shown that the amorphization front velocity observed experimentally can be reproduced with the present model.