Oxide phase development upon high temperature oxidation of γ-NiCrAl alloys

The amount of each oxide phase developed upon thermal oxidation of a γ‐Ni‐27Cr‐9Al (at.%) alloy at 1353 K and 1443 K and a partial oxygen pressure of 20 kPa is determined with in‐situ high temperature X‐ray Diffractometry (XRD). The XRD results are compared with microstructural observations from Scanning Electron Microscope (SEM) backscattered electron images, and model calculations using a coupled thermodynamic‐kinetic oxidation model. It is shown that for short oxidation times, the oxide scale consists of an outer layer of NiO on top of an intermediate layer of Cr2O3 and an inner zone of isolated α‐Al2O3 precipitates in the alloy. The amounts of Cr2O3 and NiO in the oxide scale attain their maximum values when successively continuous Cr2O3 and α‐Al2O3 layers are formed. Then a transition from very fast to slow parabolic growth kinetics occurs. During the slow parabolic growth, the total amount of non‐protective oxide phases (i.e. all oxide phases excluding α‐Al2O3) in the oxide scale maintain at an approximately constant value. The formation of NiCr2O4 and subsequently NiAl2O4 happens as a result of solid‐state reactions between the oxide phases within the oxide scale.