Stress relaxation in neutron irradiated copper single crystals

Abstract

The thermally activated slip mechanism in neutron irradiated copper single crystals has been investigated with stress relaxation measurements from 77 to 420°K. The stress dependence of the strain rate at constant temperature and the temperature dependence of the flow stress at constant strain rate have been established. Using these two quantities, the activation enthalpy and activation volume are calculated as a function of effective stress. The experimental observations of these activation parameters are in good agreement with the theoretical stress dependencies predicted by the Fleischer theory for strengthening by tetragonal distortions. An experimental force-distance curve for the dislocation-defect interaction responsible for neutron irradiation strengthening is also in excellent agreement with Fleischer's theory. The results indicate that the thermally activated, rate controlling mechanism for slip in these crystals can be expressed entirely through the relationship, γ ̇ = γ ̇ 0 exp − H 0 kT [1 − ( τ ∗ τ 0 ∗ ) 1 2 ] 2 , where γ is the strain rate, γ ̇ 0 a constant, H 0 (= 3.0 eV) the activation enthalpy at zero effective stress, τ ∗ the effective stress, γ ̇ 0 the effective stress at 0°K, and kT has the usual meaning.