High–temperature sulfidation behavior of refractory TaMoCrTiAl(–3 W) high entropy alloys

Despite the excellent sulfidation resistance of binary refractory metal–aluminum alloys, their rapid oxidation above 900°C remains a critical limitation for high–temperature applications. This study aims to bridge this gap by evaluating refractory high–entropy alloys (RHEAs), which offer the potential to simultaneously exhibit oxidation and sulfidation resistance. The high–temperature sulfidation behavior of TaMoCrTiAl and TaMoCrTiAl–3 W RHEAs was investigated in the temperature range of 800–1000°C at a sulfur vapor pressure of 1000 Pa. Kinetic measurements revealed diffusion–controlled mechanism. The sulfidation rates were found to be exceptionally low (k$_p$ in the range of 10$^{–13}$–10$^{–11}$ g$^2$ cm$^{–4}$ s$^{–1}$), outperforming pure molybdenum and conventional amorphous Al–Mo alloys, as well as the oxidation rates of standard chromia–formers. The superior corrosion resistance is attributed to the formation of complex, protective scales composed of a mixture of oxides (Al$_2$O$_3$, Cr$_2$O$_3$, TiTaO$_4$, TiO$_2$) and sulfides (CrS and refractory metal sulfides). The presence of highly protective oxides and sulfides in the scale provides a highly effective barrier against reagent transport, making these RHEAs promising candidates for structural applications in multi–oxidant, harsh environments.