Formation of a hexagonal closed-packed phase in Al0.5CoCrFeNi high entropy alloy

We report that a hexagonal closed-packed (HCP) phase with high cobalt content precipitates in Al0.5CoCrFeNi high entropy alloy (HEA) after 650 °C/8 h heat-Treatment. The precipitate with the shape of plate is completely located at the interdendritic region. Results of electron diffraction and high resolution transmission electron microscopy show that the HCP phase was transformed from the body-centered cubic phase through a simple shear and the two phase obey an orientation relationship. The thermodynamic stability of Al0.5CoCrFeNi HEA should be carefully reevaluated, especially at the vulnerable temperature.