Formation mechanism of Cr precipitates in FeCrAl alloys

FeCrAl alloys are promising materials for fuel cladding in light water reactors; however, the formation of α’ precipitate can lead to embrittlement. A clear understanding of the formation mechanism of α’ precipitates in FeCrAl alloys is important, as these brittle particles significantly impact the material's mechanical properties. The density, size, and concentration gradients of the α’ precipitate have been measured by previous experiments, but the microstructure information of the precipitate is rarely understood. Although the α’ precipitate shares the same type of lattice structure as the matrix, differences in concentration between the precipitates and the matrix result in lattice parameter mismatches, creating stress fields around the precipitates. Atomistic simulations were used in this work to study the energetic stability of the precipitate and associated stress field as a function of sizes and concentration. Furthermore, the impact of pre-existing radiation-induced defects on the stability of the precipitate was investigated. The results suggest that the α’ precipitate experience tensile stress in the Fe-20%Cr-10%Al alloy, whereas it exhibits compressive stress in the Fe-20%Cr alloy. In addition, the enrichment of Cr atoms near vacancies was observed. This work predicts that a higher density of vacancies can lead to an increased concentration of Cr atoms, suggesting that vacancies may serve as preferential sites for Cr atom accumulation. The current findings on the formation mechanism of brittle α’ precipitates are expected to assist the development of FeCrAl cladding materials by providing insights into controlling precipitate formation, which may enhance the material's mechanical properties.