Effects of Cr-to-Al ratios on the phase equilibria, microstructure and competing oxide growth of CoNiCrAl bond coat alloys at 1100 °C

This paper studied the effects of Cr-to-Al ratios on the phase equilibria, microstructure and high temperature oxidation behaviour of CoNiCrAl alloys at 1100 °C. As-cast CoNiCrAl alloys at five different Cr-to-Al ratios were prepared by vacuum arc melting and were isothermally oxidised in air at 1100 °C for periods up to 100 h. Prior to oxidation, an initial vacuum heat treatment was conducted to obtain homogeneous phase distributions from as-cast conditions. The phase distribution was calculated by Thermo-Calc and the microstructure evolution before and after oxidation were analysed 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-NiAl phase structure at a low Cr-to-Al ratio of 0.3, followed by a γ+β two-phase structure at intermediate Cr-to-Al ratios of 0.8 and 1.3, and eventually became a single γ phase structure at high Cr-to-Al ratios of 1.8 and 2.8. At the early stage of oxidation, it was found that Al₂O₃ was formed at low and intermediate Cr-to-Al ratios (0.3–1.3) when the β-NiAl phase was dominant. A mixture of Cr₂O₃ and Al₂O₃ was mainly observed at high Cr-to-Al ratios (1.8–2.8), which was due to that insufficient Al content at high Cr-to-Al ratios failed to facilitate the continuous growth of Al₂O₃ and high Cr content allowed the growth of Cr₂O₃ at the surface. Furthermore, the formation of less protective Cr₂O₃ and limited Al content resulted in significant internal oxidation and nitridation of CoNiCrAl alloys at high Cr-to-Al ratios.