Modelling amorphization and dissolution of Zr(Fe,Cr)2 secondary phase precipitates in Zircaloys

Second phase Zr(Fe,Cr)2 precipitates (SPPs) in Zircaloys can undergo a number of structural changes in-reactor including amorphization and dissolution. These changes have a direct effect on the mechanical and corrosion properties of the alloy, so understanding them is critical to improving performance. This paper describes a classical model that predicts SPP evolution over the whole range of observed behaviour, from complete amorphization at low temperature through formation of an amorphous rim, iron loss, and dissolution at intermediate temperatures, and a return to stability at high temperatures. A single set of assumptions is used, with the different regimes of behaviour arising from a natural competition between mechanisms. The model suggests that there is significant overlap in regimes, and for the same temperatures formation of an amorphous rim and SPP dissolution can occur simultaneously. The most interesting behaviour is seen at intermediate temperatures (520–620 K), where there is a transition from irradiation to thermally dominated processes. The model qualitatively captures experimental observations, but for more accurate quantitative predictions a number of unknown parameters require further study. These include the effect of irradiation on the solubility and diffusion rates of alloying elements.