Tuning Oxidation and Mechanical Behavior of Refractory High-Entropy Alloys via Minor Alloying Elements and Surface Segregation

Refractory high-entropy alloys (RHEAs) are an intriguing class of materials which can possess excellent strength at elevated temperatures, but often at the expense of oxidation resistance and ductility. Herein, we leverage a threefold approach of data-driven modeling, first-principles simulations, and experimental data to study the impact of minor alloying elements in CrMoNbTaV and HfNbTaVW, two RHEAs which possess excellent strength and ductility synergy, but lack sufficient oxidation resistance. An atomistic representation of the RHEA structure is obtained via calculation of the Warren-Cowley short-range order parameters. CrMoNbTaV with a representative coinage metal (Ag), platinum group metal (Pt), and metalloid (Si) are selected to study their elemental surface segregation tendencies and oxygen adsorption properties. The results reveal both strong surface segregation and a substantial impact on oxygen adsorption for each of the minor alloying elements, which is deemed to be beneficial for improving oxidation resistance. The trends in the modified RHEA's oxygen adsorption properties are attributed to the shift in the local atomic density of states at the RHEA surface. Finally, we discover that at atomic concentrations of 5% and above, minor alloying elements can have significant impacts on both the strength and ductility, and clear trends in the directionality of the observed changes in strength and ductility are observed based on the type of minor alloying element present.