High-temperature oxidation behavior of Ti-rich TaMoCrTiAl refractory high-entropy alloys (RHEAs) with room temperature plasticity

Refractory high-entropy alloys (RHEAs) are emerging as promising high-temperature structural materials due to their high melting points and exceptional high-temperature strength. However, their development and deployment face specific challenges, particularly low-temperature brittleness, high density and poor oxidation resistance. Our recent studies highlighted that equimolar TaMoCrTiAl RHEAs exhibit notable high-temperature oxidation resistance, primarily due to the formation of rutile (Cr,Ta,Ti)O2 oxide layer. The (Cr,Ta,Ti)O2 layer effectively suppresses the outward diffusion of cations, and its growth rate falls between those of typical chromia and alumina scales. However, this equimolar alloy which comprise brittle B2 phase matrix and Laves phase (TaCr2), like many Al-containing RHEAs, lacks ductility.

To retain the good oxidation resistance while mitigating low-temperature brittleness, new compositional variants with high Ti content, designed to achieve a solely disordered A2 structure and relatively low density, are predicted using thermodynamic modeling. Three alloys with varying Ti and Mo concentrations (i.e. Ta12Mo25Cr10Ti48Al5, Ta15Mo20Cr15Ti44Al6, Ta15Mo10Cr15Ti54Al6, in at.%) have been identified and produced, all demonstrating over 6% plastic deformation during room-temperature compression testing. ... mehrTheir high-temperature oxidation behaviors at 800-1100°C in air are investigated by thermogravimetry. The oxidation performance exhibits strong temperature and composition dependence, resulting from the synergistic effects of kinetics aspects, molybdenum oxide volatilization, and internal TiN precipitation. All alloys form a multilayered oxide scale after oxidation, with outer layer consisting of Ti, Cr, Al, and inner complex oxide layer primarily comprising Cr, Ta, Ti, and Mo (besides O). Volatilization of Mo oxide becomes noticeable at temperatures higher than 900°C, occurring more localized the lower Mo content. The alloy with the highest Ti (54%) exhibits significantly internal TiN precipitation, which compromises scale adherence and results in scale spalling at 1100°C. Among the alloys, Ta15Mo20Cr15Ti44Al6 demonstrates the best oxidation performance, with kinetics comparable to the equimolar TaMoCrTiAl. In addition, the mass gains of these Ti-rich RHEAs are generally several times (at 800° and 900°C) to one magnitude lower (above 900°C) than common TiAl-based alloys (like 4822 alloys), highlighting their potential as replacement for traditional titanium alloys. The mechanisms of oxide scale growth and failure are discussed based on the thermogravimetric results along with thermodynamic calculations and detailed oxide scale characterization.