Accelerated kinetic Monte Carlo method for simulations of helium bubble formation in metals

Due to the tritium-to-helium decay, materials containing tritium often suffer from aging by the formation of internal helium bubbles. A fundamental understanding of how helium bubble microstructure evolves can guide improved application of these materials. Molecular dynamics simulations can accurately reveal bubble microstructure evolution, but can typically only reach 100 ns time scales and 0.01 mu m length scales. An acceleration factor of 1022 is needed to reach the 10+ year and 1+ mu m scales relevant to experiments. Here, we show that such an acceleration factor is unlikely to be achieved by the classical kinetic Monte Carlo methods. To overcome this difficulty, we have developed an accelerated kinetic Monte Carlo method. Starting from He diffusivities obtained with molecular dynamics, we demonstrate that the results from the accelerated and classical kinetic Monte Carlo methods are indistinguishable at short time and length scales, and both reproduce the rate of bubble nucleation and growth observed in molecular dynamics. We have also applied the method to helium bubble formation in palladium tritides at experimental time / length scales. To provide reliable experimental comparison, we further report our recent experimental analysis of bubble densities in tritium-exposed samples. Our studies reveal that to account for the experimentally observed helium bubble densities, either helium trap sites are necessary, or helium cluster diffusivities are significantly lower than those predicted from molecular dynamics. These new developments are against the previous assumptions and can impact future material improvements and aging predictions.