Effects of Si and Al on the Microstructure and Oxidation Resistance of Mo0.5NbHf0.5Ti Refractory High Entropy Alloy

Mo0.5NbHf0.5Ti, Mo0.5NbHf0.5TiSi0.5 and Mo0.5NbHf0.5TiSi0.4Al0.4 (molar ratio) refractory high-entropy alloys (RHEAs) were fabricated by vacuum non-consumable arc melting. The effects of Si and Al additions on the microstructure and oxidation resistance at 1473 K were systematically investigated. The results indicate that the Si- and Al-free RHEA exhibits a microstructure primarily consisting of body-centered cubic (BCC) dendrites, whereas the Si-containing and Si+Al-containing RHEAs are mainly composed of a BCC+M5Si3 eutectic with a minor fraction of primary BCC phases. Further Al addition in the presence of Si does not significantly alter the silicide content. The oxide scale formed on the RHEAs comprises TiO2-, TiNb2O7-, HfO2- and SiO2-type oxides, with no evident Al2O3 or MoO3 detected. The oxidation resistance is markedly improved by Si addition, which is attributed to the excellent oxidation resistance of the M5Si3 phase and the formation of a relatively continuous and dense TiNb2O7-type oxide matrix resulting from the selective oxidation of Ti. However, with additional Al in the presence of Si, the selective oxidation of Ti is inhibited, preventing the TiNb2O7-type oxide from forming a continuous matrix. Consequently, the compactness of the oxide scale decreases, leading to degraded oxidation resistance.