Microstructure and mechanical behavior of as-cast AlMo_0.5NbTa_0.5TiZr refractory high-entropy alloy after aging at elevated temperatures

In this study, the effect of aging heat treatment on the microstructure, phase transformation, and mechanical properties of as-cast AlMo_0.5NbTa_0.5TiZr refractory high-entropy alloy has been investigated. The samples were directly aged (aging of the as-cast samples without prior homogenization) at 900, 1000, and 1100 °C for 2, 5, and 10 h. The results showed that increasing the aging time or temperature leads to a gradual homogenization of dendritic (DR) and interdendritic (IDR) regions, removing the elemental segregated regions as the coring effect and the formation and growth of secondary precipitates (mainly the Zr/Al-rich HCP phase). The X-ray diffraction analyses and thermodynamic simulations confirmed that the light elements (Al, Zr) tend to diffuse towards the IDR regions, while the heavy elements (Mo, Nb, Ta) remained inside the DR regions with BCC structure. The hardness measurements indicated that the maximum hardness (754 HV) is achieved after aging at 1000 °C for 5 h. The formation of fine HCP precipitates inside the DR phase and an increase in the fraction of IDR phase were found to be responsible for the increase in hardness. Further increase in the aging temperature or time caused the excessive coarsening of secondary precipitates and a dramatic decrease in the fracture toughness. This study indicated that the as-cast AlMo_0.5NbTa_0.5TiZr is thermally unstable and tends to decompose to a multi-phase structure.