Structural evolution in TiZrHfNb high-entropy alloy

Among the multiplicity of single-phase high-entropy alloys explored up to now, the TiZrHfNb one is accepted as thermally stable material that crystalizes into a single-phase body-centered cubic (BCC) solid solution. This HEA demonstrates exceptional structural stability at high temperatures, while its behavior under moderate thermal conditions (700 1300 K) has not been examined extensively so far. Here we address structure formation, phase transitions, thermal stability, and properties of the HEA under different thermal treatments. To do that, we synthesize the single-phase BCC alloy and analyze its structural evolution during long-time isothermal annealing as well as slow heating. In this study, we cover the temperature interval up to 1300 K and the annealing times up to 700 hours. In-situ X-ray diffraction combined with high-temperature measurements allow us to identify phase transformations in the HEA. The results reveal that BCC single-solid solution decomposes into several phases after isothermal annealing at 673 K for less than 200 hours. After prolonged homogenization, the system is a three-phase material consisting of two BCC and one HCP solid solutions. Ab initio simulations performed for the liquid phase indicate weak chemical interaction in the studied system and indirectly confirm the ability of the HEA to form single-phase solid solutions.