Effect of confinement of SIA cluster diffusion by impurities on radiation defect accumulation due to 14 MeV neutrons in tungsten

Abstract

Impurities acting as traps reduce the self-interstitial atom (SIA) cluster mobility, thereby increasing recombination and, in turn, enhancing radiation-resistance of polycrystalline materials. However, at elevated temperatures, depending on SIA-trap (impurity) binding energy, SIA clusters may trap and detrap (detach) numerous times. In tungsten, SIA clusters of all sizes glide one-dimensionally (1D) along their Burgers vector direction. This can result in detrapped SIA clusters to retrace their original 1D path, confining their 1D-glide between traps. However, SIA clusters can change the direction of their 1D-glide by overcoming a rotation barrier and impurities or solutes are known to reduce this barrier. A lower rotation barrier can result in detrapped SIA clusters to diffuse in a random 1D direction, effectively leading to 3D (net-3D) diffusion. Here we investigate the effect of two cases of SIA diffusion, namely the confined-1D diffusion and the net-3D diffusion, on the damage accumulation in polycrystalline tungsten under 14 MeV neutron irradiation using the object kinetic Monte Carlo method. Simulations were performed using cascade debris obtained from molecular dynamics simulations with primary knock-on atom (PKA) energies corresponding to 14 MeV neutrons. In the simulations with net-3D diffusion, SIA clusters of all sizes diffuse in a random 1D direction after the clusters detrap. While for the confined 1D diffusion case, clusters larger than size 5 retain their original direction of 1D glide. In both types of simulations, trapped vacancies are assumed to be permanently immobilized. Unexpectedly, the damage accumulation was lower with confined 1D diffusion than that with net-3D. We present a systematic comparison of the influence of the two cases of SIA diffusion on the radiation damage accumulation as a function of dose rate, detrapping barrier, and impurity concentration at 1025 K.