The effect of irradiation temperature on damage structures in proton-irradiated zirconium alloys

A study into the effects of irradiation temperature on the damage structures that form during proton-irradiation has been carried out on two commercial Zr alloys in order to develop a more mechanistic understanding of the effect of niobium on dislocation loop evolution. The two Zr alloys (Zircaloy-2 and Low-Sn ZIRLOTM) were proton irradiated to a damage level of ~2 dpa at 280°C, 350°C and 450°C. Detailed dislocation analysis was carried out using on-axis bright-field scanning transmission electron microscopy combined with spectral imaging and synchrotron x-ray line profile analysis. The analysis revealed a significant difference in the effect of irradiation temperature on loop size between the two alloys. In the case of the Nb-free Zr-alloy (Zircaloy-2), an increase in irradiation temperature results in a marked increase in a-loop diameter, by a factor of ~7.5 from 280 to 450C, and a stark decrease in the dislocation line density. In contrast, the Nb-containing Zr-alloy (Low-Sn ZIRLOTM) showed very little variation of loop size and line density over the same radiation temperature range. The STEM-based spectral imaging revealed irradiation-induced nano-clustering found throughout the matrix in Low-Sn ZIRLOTM, which is not present in the case of Zircaloy-2. Therefore, it is proposed that Nb plays a crucial role in the evolution of dislocation loops in Zr through the formation of irradiation precipitation throughout the matrix.