Microstructural evolution and mechanical response of ion-irradiated Fe-9Cr alloys: Insights from nanoindentation

• Smaller indenters measure higher shear stress than larger spherical ones. • RT irradiation increases loop density, strengthening and hardening the material. • Dense loops from RT irradiation raise critical stress, hindering dislocation motion. • At 300 °C, loops near dislocations enable channels, easing dislocation glide. Understanding the mechanical behavior of Fe-Cr alloys under irradiation is crucial for their application in nuclear environments. This study investigates the evolution of dislocation structures and their impact on the nanoindentation response of Fe-9Cr alloys in five distinct conditions: (1) non-irradiated pristine material, (2) non-irradiated material annealed at 300 °C, (3) material irradiated with 10 MeV Fe²⁺ ions to 5 dpa at room temperature, (4) material irradiated with 10 MeV Fe²⁺ ions to 1 dpa at 300 °C, and (5) material irradiated with 10 MeV Fe²⁺ ions to 5 dpa at 300 °C. Transmission electron microscopy (TEM) analysis revealed that irradiation at RT leads to a dense distribution of dislocation loops, which act as strong obstacles to dislocation glide, significantly increasing the critical stress required for plastic deformation. In contrast, irradiation at 300 °C results in a lower density of defects in the matrix, with dislocation loops observed near pre-existing dislocation lines. This defect configuration facilitates the formation of dislocation channels, reducing overall obstruction to dislocation motion and leading to a decrease in pop-in stress compared to RT-irradiated samples. However, despite the apparent increase in dislocation mobility, Cr-decorated dislocation loops in the 300 °C-irradiated sample act as pinning sites, impeding the contribution of pre-existing dislocations to plastic deformation and necessitating the nucleation of new dislocations. Recorded mechanical properties, together with microstructural evolution, provide critical insights into the mechanical response of Fe-Cr alloys, offering valuable implications for their performance in nuclear applications.