Theoretical study on the element distribution characteristics and the effects of oxygen in TiZrHfNb high entropy alloys

Short range order and oxygen both have great effects on the mechanical properties like yield strength and ductility of refractory high entropy alloys. In this work, we studied the element distribution characteristics and oxygen occupation properties in TiZrHfNb high-entropy alloy based on first principles, molecular dynamics and Monte Carlo calculations. Two thousands of TiZrHfNb crystal models were firstly calculated based on first principles calculations to analyze the element distribution characteristics. The Warren-Cowley parameters were then evaluated, the Nb-Nb and Ti-Zr atomic pairs tend to form in TiZrHfNb. Hybrid of molecular dynamics and Monte Carlo calculations at 300 K and 1000 K are further carried out to study the possible short range order and phase transition in TiZrHfNb. The element distributions are not uniform, and the transition from body-centered cubic to hexagonal close packed structures are observed. The oxygen atoms are then placed into the enriched regions of Ti, Zr, Hf, and Nb in high-entropy alloy, respectively. The ab initio molecular dynamics calculations of these four configurations are carried out and the diffusion coefficients are evaluated. The oxygen owns the largest diffusion coefficient in the Nb-rich zone. Furthermore, the tensile simulations reveal the great effects of element distributions and oxygen occupations on the mechanical properties of TiZrHfNb. These findings will help us to understand the short range order characteristic and oxygen occupation effects on the phase stability and to regulate the mechanical properties of TiZrHfNb high-entropy alloy.