The thermal activation of slip in neutron irradiated copper

Abstract

The activation energy and activation volume of a slip process in which dislocations cut obstacles by a combination of stress and thermal activation can be obtained at any temperature by measurements of the local temperature, and strain rate, dependence of the flow stress. Results are given of such measurements at 77°K and 293°K on neutron irradiated copper crystals in both the as-irradiated condition and after a mild annealing treatment (306°C for 251 min). Constant values of (δ ln σ/∂T)ε and (∂ ln σ/∂ ln ε)T were obtained during stage I of the deformation process at each temperature, enabling a single activation energy to be calculated. At the end of stage I, (∂ ln σ/∂T)ε and (∂ ln σ/∂ ln)εT decrease to the unirradiated values, the decrease being sudden at 293°K and more gradual at 77°K. In both materials the activation energy and volume are temperature dependent, being much greater at 293°K than at 77°K. This temperature variation is considerably reduced by the annealing treatment, which affects principally the results at 77°K. Comparison of the results of the analysis with existing theories of lattice hardening shows that there is no adequate theory of the as-irradiated condition. The results show that this is due to the presence of a spectrum of obstacles with a wide range of activation energies. At low temperatures all the obstacles are effective, whereas at high temperatures the smaller obstacles become transparent to dislocation movement. Annealing reduces the width of the dispersion, largely by removing the smaller obstacles, and the theory derived by Seeger for hardening by a single type of obstacle is then much more closely obeyed.