Microstructure, oxidation and nitridation behaviour of Cr_100-xAl_x alloys at 1000 °C

In this study, the microstructure, oxidation and nitridation behaviour of Cr_100-xAl_x alloys were investigated at 1000 °C. Five Cr_100-xAl_x alloys (x = 10–50) were prepared by vacuum arc melting and casting process. The microstructure and oxidation behaviour of as-cast Cr_100-xAl_x alloys were characterised by scanning electron microscopy (SEM), X-ray diffractometry (XRD) and energy dispersive spectroscopy (EDS). It was shown that both Cr₉₀Al₁₀ and Cr₈₀Al₂₀ exhibited a single Cr structure. The AlCr₂ phase was formed and became detectable in Cr₇₀Al₃₀. A eutectic structure, consisting of Al₈Cr₅ and AlCr₂, was found in Cr₆₀Al₄₀. With further increase of Al content to 50 at%, Al₈Cr₅ became the dominant phase in Cr₅₀Al₅₀, where a minor fraction of AlCr₂ was retained. After oxidation for up to 50 h at 1000 °C, a thick Cr₂O₃ layer was formed and both internal oxidation and nitridation occurred in Cr₉₀Al₁₀ and Cr₈₀Al₂₀. More significant internal oxidation and nitrogen was observed in Cr₈₀Al₂₀ compared to Cr₉₀Al₁₀, which was attributed to its larger diffusivities of O and N and more available Al to form internal Al₂O₃ and AlN. In Cr_100-xAl_x alloys with more than 30 at% of Al, a continuous Al₂O₃ protective layer was developed during initial oxidation, which significantly reduced oxide growth and effectively suppressed internal nitridation. The critical Al content required for external Al₂O₃ growth was found between 20 and 30 at%, representing the necessary minimum Al content of CrAl alloys for enhanced oxidation resistance.