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 degrees 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 con-stituents 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 tem-perature and/or time. A remarkable increase of 78% in Vickers hardness was measured for the sample heat treated at 1100 degrees 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 decom-position of the alloy, based on the microstructure and chemical composition variations. (c) 2023 Elsevier B.V. All rights reserved.

Automated summary

This paper investigates the effect of heat treatment process on the microstructure evolution and hardness variation of a cast and homogenized AlMo0.5NbTa0.5TiZr refractory high-entropy alloy (RHEA). The study reveals that phase decomposition occurs in a shorter time than reported previously, with Zr and Ti separation identified as the main cause. The decomposition leads to the formation of Zr-rich needle-like phase constituents and Ti-rich globular ones. The presence of these phases results in a transformation of crystal structure from body-centered cubic (BCC) to hexagonal close-packed (HCP). The increase in annealing temperature and/or time further enhances the proportion of HCP phase, and the hardness of the alloy increases significantly.