Model for irradiation softening of nickel-based single crystal superalloys under ion irradiation

Abstract

The phenomenon of irradiation softening has recently been noticed for nickel-based single crystal superalloys. However, corresponding theoretical analysis addressing the fundamental deformation mechanisms is limited so far. In this work, a novel irradiation softening model is developed to characterize the hardness-depth relations of ion-irradiated nickel-based single-crystal superalloys, which considers four deformation mechanisms including the irradiation softening effect, indentation size effect, damage gradient effect and soft matrix effect. Thereinto, irradiation softening can be mainly ascribed to irradiation-induced disordering of the γ′ precipitates that results in the loss of the ordered strengthening, and dissolution of the γ′ precipitated phase that mitigates the hardening behavior related to dislocation strain. In order to validate the developed model, experimental data of ion-irradiated nickel-based alloy Rene N4 is considered at different irradiation doses. A good agreement between the theoretical results and experimental data is obtained for the variation of indentation hardness as a function of the indentation depth. Further analysis of the deformation mechanisms indicates that: (1) when compared with the exiting dislocations, irradiation-induced defects have limited contribution to the hardness of ion-irradiated nickel-based alloy Rene N4. (2) It is the change of the precipitated phase dissolution that results in the variation of the extent of irradiation softening at different irradiation doses.