Atomistic simulations of athermal irradiation creep and swelling of copper and tungsten at high dose

Radiation creep and swelling are irreversible deformation phenomena occurring in irradiated materials. With thermal activation, deformations can reach several percent; at low temperature, fundamentally different and saturating mechanisms are active and are the topic of this work. Collision cascades generate and eliminate defects that interact and coalesce under internal and external stress. We investigate how copper and tungsten swell and deform under various applied stress states in the low- and high-energy irradiation limits. The two metals respond in a qualitatively similar manner, in a remarkable deviation from the fundamentally different low-temperature plastic behaviour of bcc and fcc. The deviatoric strain is particularly sensitive to applied stress, leading to anisotropic dimensional changes, contrary to the total volume change, vacancy content and dislocation density. Low- as opposed to high-energy irradiation gives rise to greater swelling, faster creep, and higher defect content for the same dose. Simulations show that even at low temperatures, with no thermal creep, irradiation results in a swelling of about 0.1%–1% and stress-dependent irreversible anisotropic deformations of up to a few percent aligned with the orientation of applied stress. To simulate the high dose microstructures, we develop an algorithm that at the cost of about 25% overestimation of the defect content is up to ten times faster than collision cascade simulations. The direct time integration of equations of motion of atoms is replaced by the energy minimisation of molten spherical regions during their solidification; multiple insertion of molten zones and subsequent relaxation steps simulate the accumulation of radiation exposure.