Microstructure, kinetics and thermodynamics of HEA Al0.5CoCrFeNi at T ≥ 800 °C

Abstract Al0.5CoCrFeNi contains a mixture of the ductile fcc (A1) phase and the hard ordered bcc (B2) phase. In this paper, the microstructure, composition, precipitation kinetics, and thermodynamic calculation of the equilibrium state of the Al0.5CoCrFeNi alloy were studied. It was found that the morphology of the B2 phase (rich Al Ni) is temperature-dependent; up to 1000 °C the dominant morphology is needle/plate-like whereas above this temperature, equiaxed morphology takes over until the needle morphology disappears completely at above 1200 °C. It was confirmed experimentally, and by thermodynamic calculation with good agreement between the two, that the phase compositions are temperature dependent. Nevertheless, there is still disagreement between the thermodynamic model and the experimental results for the A1 to B2 phase ratios and compositions at ~1200 °C and above. The inter-diffusion activation energy (Q = 187 ± 20 kJ/mol) for the B2 phase growth was found to be lower than the activation energy reported for the AlCoCrFeNi alloy. This result is attributed to the different lattice distortion parameters of the two alloys. It is further suggested that the morphology transformation of the B2 phase from needle-like to equiaxed with increase in temperature, occurs because of increased density of dislocations at the B2/A1 interface.