High temperature oxidation of single-crystal Ni-base superalloys

The isothermal oxidation behavior at 900 and 1100 degrees C in pure oxygen of several commercial single-crystal Ni-base superalloys (CMSX2, CMSX4, CMSX6, AM3, MC2, SRR99) and modified versions of some of them has been investigated. Oxide scales formed at 900 degrees C are adherent and reveal the dendritic microstructure of the subjacent alloy. These scales are composed of three layers 1- (Ti,Co,Ni)-rich oxides, 2- Cr-rich oxides and 3- (Ta,W,Ti)-rich oxides. Al-internal oxidation zone and y' depleted zone are identified. Oxide scales formed at 1100 degrees C are not adherent and spall off extensively regardless of cooling rate. The critical temperature drop to initiate spallation (Delta T) depends on the oxide layer thickness (xi) and the interfacial rupture energy (y(F)). For a given oxide/metal couple (Delta T)(2) xi/y(F), is constant (EVANS and LOBB model). AM3 and CMSX4 alloys follow this model while AM3 and MC2 modified alloys have different behavior. Addition of Si and Hf and substitution of Mo by Re or Pt and Ti by Ta improve oxidation resistance at 900 degrees C and scale adherence at 1100 degrees C.