Low Temperature Localized Deformation Mechanisms in Irradiated Alloys

This talk will illustrate the effects of irradiation defect microstructures on low temperature localized deformation mechanisms, specifically twinning and martensitic phase transformations, in face centered cubic (fcc) alloys. The presence of irradiation-induced point defects, dislocation loops, and voids, can have a profound effect on deformation twinning and martensitic phase transformations, and may activate these deformation modes over a wider range of temperatures. This talk will present our experimental and computational efforts aimed at understanding these effects in several fcc alloys spanning a range of stacking fault energies (SFE) and phase transformation reversibility (i.e., stress-induced versus strain-induced). We begin by first examining the role of defects in ferrous alloys, including stainless steel 304L, and model Fe-Mn and Fe-Ni systems, using SEM in situ mechanical testing with post mortem TEM characterization, coupled with molecular dynamics (MD). Cavity-type defects tend to have the most profound effect on these alloys, and tend to enhance the tendency for twinning and strain-induced martensitic phase transformations to occur. We then consider higher-SFE Ni-based fcc Alloy 625, which shows unprecedented evidence of deformation-induced martensitic phase transformations. Complementary MD simulations suggest a stacking fault-based initiation mode. Preliminary results suggest irradiation defects have less influence on this deformation mode than they do in ferrous alloys. Finally, Ni-Ti is studied for its reversible stress-induced martensitic transformation. MD shows that point defects suppress the forward and reverse transformations. We will then synthesize all results to draw conclusions about the roles of SFE and stress- versus strain-induced transformation mode, on the localized deformation mechanisms in irradiated fcc alloys.