An integrated in-situ HTXRD, Scheil–Gulliver, and CALPHAD approach to investigate solidification, thermal phase stability, and oxidation behaviour in AlₓCrCoFeNi high-entropy alloys (HEAs)

This study employs an integrated approach combining Scheil–Gulliver model, CALPHAD, and in-situ high-temperature X-ray diffraction (HT-XRD) to investigate the effect of Al on solidification, microstructure development, phase evolution, and oxidation behaviour of AlₓCrCoFeNi (x = 0.5, 1.0, 1.8) HEAs. Scheil simulations revealed that increasing Al content reduces the solidification temperature depression (T_D) from 76 °C to 19 °C, showing a transition from non-equilibrium dendritic to near-equilibrium solidification with reduced micro-segregation. The higher temperature depression (76 °C) due to pronounce segregation in lower Al HEAs prevented the development of CALPHAD-predicted equilibrium phases during in-situ HTXRD as opposed to AlCrCoFeNi and Al_1.8CrCoFeNi with lower T_D of 41 °C and 19 °C respectively. CALPHAD predicted oxidation of HEAs supported by in-situ HT-XRD in air confirmed Al preferential oxidation in all compositions while the formation of diffusion control oxide scale found strongly dependent on Al content. The preferential oxidation of Al leads to depletion of ordered B2 phase and the formation of a sublayer of disordered BCC and FCC phases beneath the oxide layer. In lower-Al HEAs, this results in an internally oxidised sublayer with chromia-based top oxide scale, whereas higher-Al HEAs form a dense alumina (Al₂O₃) scale with a preserved sublayer region.