Effect of heat treatment regime on microstructure and phase evolution of AlMo0.5NbTa0.5TiZr refractory high entropy alloy

This paper reports the effect of heat treatment process on the ensuing microstructure evolution, and hardness variation of a cast and homogenized AlMo0.5NbTa0.5TiZr refractory high-entropy alloy (RHEA). Heat treatments were carried out on the homogenized samples at 1000, 1100, and 1200 °C for 10, 24, and 48 h. It was revealed that the phase decomposition in the alloy took place in a much shorter time than that reported in the literature. The significant tendency of Zr and Ti in respect of separation from the solid solution was found as the main reason for the formation of multi-phase structure and causing thermal instability. The phase decomposition occurred though the formation of Zr-rich needle-like phase constituents and Ti-rich globular ones. The semi-stable Zr-rich phase constituents are believed to form on B2 lamellas, and during their growth, the crystal structure gradually transforms from body-centered cubic (BCC) to a hexagonal close-packed (HCP) configuration. The electron back-scattered diffraction (EBSD) analyses confirmed that the HCP phase fraction increased significantly with increase in annealing temperature and/or time. A remarkable increase of 78% in Vickers hardness was measured for the sample heat treated at 1100 °C for 24 h compared to that of the homogenized state. It turned out that the morphology, size, interface coherency, and volume fraction of the needle-like precipitates played the leading role in influencing the alloy’s hardness. A comprehensive mechanism has been proposed for the phase decomposition of the alloy, based on the microstructure and chemical composition variations. •Initial microstructure of the alloy is composed of BCC/B2 network.•Zr shows non-sluggish diffusion and is responsible for the thermal instability.•In annealing, B2 lamellas grow by absorbing Zr and finally change to hcp.•Zr-rich hcp needle-like phases form by uphill diffusion and spinodal decomposition.•Strength of the alloy rises by the formation of needle-like hcp phases.