The effect of proton irradiation dose rate on the evolution of microstructure in Zr alloys: A synchrotron microbeam X-ray, TEM, and APT study

Protons are increasingly used as a surrogate for neutrons to study radiation damage of engineering alloys used in the core of a nuclear reactor, enabling high fluences in comparatively short times. However, the accelerated damage rate of protons is usually compensated by an increased irradiation temperature to assist diffusion. To better understand dose rate effects on microstructure evolution during radiation damage, recrystallized Low-Sn ZIRLO and Zircaloy-2 were proton-irradiated to 0.15 dpa at 320 degrees C using nominal dose rates of 1.3, 2.5, and 5.2 x 10-5 dpa/s. Depth profiling using microbeam synchrotron XRD was conducted across the 30 mu m deep irradiated regions for line profile analysis, enabling dislocation line density determination. We found no significant difference in dislocation density among the different dose rates for Zircaloy-2 while Low-Sn ZIRLO displayed dose rate sensitive microstructural evolution. However, Low-Sn ZIRLO exhibited a significantly lower overall dislocation density compared to Zircaloy-2 samples at all dose rates. (S)TEM analysis of the samples showed clear (a) loop alignment in Zircaloy-2, while this was less pronounced in Low-Sn ZIRLO. APT analysis conducted on Low-Sn ZIRLO specimens showed the early onset of irradiation induced nanoclusters of Nb, where the clusters were observed to be comparatively smaller in the sample exposed to high dose rate irradiation. Overall, the integration of different techniques has provided a more comprehensive understanding of the early-stage damage evolution under differing damage accumulation rates.