Fundamental reactions between prismatic loops in stochastic dislocation dynamics

Body-centred cubic metals exposed to irradiation by energetic particles form highly mobile prismatic loops of <111> type, that in many cases represent the dominant type of radiation defects. As the microstructure of materials in a fusion reactor is expected to coarsen and evolve significantly over the service lifespan, one of the desired outcomes of fusion materials research is predicting the fundamental laws of evolution governing this process.

By virtue of their high mobility, prismatic loops often assemble into mutually trapped loop rafts at lower temperatures, while separating and dispersing at higher temperatures, with consequences to the mean obstacle distance and thus plasticity. The stochastic motion of individual loops and interacting clusters of loops, including reactions between the loops, is one of the central questions that needs resolving to enable the simulation of microstructure driven by stochastic forces associated with ambient thermal fluctuations. We find that the stochastic thermal force in conjunction with internal degrees of freedom, and the re-orientation of the loop habit-planes, is a factor fundamentally affecting the dynamics of interacting dislocation loops. The trapping reaction between the loops depends critically on the internal re-orientation of the loop habit plane during the reaction: the barrier to enter the bound state is lowered substantially to nearly athermal level, whereas the lifetime of the collective bound state is increased by several orders of magnitude.

In conclusion, we find that even within the simple (but elegant) theory of linear elasticity, the inclusion of internal degrees of freedom makes the fundamental difference between loop rafts remaining bound and stable, or separating on the experimentally relevant timescales.