Evaluation of irradiation hardening of ion-irradiated V–4Cr–4Ti and V–4Cr–4Ti–0.15Y alloys by nanoindentation techniques

Abstract Irradiation hardening behavior of V–4Cr–4Ti and V–4Cr–4Ti–0.15Y alloys after Cu-ion beam irradiation were investigated with a combination between nanoindentation techniques and finite element method (FEM) analysis. The ion-irradiation experiments were conducted at 473 K with 2.4 MeV Cu 2+ ions up to 7.6 dpa. For the unirradiated materials, the increase in nanoindentation hardness with decreasing indentation depth, so-called indentation size effect (ISE), was clearly observed. After irradiation, irradiation hardening in the measured depth was identified. Hardening behavior of bulk-equivalent hardness for V–4Cr–4Ti–0.15Y alloy was similar to that for V–4Cr–4Ti alloy. Y addition has little effect on irradiation hardening at 473 K. Adding the concept of geometrically necessary dislocations (GNDs) to constitutive equation of V–4Cr–4Ti alloy, the ISE was simulated. A constant value of α  = 0.5 was derived as an optimal value to simulate nanoindentation test for ion-irradiated V–4Cr–4Ti alloy. Adding the term of irradiation hardening Δ σ irrad . to constitutive equation with α  = 0.5, FEM analyses for irradiated surface of V–4Cr–4Ti alloy were carried out. The analytic data of FEM analyses based on neutron-irradiation hardening equivalent to 3.0 dpa agreed with the experimental data to 0.76 dpa. The comparison indicates that irradiation hardening by heavy ion-irradiation is larger than that by neutron-irradiation at the same displacement damage level. Possible mechanisms for extra hardening by heavy ion-irradiation are the processes that the injected Cu ions could effectively produce irradiation defects such as interstitials compared with neutrons, and that higher damage rate of ion-irradiation enhanced nucleation of irradiation defects and hence increased the number density of the defects compared with neutron-irradiation.