Thomas Jourdan

In order to assess the microscopic mechanisms at the origin of irradiation creep deformation, a combined experimental and numerical study has been undertaken. Pure aluminum has been irradiated in-situ with electrons, in a transmission electron microscope, under an applied stress. Experiments show a strong effect of the applied stress on relative loop densities depending on the orientation of the loop habit plane with respect to the tensile direction. However, no effect of stress is observed on the loop growth rate, whatever the local relative loop densities. This suggests that the stress induced preferential absorption (SIPA) of point defects by dislocation loops is not significant.Object kinetic Monte-Carlo numerical simulations that account only for the SIPA mechanism indicate that the effect of the applied stress on loop growth rate is negligible compared with the scatter induced by the loop size distribution or the environment, in good agreement with the experiments. From this study, it is concluded that the applied stress only affects the loop nucleation process and not its growth. Furthermore, it is proved that the stress induced preferential absorption of point defects by dislocations and dislocation loops is in fact very limited and should have a negligible contribution to irradiation creep deformation.

We show how to adapt and improve the stochastic simulation algorithm (SSA), also known as the Lanore–Gillespie algorithm, to exactly and efficiently simulate a large, fully connected network of chemical reactions. By combining a low-rank decomposition of an upper bound of the propensity matrix with rejection sampling, we are able to significantly reduce the time and memory costs of manipulating the reactions’ priority queues. The resulting algorithms exhibit logarithmic time and linear space complexity in the number of involved chemical species, outperforming the original SSA and subsequent stochastic methods on a benchmarking model. As a physical application, we simulate the time evolution of solute precipitation in a FeCu1.34% alloy under thermal aging. The substantial speed-up and significantly reduced memory consumption enable us to reach physical times and system sizes that were unattainable with previously employed deterministic and stochastic methods. The temporal evolution of the simulated sizes and number densities of Cu precipitates also matches very well with small-angle neutron scattering experiments.

The asymmetric distribution of geometrically equivalent defects is a long-standing problem in materials science. In this study, we investigate the preferential nucleation of interstitial dislocation loops in specific planes in stressed aluminum, commonly observed experimentally, and seek to clarify the underlying mechanism. For this purpose, we consider a structural change in the geometry of defects, specifically the transformation of 3D compact A15 clusters into 2D Frank loops. Using object kinetic Monte Carlo and ab initio calculations, we show that a symmetry breaking in the transformation of A15 clusters significantly impacts the dislocation loop distributions, resulting in the emergence of a preferential orientation when the material is under stress. This discovery not only calls for a critical revision of established theories but also has tangible applications for materials under extreme conditions.

Elastic interactions between point defects and sinks, such as dislocations and cavities, affect the diffusion of point defects and are responsible for some of the features observed in microstructures under irradiation. It is therefore necessary to include elastic interactions in kinetic simulations for a quantitative prediction of material properties. In this work a method is presented to accurately and efficiently evaluate the strain field in object kinetic Monte Carlo simulations. It can handle any strain field which is biharmonic, such as the one generated by a dislocation segment or a cavity in isotropic elasticity. A speed-up of several orders of magnitude is obtained compared to the direct summation over strain sources, so that simulations over experimental time scales can be performed within reasonable computation times. The case of a thin foil containing a high density of loops under irradiation is investigated. Loop growth rates are found to depend on the loop radius, as shown experimentally, but more complex effects due to the surrounding microstructure are also highlighted.