Study of mechanical properties evolution and intergranular cracking susceptibility regarding irradiation microstructure in heavy ion irradiated 304 stainless steel

IASCC is a phenomenon of embrittlement observed in stainless austenitic steels used as internal structural elements of PWRs. The magnitude and the characteristics of the phenomenon depend on the material, the PWR environment, the neutron irradiation and the mechanical stresses that these elements undergo. The degradation of one of these contributors makes it possible to limit or eliminate the sensitivity to intergranular cracking. Although contributors such as deformation localization and intergranular segregation have been identified, the role of the irradiation microstructure on the susceptibility to this phenomenon of a material is not fully established. The role of irradiation nano-cavities on IASCC, especially those located at grain boundaries is questioned. In this thesis, a 304L stainless steel austenitic was subjected to different irradiation conditions in order to determine the influence of the hardening, the localization and the effect of the irradiation cavities on the embrittlement of this material. The steel samples were irradiated with 10 MeV iron ions with or without helium and under two different temperatures, 450 C and 600 C. These parameters made it possible to modify the microstructures of irradiation. The size and density of the cavities and the Frank loops generated are linked to the irradiation conditions employed. The study of the susceptibility of the irradiated samples has shown that the microstructure that underwent the strongest hardening is the most sensitive to intergranular cracking, although this susceptibility may appear after a weak hardening. The irradiation microstructure, in particular the presence of cavities, seems to reduce the sensitivity to cracking by limiting the location of the deformation or by their softening effect. In this study, the presence of cavities at grain boundaries does not increase sensitivity to intergranular cracking. For the study of radiation-induced hardening, we have developed and realized in situ micro compression tests in SEM on ion-irradiated materials. These simple slip tests show a significant effect of the size of unirradiated micro-pillars, which is attenuated or completely annihilated by the hardening effect of the irradiation microstructure. These micro-compression tests lead to results consistent with those established by nano-indentation. To better understand the localization effect of the deformation, we varied the grain size of the material under study. When this decreases, the deformation concentrates even more on grains with strong Schmid factors. In addition, the sensitivity to cracking decreases as the grain size increases, showing an effect of the ratio between the irradiation depth and the average grain size. (author)