Phase equilibria, microstructure, and high temperature oxidation resistance of novel refractory high-entropy alloys

A new refractory high-entropy alloy system Mo-W-Al-Cr-x is proposed as a family of candidate materials for structural applications at high temperatures. Thermodynamic assessment was used to set the chemical composition of the first alloy as 20Mo-20W-20Al-20Cr-20Ti (at.%) with a calculated melting temperature of about 1700 °C. A single disordered BCC phase should be stable at high temperatures between 1077 °C and 1700 °C. Microstructural examination and XRD results clearly show that the alloy in the as-cast condition exhibits a non-homogeneous microstructure with pronounced dendritic and interdendritic regions. Heat treatment processes, however, reveal a strong tendency of the alloy 20Mo-20W-20Al-20Cr-20Ti to homogenize. While possessing a high hardness of around 800HV, the crack-free indents allow the assumption that the alloy studied may be intrinsically ductile at room temperature. Despite the fact that the alloy possesses 40 at.% of refractory elements, high temperature oxidation tests show a surprisingly good oxidation resistance. Strategies to enhance the long-term stability of the disordered BCC phase aiming at achieving the required mechanical properties as well as optimizing the alloy's chemical composition in terms of high temperature oxidation resistance are discussed.