High temperature oxidation behavior of γʹ-strengthened CoNi-base superalloys

• 30Ni-10Al-5W-4Ti-1Ta (5W1Ta) alloy has better oxidation resistance than 30Ni-10Al-4W-4Ti-2Ta (4W1Ta) alloy at 800 °C and 900 °C. • The oxide films formed at 800 °C and 900 °C for both alloys show a three-layer structure, but the oxidation mechanism is different at different temperatures. • The outer oxide is composed of Co 3 O 4 , CoO and NiO at 800 °C, however, at 900 °C, the outer oxide layer consists of only CoO. Oxidation behavior of γ ʹ-strengthened CoNi-base superalloys was investigated with isothermal oxidation tests conducted in air at 800 and 900 °C, respectively, for 100 h. It is observed that both 30Ni-10Al-4W-4Ti-2Ta (4W2Ta) and 30Ni-10Al-5W-4Ti-1Ta (5W1Ta) alloys exhibit a three-layered oxide scale structure after oxidation. Addition of Ta increases oxidation mass gain of CoNi-based high-temperature alloys, but the effects on the type and distribution of oxidation products are not significant. However, the oxidation temperature has a great influence on oxide surface morphology and oxidation mechanisms for those two alloys. After oxidation at 800 °C, the outermost oxide film contains Co 3 O 4 , CoO, and NiO, and when the oxidation temperature is 900 °C, the outermost oxide film contains only CoO.