Effects of diffusion and primary creep on intergranular cavitation at high temperatures

Next-generation reactors are expected to play a crucial role in power production in the foreseeable future. Due to the extreme anticipated operating temperatures of next-generation plants, a major concern for candidate materials is failure by creep cavitation. Indeed, many commonly used component lifetime estimates are based on how quickly intergranular voids grow. Void growth is known to be caused by three processes: diffusion along the void surface, diffusion along the grain boundary, and creep of the surrounding grains. However, until now, previous creep cavitation models have neglected to account explicitly for both the surface diffusion process and primary creep effects. More precisely, previous models assume that surface diffusion occurs rapidly enough to sustain quasi-equilibrium void growth, and that the creep response of the grains is accurately modeled by power-law secondary creep. To illustrate the potential ramifications of these assumptions, we present here novel finite element simulations of intergranular void growth under the combined effects of surface diffusion, grain boundary diffusion, and bulk primary/secondary creep. Our results indicate that crack-like void growth may be more prevalent at high temperatures than previously assumed, and that void growth of any kind is substantially accelerated during the primary creep regime. This could have serious implications for previous creep rupture models, which may underestimate the rate of void growth by almost two orders of magnitude or more. Based on our results, we establish quantitative criteria for quasi-equilibrium and crack-like void growth, and we suggest quantitative improvements to the existing models.