Phase stability of Ti-containing high-entropy alloys with a bcc or hcp structure

Solid-solution-strengthened alloys with a hexagonal close-packed (hcp) structure are used as high-temperature structural materials; for example, hcp and body-centered cubic (bcc) two phase Ti alloys are used in aircraft jet engines. The temperature limit of Ti alloys is ~873 K because of deterioration of their creep properties and oxidation resistance at higher temperatures. Greater solid-solution strengthening will be beneficial to improve the strength of Ti alloys at temperatures greater than 873 K. High-entropy alloys (HEAs) are expected to exhibit superior mechanical properties because of the sluggish diffusion of their constituent elements and their large lattice distortion. Hcp HEAs are expected to result in improved high-temperature mechanical properties. However, making the hcp structure stable in HEAs is difficult. To understand the phase stability of Ti-containing HEAs, the present study focuses on two alloys: Ti34Al33V33 and Ti34Zr33Hf33. Al and V are used in commercial Ti alloys. In Ti34Al33V33, a bcc structure is stable at high temperatures and a hcp phase precipitates during cooling. Hcp-structured Ru, Sc, Co, and Zr were selected as additional elements to investigate the change in phase stability of a bcc phase of TiAlV. In Ti34Zr33Hf33, a hcp structure is stable at room temperature. Ru with a hcp structure, Al with a face-centered cubic structure, and V with a bcc structure were selected as additional elements to investigate the phase stability of TiZrHf. Phase stability is discussed on the basis of binary phase diagrams, the formation energy of phases, and the valence electron concentration of the constituent elements.