Designing sustainable high-entropy alloys directly from mixed oxides: A thermodynamic perspective

Hydrogen-based direct reduction (HyDR) of mixed oxide precursors offers an alternative route to produce sustainable alloys, bypassing the sequential extraction, refining, and remelting steps that dominate the carbon footprint of conventional metallurgy and individual metal production. However, the thermodynamic limits and most effective H$_2$/H$_2$O ratio needed for full metallization of multicomponent oxides are not yet established. This is because the formation of metallic and oxide solid solutions alters the chemical activity of both the reduced metallic phases and the unreduced oxides, coupling their equilibria in ways that cannot be deduced from the stability of the pure individual oxides. Herein, we develop a CALPHAD-based thermodynamic framework to establish the compoistion-temperature-H$_2$/H$_2$O window for complete metallization of CoCrFeMnNi-based oxide mixtures. Mn and Cr generally increase oxide stability, while Co, Fe, and Ni decrease it, allowing reduction at lower H$_2$/H$_2$O ratios. The effect of each element is not fixed; it depends on the starting mixed oxide composition and can reverse when the last oxide to reduce changes. Furthermore, we show that the reduction pathway complexity does not correlate with the difficulty of removing the final oxygen: a system with few intermediate transformation steps can still terminate as a highly stable residual oxide, whereas extensive partial reduction may leave the stability of the final residual oxide essentially unchanged. By linking residual-oxide stability to the H$_2$/H$_2$O ratio, the framework identifies the thermodynamic limits and provides guidelines for designing sustainable multicomponent alloys directly from mixed oxide precursors.