Oxygen-Induced Hierarchical Heterogeneities and Enhanced Hardness in RMPEAs

Refractory multiprincipal element alloys (RMPEAs) offer superiority to incumbent high-temperature structural alloys due to high melting points and retained strength at elevated temperatures. Of this class of alloys, those containing Group IV and V elements possess adequate ductility, low density, and the necessary formability. However, these elements have dramatically different interactions with oxygen, which creates uncertainty in predicting oxide evolution and in alloy design for oxidation resistance. We used advanced characterization and Monte Carlo simulations to decipher the complex sub-surface phase evolution during high-temperature oxidation of Group IV-V RMPEAs. We found that a refined hierarchical microstructure of nanoscale suboxides and oxides forms, which leads to a gradient hardness increase up to 23 GPa while preserving the plasticity of the base metal. Criteria for expansion of this alloy design strategy to other composition spaces and interstitial elements will be discussed.