A self-consistent model for polycrystals undergoing simultaneous irradiation and thermal creep

A self-consistent model is presented that describes the creep response of a polycrystalline aggregate consisting of grains which exhibit a single crystal deformation law expressed as a sum of stress powers. A number of limitations of the self-consistent formulation are discussed and an approximate scheme is proposed for solving the mixed creep problem. In particular the model is applied to simulate a material that deforms simultaneously under neutron irradiation and thermal creep. In this case the single crystal creep rate has two components, one due to irradiation creep, which depends linearly on stress, and another, due to thermal creep, that depends nonlinearly on stress. Using the approximate scheme mentioned above and scanning the stress space, it is shown that, at the macroscopic level, the coupling between thermal creep and irradiation creep is significant in a range of applied stress which is a function of the irradiation and thermal creep parameters. In general the coupling of both mechanisms can be neglected in the macroscopic level when the stress magnitude exceeds a given value. This result greatly simplifies the calculation of the deformation rate of the polycrystalline aggregate, since it defines the range of conditions at which the linear superposition of irradiation creep and thermal creep in the macroscopic level gives an accurate estimate of the overall response.