High-temperature oxidation and β-phase depletion kinetics of CoNiCrAl bond coat alloys: effects of Al content

This study investigates the high-temperature oxidation and β-phase depletion kinetics of CoNiCrAl bond coat alloys at 1100 °C. Three CoNiCrAl alloys with an Al content range from 8–12 wt%, namely CoNiCr8Al, CoNiCr10Al and CoNiCr12Al, were subjected isothermal oxidation in air at 1100 °C for periods up to 100 h. The microstructure, oxidation behaviour and β-phase depletion kinetics of these CoNiCrAl alloys were characterised by x-ray diffractometry (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). It was shown that the CoNiCrAl alloys exhibited a predominant β + γ two-phase structure, with the proportions of the β-NiAl phase increasing linearly with Al content. It was found that the growth of external alumina scale was favoured during the early stage of oxidation when the β-NiAl phase was more dominant at an Al content of 12 wt%. Some NiCr₂O₄ spinel oxides were formed as an outer layer but their growth was gradually restricted at high Al content. Meanwhile, a corresponding β-phase depletion zone was developed at the sub-surface due to the consumption of Al during oxidation. Since the growth of alumina scale was limited and the NiCr₂O₄ spinel oxides were facilitated at low Al content (8 wt%), a pronounced β-phase depletion zone was formed. An analytical model was developed to correlate the β-phase depletion with Al content and reasonable agreements were obtained between experimental and calculated results in CoNiCr10Al and CoNiCr12Al alloys. It was found that the β-phase depletion kinetics exhibited a parabolic behaviour with oxidation time and were inversely proportional to the Al content in CoNiCr10Al and CoNiCr12Al alloys.