Implications for the Nb aggregation inherited from melt to γ phase of U-Nb alloy

Abstract

By means of ab initio molecular dynamics (AIMD) simulations, Nudged Elastic Band (NEB) method and cluster expansion (CE) combined with the Monte Carlo (MC) techniques, we have investigated that the elemental interactions and local structural dynamics in the melt and γ phase of U-Nb alloy and hydrogen diffusion behavior in γ-U-Nb alloy. It has been found that the aggregation of Nb atoms inherited from melt to γ phase plays a significant role in improving mechanical property and protecting γ-U-Nb alloy from hydrogen damage. The analyses of chemical short-range order parameters attest that there exists the similarity of element interactions between the melt and γ phase of U-Nb alloy. The U-Nb pairs in both melt and γ phase exhibit the repulsive interactions, whereas the attractive U-U and Nb-Nb interactions exits. These results demonstrate that the U and Nb atoms prefer to stay in their own shells to form the localized clusters in both melt and γ phase of U-Nb alloy. Furthermore, the analyses of hydrogen diffusion behavior in γ-U-Nb alloy, our NEB calculations have further proved that the hydrogen atoms prefer to be trapped by Nb atoms and, this trend is further reinforced by the aggregation of Nb atoms. It implies that, once Nb is added into U alloys, the Nb addition plays an important role in slowing down the diffusion of hydrogen atoms in γ-U-Nb alloy. The uniform distribution of Nb clusters helps to avoid the occurrence of excessive local hydrogen concentration.