First-principles study of vacancy defects in TiVTa and TiVTaNb concentrated solid-solution alloys

The formation and migration energies of vacancy defects are studied systematically by first-principal calculations method in three concentrated solid-solution alloys (CSAs), namely as equimolar TiVTa and TiVTaNb alloys, and TiVTa 50 alloy with Ta content increased to 50%. It is found that the vacancy formation and migration in TiVTaNb alloy are more difficult than that in TiVTa and TiVTa 50 alloys. The TiVTa alloy has the larger local lattice distortion than TiVTa 50 and TiVTaNb alloys, which would lead to an irregular energy landscape and make it easier to form vacancies. Vacancies are easily formed in Ti-rich environments and the trend reverses for in V-rich environments in these three alloys, originating from the large local lattice distortion in Ti-rich environments and strong binding interactions between Ti atoms and vacancies. Ti atoms with the largest size tend to migrate to neighboring vacancies in these three alloys. The addition of Nb enhances the electron interactions between atoms in TiVTaNb alloy. Compared to TiVTa and TiVTa 50 alloys, the migration barrier of atoms diffusing into vacancies in TiVTaNb alloy is improved due to the combination of strong electron interactions between atoms and lattice distortion. It is implied that the influence of alloy element combination on vacancy formation and migration energies may be greater than that of alloy element content in Ti-V-Ta and Ti-V-Ta-Nb alloys. These results provide fundamental insights into the defects evolution of CSAs with body-centered cubic (bcc) structure and supply the scientific basis for the composition design of bcc CSAs.