RADIATION HARDENING IN SINGLE CRYSTAL NIOBIUM - TEMPERATURE DEPENDENCE OF YIELDING

Neutron-irradiated single crystals of niobium were tensile tested over a range of temperatures from 93 to 298°K. The yield and flow stresses were increased upon irradiation, and the rate of work hardening, the uniform strain, and the fracture strain were reduced. Slip markings on polished surfaces were coarser and more inhomogeneous for the irradiated samples. The increase in the critical shear stress upon irradiation was approximately the same over the entire range of test temperatures, with perhaps a slight tendency for greater hardening at lower temperatures. The shape of the stress-temperature (τ vs. T) curve was analyzed according to several expressions discussed in the literature. A reasonably good fit to the Fleischer expression, (τ/τ0) 1 2 = 1 - (T/T0) 1 2, was obtained, but an equally good fit was found to the simpler relation (τ/τ0) = 1 + (T0/T). It is suggested that radiation-produced obstacles exert long-range stresses, giving rise to an athermal inhibition to dislocation motion. A simple analytical treatment is presented that illustrates conditions under which the radiation-hardening is expected to be athermal. © 1969.