The role of molybdenum in the passivating oxidation of CoFeNiMo high entropy alloys

In recent years, with the increasing requirement for coating materials, researchers have shown growing interest in the high-temperature (HT) oxidation performance of high entropy alloys (HEAs). To understand the contribution of alloying element to the passivation and dissolution characteristics of HEAs is crucial for comprehending the synergistic effects of these elements. As a common refractory metal employed in HT environments, molybdenum (Mo) has faced controversy due to issues such as the formation of secondary phases and brittle oxide. To delve deeper into the mechanistic role and associated impacts of Mo in passivating oxidation, we conducted a comprehensive analysis of Mo's characteristics at electronic, atomic and macroscopic thermodynamic levels, which reveals that Mo's application scenario in passivating oxidation differs from the acknowledged ability of Cr to form stable protective oxide. This analysis was based on Density Functional Theory, Monte Carlo (MC) and Local Self-consistent Multiple Scattering (LSMS) methods. Although there is significant local lattice distortion induced by Mo and lattice expansion induced by oxygen (O) atom in bulk structures, thereby reducing the mechanical properties of Mo-containing HEAs, it exhibits stronger ductility in grain boundary (GB) regions compared to Cr-containing HEAs. From this, we propose design principles for enhancing the passivating oxidation quality of Mo-containing transition HEAs, encompassing factors such as chemical affinity, atomic size and microstructure. [Display omitted] •Mo stabilizes the bulk structure in the equilibrium state by occupying sublattice sites and exhibiting negative atomic stress.•Mo's larger atomic size and bonding anisotropy due to unique t2g/eg ratio contribute to greater local lattice distortion.•Mo's stronger chemical affinity for oxygen is attributed to d orbital character.•Grain boundaries increase Mo concentration due to large excess volume and facilitate energetically stable oxygen occupation.