Oxidation and Mechanical Behavior of Nb-Ti-(Cr)-(Al) Refractory Multi-principal Element Alloys at 1000 °C

Abstract The oxidation and mechanical behavior at 1000 °C of five single-phase body-centered cubic (BCC) Nb-Ti-(Cr)-(Al) Refractory Multi-Principal Element Alloys (RMPEAs), with low density of ~ 6.5 g cm −3 and intrinsic room-temperature ductility, was systematically investigated. Isothermal oxidation tests conducted in air for up to 100 h revealed a strongly composition-dependent response. The binary NbTi alloy exhibited near-linear oxidation kinetics and an extensive internal reaction zone (IRZ). Conversely, Cr- and Al-containing alloys demonstrated reduced mass gain, narrowed IRZ and altered kinetic regimes, attributed to the formation of chemically complex oxide scales. Although Cr and Al additions reduced both mass gain and IRZ, oxide-scale spallation persisted across all compositions. Compression tests at 1000 °C confirmed that all alloys retained ductile behavior, with Cr-containing compositions exhibiting superior yield strength. Among the evaluated alloys, Nb 3 Ti 3 Cr 1 Al 1 achieved the most favorable balance between oxidation resistance (mass gain of 14.4 mg cm −2 after 100 h of exposure) and mechanical stability (yield strength 229.0 MPa at 1000 °C). The combined addition of Cr and Al to the equimolar NbTi alloy improves oxidation resistance and mechanical performance, advancing the development of lightweight RMPEAs for high-temperature structural applications.