Investigation of the strengthening mechanisms under irradiation of ferrite by dislocation dynamics simulations

To contribute to a better understanding and prediction of the behaviour of vessel steel of pressure water reactor under irradiation, a European project (FP7-IP PERFECT) which associates experimental characterization and multi-scale modeling has been operated. In this framework, the modeling of the plastic deformation of ferritic lathes, main constituent of these steels, has been done. At the non-irradiated state, the flow stress of the lathes depends on different microstructural elements: dislocation density, carbides distribution and alloy friction. The technique of Dislocation Dynamics (DD) appears to be the single quantitative tool allowing to explicitly account for the material microstructure and to estimate the effect of each element. After a detailed literature review, we have defined the stress associated to alloy friction inside ferritic lathes. The DD simulation of a tensile test shows that forest strengthening is the dominant mechanism of the flow stress. Latent hardening simulations allow a quantitative estimation of the interaction strength between slip systems. Besides, simulations including carbides inside lathes point out that these carbides affect mainly the density of dislocations stored during deformation. Finally, we have checked that a quadratic mixture law reproduces well the effect of the different strengthening mechanisms. The effect of cross slip has also been studied. Considering the experimental observations, cross slip is supposed to be easy in ferrite and depends only on the local stress state. DD simulations reveal the complex role played by this mechanism. On one side, the appearance of numerous cross slip segments leads to the pinning of primary dislocations, inducing an hardening. On the other side, at larger deformation, deviation of screw dislocations relaxes the microstructure stress concentrations, which is a recovering mechanism. At the irradiated state, for the sake of simplicity, voids are the only supplementary strengthening considered. Voids density and size are given by rate theory simulations. Starting from molecular dynamics results, a scale transition law enables the determination of activation energies needed to shear voids. Finally, a simple methodology is implemented in DD simulations. It takes into account the effect of the temperature and the deformation rate on the shear. The most recent simulations, starting from a realistic microstructure of voids, leads to a relative strengthening comparable to the ones observed during experiments. Thus, voids are probably the main source of strengthening induced by the irradiation in these steels