High-temperature oxidation behaviour of novel Co-Al-W-Ta-B-(Mo, Hf, Nb) alloys with a coherent γ/γ'–dominant microstructure

In this work, 2 at% Mo, 2 at% Nb and 2 at% Hf were substituted for the same amount of W into a Co-9Al-9W-2Ta-0.02B alloy (hereafter referred as to 2Mo, 2Nb and 2Hf alloys, respectively, while the original alloy is denoted as 0Me alloy). The effect of the Mo, Hf and Nb additions on the isothermal oxidation resistance, oxide scale evolution and failure mechanism, of the Co-9Al-9W-2Ta-0.02B alloy when exposed at 800 °C and 900 °C for 100 h was investigated. It was found the Mo, Hf and Nb additions degraded the oxidation resistance of the Co-9Al-9W-2Ta-0.02B alloy, while the 2Mo alloy always displayed the poorest oxidation resistance, resulted from heavy spallation of the oxide scale. An oxide scale composed of an outer Co₃O₄+CoO layer, a middle complex oxide layer enriched with Al, W and Ta, and a γ/needle-like Co₃W zone adhering to the γ/γ’ substrate was gradually formed; moreover, a continuous or discontinuous Al₂O₃ layer and dispersive Al₂O₃ dots or slices were observed within the γ/needle-like Co₃W zone, depending on the oxidation temperature and added elements (Mo, Hf and Nb). The formation of volatile MoO₃ in the oxide scale of the 2Mo alloy enhance the exfoliation of the oxide products, resulting in severe spallation and poor oxidation resistance.