Decoupling irradiation effects on deformation-induced martensitic transformations in commercial Ni-Cr-Fe alloys

Abstract

This study deconvolutes the roles of irradiation-induced cavities and dislocation loops on deformation-induced martensitic transformations in Ni-Cr-Fe alloys. Until recently, martensitic transformations were not thought possible in these materials due to their high stacking fault energy. But mechanical martensitic transformations can be activated by nanoindentation, prompting a need to understand the role of defects such as those which would be generated during service in high temperature irradiation environments, on the extent of the transformations. Here, commercial Alloy 625 and Alloy 690 are irradiated with neutrons, protons, or He ions at 400 °C to create microstructures containing loops and cavities, predominantly loops, or predominantly cavities, respectively. Nanoindentation on irradiated {101} grains are dissected, and their deformation microstructures are characterized using post-mortem transmission electron microscopy (TEM). In Alloy 625, all irradiation types either partially or fully suppress the fcc → hcp → bcc transformation, whereas in Alloy 690, irradiation promotes the transformation, with neutron irradiation exhibiting the most mature fcc → hcp transformation. Irradiation defects compete to influence the martensitic transformation through a competition between increasing free energy and increasing hardening. Cavities increase free energy more than they increase hardening, thus promoting transformation, whereas loops hinder transformation because they increase hardening more than free energy. These findings have significant implications on the evolution of mechanical behavior and thus the safety factor of Ni-Cr-Fe alloy components under irradiation.