Mutual FCC-BCC phase transition driving surface homogenization in a novel core-shell medium-entropy alloy after exposure to supercritical water

A novel medium-entropy alloy (Al 7.7 Cr 25 Fe 25 Ni 42.3 , wt%) was developed for supercritical water reactor (SCWR) applications. Featuring a stable core-shell structure with an FCC outer shell, B2 inner shell, and BCC core, the alloy exhibits excellent mechanical properties at room temperature and 600 °C. After 1000 h in 600°C supercritical water, the alloy formed a dense, uniform oxide scale composed of Al 2 O 3 , Cr 2 O 3 , Fe 3-x Cr x O 4 , and NiFe 2 O 4 . Beyond the protective effect of the oxide scale , the alloy's outstanding corrosion resistance is largely attributed to mutual phase transformations that promote a homogenized and fine-grained microstructure, ensuring uniform oxide growth . This unique combination makes the alloy a promising SCWR candidate. • A novel Al 7.7 Cr 25 Fe 25 Ni 42.3 alloy was proposed for SCWR applications. • The alloy exhibits excellent corrosion performance and mechanical properties. • The oxidation scale in steady-state consists of Al 2 O 3 , Cr 2 O 3 , Fe 3-x Cr x O 4 , and NiFe 2 O 4 . • The mutual BCC-FCC transition inducing near-surface homogenization is revealed.