EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Diffusion in high entropy alloys (HEAs) has been attracting significant attention in the community of developing advanced HEAs. As vacancies act as one of the most critical vehicles for atom transport, the knowledge of vacancy properties is of great importance for gaining physical insight into the related diffusion mechanisms. Theoretical prediction of vacancy properties in concentrated alloys, however, has always been a challenge since it has been shown that the formation and migration of thermal vacancies depend strongly on the local chemical environment, even in a simple binary alloy. In multicomponent alloys, ab initio simulations of thermal vacancies become even more challenging due to the explosive configurational space.

In the present work, we perform ab initio density-functional-theory (DFT) based calculations to investigate the vacancy formation and migration energies in HCP AlHfScTiZr HEAs and their sub-systems from binaries to quinaries. For the vacancy formation properties, we resort to the special quasi-random structure (SQS) supercell approach in conjunction with statistical analysis, from which temperature-dependent formation Gibbs energies as well as the average atomic environment are nicely extracted. We show that the temperature-dependent vacancy formation Gibbs energy due to the “configurational excitation” has a negative configurational entropy contribution. For the vacancy migration barriers, with the energy data from the nudged elastic band calculations, we additionally applied the local cluster expansion technique to the so-called kinetically resolved activation (KRA) barriers for improving the statistics. The corresponding local chemical environment effect and the general trend in terms of the “high entropy” effect are also analyzed.