Microstructure evolution in ultra-high purity Fe-Cr alloys under low-PKA proton irradiation

Abstract

Fe-Cr based ferritic steels are essential structural materials for nuclear reactor core applications. This study investigates how Cr content and irradiation temperature affect the formation of dislocation loops and the chemical segregation in Fe-Cr binary alloys under proton irradiation with low cascade mixing. Ultra-high purity Fe-Cr alloys containing 8, 10, and 18 wt. % Cr were irradiated by 800 keV protons at a dose rate of 2 × 10⁻⁵ dpa/s up to 2 dpa at 3 μm depth at 250, 350, and 450 °C. Scanning transmission electron microscope (STEM) analysis revealed the formation of “large” and “regular-sized” dislocation loops, which are likely attributed to the direct overlap of displacement cascades and accumulation of interstitials, respectively. The large dislocation loops appear to be more stable at lower irradiation temperatures and with higher Cr concentrations. The size of regular-sized dislocation loops increased, and the number density decreased, with increasing irradiation temperature or decreasing Cr content. The ratio of a〈100〉 to a/2〈111〉 dislocation loops increased at higher irradiation temperatures, while the ratio was independent of Cr content at 250 °C. Clustering and coalescence of a〈100〉 dislocation loops occurred at 350 °C and 450 °C, respectively. STEM coupled with energy dispersive X-ray spectroscopy revealed a transition from heterogeneous Cr segregation around dislocation loops to homogeneous Cr-rich α′ precipitates in the matrix in alloys with Cr content >8 wt.% at 250 °C.