Driven Diffusion of Radiation Defects in α-Uranium by Temperature/Stress Gradients: Molecular Dynamics Insights into Anisotropy and Directional Migration

Abstract: The anisotropic diffusion of irradiation defects in α-uranium (α-U) has been investigated using classical and nonequilibrium molecular dynamics methods, with the aim of better understanding the complex response of the microstructure to irradiation damage, which depends on temperature and internal stress. Our results show that (i) both vacancy and self-interstitial atoms exhibit significant anisotropic diffusion characteristics at elevated temperatures, arising from their unique diffusion mechanisms. The difference in anisotropic diffusion rates between the two species strongly supports the previously proposed hypothesis of the irradiation growth mechanism. (ii) When a temperature gradient exists, the additional driving force introduced by the gradient accelerates the diffusion of vacancy and interstitial atoms, causing both to move directionally toward the high temperature region. Kinetic modeling suggests that the direction of this driving force is primarily determined by the sign of the ‘heat of transport Q’ and the direction of the temperature gradient together. (iii) The stress gradient field similarly promotes the anisotropic diffusion of vacancies and interstitial atoms. Additionally, the stability and migration behavior of defects change with the stress state, leading to species separation under stress gradients. We showed that the stress effect can be well predicted by the continuum model based on the formation volume tensor of point defects. (iv) A change in the sign of the migrating volume tensor alters the monotonicity of the migration barrier with respect to stress. This study provides valuable new insights into the complex irradiation damage behavior of α-U and, for the first time, reveals how the gradient environment within the reactor interacts with irradiation defects.