Atomistic simulations of athermal irradiation creep and swelling of copper and tungsten in the high dose limit

Radiation creep and swelling are the macroscopic irreversible deformation phenomena, occurring in materials exposed to energetic particle irradiation even at low temperatures. On the microscopic scale, energetic particles initiate collision cascades, generating and eliminating defects that then interact and coalesce in the presence of internal and external stress. We investigate how copper and tungsten swell and deform under various applied stress states in the low- and high-energy radiation exposure limits. Simulations show that the two metals respond to irradiation and stress in a qualitatively similar manner, in a remarkable deviation from the fact that the low-temperature plastic deformation modes of bcc and fcc crystals are fundamentally different. The deviatoric part of plastic strain is particularly sensitive to applied stress, leading to anisotropic dimensional changes. At the same time, the total volume change as well as vacancy and dislocation densities are almost independent of the applied stress. Low- as opposed to high-energy irradiation gives rise to greater swelling, faster creep, and higher defect content for the same radiation exposure. Simulations show that even at low temperatures, where thermal creep is absent, irradiation results in a stress- dependent irreversible anisotropic deformation of considerable magnitude, with the orientation aligned with the orientation of applied stress. To model the high dose microstructures, we develop an algorithm that at the cost of about 25% overestimation of the defect content is about ten times faster than collision cascade simulations. Direct time integration of equations of motion of atoms in cascades is replaced by the minimization of energy of molten spherical regions; multiple inser- tion of molten zones and the subsequent relaxation steps simulate the increasing radiation exposure.