Irradiation energy dependence of stress relaxation rate and activation volume in polycrystalline nickel

Stress relaxation in un-irradiated and electron beam irradiated polycrystalline nickel (99.99%) was studied at room temperature. The specimens were irradiated by high-energy electrons with energies in the range 8-18 MeV for 12 min at 300 K. The tensile tests of both un-irradiated and irradiated specimens were carried out at room temperature using a universal testing machine. During deformation, straining was frequently interrupted to observe stress relaxation at fixed loads. Measurements were made over the entire stress-strain curves up to fracture. The stress-relaxation was found to be logarithmic with time and its rate was found to decrease with an increase in the energy of incident electrons. The activation volume Vσ was observed to decrease with an increase in stress levels (σ0) both in un-irradiated and irradiated specimens, however Vσ values were higher in irradiated specimens than those of un-irradiated specimens. The intrinsic height of energy barrier (Uo) to the movement of dislocations increased with an increase in incident electron energy. The results are analysed in terms of a single barrier model of stress relaxation proposed by Feltham