In situ measurements of a homogeneous to heterogeneous transition in the plastic response of ion-irradiated 〈1 1 1〉 Ni microspecimens

We report on the use of quantitative in situ microcompression experiments in a scanning electron microscope to systematically investigate the effect of self-ion irradiation damage on the full plastic response of 〈1 1 1〉 Ni. In addition to the well-known irradiation-induced increases in the yield and flow strengths with increasing dose, we measure substantial changes in plastic flow intermittency behavior, manifested as stress drops accompanying energy releases as the driven material transits critical states. At low irradiation doses, the magnitude of stress drops reduces relative to the unirradiated material and plastic slip proceeds on multiple slip systems, leading to quasi-homogeneous plastic flow. In contrast, highly irradiated specimens exhibit pronounced shear localization on parallel slip planes, which we ascribe to the onset of defect free channels normally seen in bulk irradiated materials. Our in situ testing system and approach allows for a quantitative study of the energy release and dynamics associated with defect free channel formation and subsequent localization. This study provides fundamental insight into the nature of interactions between mobile dislocations and irradiation-mediated and damage-dependent defect structures.