The Oxide Scales Formed on Different Co-Ni Based Superalloys During Isothermal Oxidation at 800 and 900°C

ISOTHERMAL OXIDATION AT 800 AND 900°CIn this investigation, the formation of oxide scales on different Co-Ni based superalloys of γ-γ′ type was analyzed.Co-20Ni-7Al-7W (at.%) alloy as well as its W-free modifications based on Co-Ni-Al-Mo-Nb and Co-Ni-Al-Ta systems was analyzed under conditions of high temperature oxidation at 800 and 900°C.Therefore, the alloys were isothermally oxidized at selected temperatures for 100 h in laboratory furnace.Afterwards, the oxidation products were evaluated by means of X-ray diffraction (XRD), optical microscopy (OM) and scanning electron microscopy (SEM).The performed tests showed that W-free alloys exhibit worse oxidation resistance compared to those of Co-Ni-Al-W alloys.After oxidation at 900°C, all alloys were prone of oxide spallation.The scales characterized by oxide peeling were mostly composed of complex Co-based oxides, including CoWO 4 , CoTa 2 O 6 , Co 2 Mo 3 O 8 , CoNb 2 O 6 .Keywords: γ-γ′