Deformation Mechanisms of Zirconium Alloys after Irradiation Studied by Dislocation Dynamics Simulations and In Situ Straining Experiments in TEM

Abstract

Zirconium alloys, used in the nuclear industry, are subjected to neutron irradiation that affects their mechanical properties. At the microscopic scale, neutron irradiation creates small dislocation loops that act as obstacles against dislocation glide, explaining the irradiation hardening. Transmission electron microscopy observations performed after post-irradiation mechanical tests have shown that loops are swept out by gliding dislocations, creating thin zones free of defects. Observations have proven that slip occurs preferably in the basal plane, a puzzling fact as dislocations mainly glide in the prismatic plane in unirradiated conditions. In order to understand this phenomenon, discrete dislocation dynamics (DD) simulations, on complex configurations, have been performed. The input parameters of this code have been adjusted on molecular dynamics simulations. Then interactions between loops and mixed dislocations gliding either in the prismatic or basal plane have been simulated. These simulations show that prismatic glide is always impeded in the mixed-screw direction, whereas for basal slip, clearing or a weak interaction occurs in the mixed-screw direction, allowing an easy glide of basal dislocations. Furthermore, all three basal systems can contribute to clearing in the basal plane contrary to prismatic slip. These two reasons explain the easy basal glide and clearing of loops after irradiation. Moreover, in situ straining experiments inside a transmission electron microscopy have been conducted on ion-irradiated recrystallized Zircaloy-4. Several interactions between dislocations and loops have been observed in situ. The DD code has been used to simulate these interactions. A fair agreement is obtained between simulations and experiments, showing the relevance of the DD numerical tool.