High-temperature oxidation behaviour of minor Hf-doped β-NiAl single crystals in dry and humid atmospheres

Oxidation behaviour of undoped β-NiAl and β-NiAl-0.05Hf single crystals were investigated at 1100 °C in 20 vol% O₂ + Ar and 15 vol% H₂O + Ar, respectively. On contrary to the well-known beneficial effect of Hf additions on the oxidation behaviour of polycrystalline β-NiAl, severe-scale spallation is observed for β-NiAl-0.05Hf single-crystal alloy, especially in dry atmosphere. It is proposed that the presence of the grain boundaries in the polycrystalline substrate enables proper content of Hf segregating at the grain boundaries of the oxide scale and at the scale–substrate interface, which promotes the conditions required for reactive element effect. Once the reactive element effect of Hf does not work, its strong affinity with oxygen will accelerate the process of high-temperature oxidation. However, the absence of the grain boundaries as fast diffusion paths partially compromises the adverse influence of water vapour on cyclic oxidation resistance of β-NiAl single crystal, resulting in considerable suppression of the oxidation rate and less loss of protection compared with that in dry atmosphere. In addition, interfacial cavities are observed in dry and humid atmospheres, which may eventually result in the spallation of the oxide scale.