Metastable phase stability in the ternary Zr–Fe–Cr system

As the existing literature reports, when Zircaloy-2 and Zircaloy-4 are irradiated at intermediate temperatures with high-energy neutrons, a crystalline to amorphous phase transition occurs, coincidental with a preferential depletion of Fe at the periphery of the Zr(Fe,Cr)2 intermetallic particles. In order to describe these phenomena, relative phase stability criteria combined with a balance between the rate of irradiation damage and irradiation enhanced diffusion leading back to a modified or new state should be considered. Aiming at this suggested study, in this work we calculate the metastable free-energy diagram of the ternary Zr–Fe–Cr system. The free energies of formation of the metastable amorphous and solid solution phases and of the stable Zr(Fe,Cr)2 compound have been calculated using Miedemaʹs model. By solving equations of analytical geometry, regions of metastable equilibria and their tie-lines are defined, linking the hcp-Zr-rich solid solution with the Zr–Fe–Cr amorphous phase, and the latter with the irradiation-energized compound. Composition profiles, reported in the literature, were measured on partially and completely amorphized particles, and also in the case when the latter experienced microcrystallization with solute reversion, that is, with the Fe concentration increased during post-irradiation heat treatments. Those composition profiles define a path along a calculated tie-line, deviating near the crystalline core/amorphous crown interface toward low Fe concentrations. The analysis in the present work suggests that the microstructure in irradiated Zircaloy tends to reach a new radiation-induced metastable co-existence of phases.