Stress dependence of dislocation velocity in single crystal niobium

Abstract

The stress dependence of dislocation velocity was measured by etching in niobium single crystals grown by an electron-beam floating-zone technique. The velocity-stress relation was determined with temperature, purity, and low neutron dose levels as variables. The data can be expressed by an equation of the form v ̄ = (τ ∗ τ 0) m∗ in all cases. The exponent m ∗ was found to increase slightly with decreasing temperature, increase strongly with greater impurity content, and to decrease with neutron irradiation up to 8.3 × 10 17 n/cm 2 ( E > 1 MeV). Measurements of the strain-rate sensitivity and the stress relaxation indicated that these techniques were unsatisfactory in determining the exponent m ∗ in the velocity equation. Correlations could be made between the data from etching and mechanical measurements in irradiated samples which suggest that the increase in the upper yield stress upon neutron irradiation is associated with a dynamic resistance to dislocation motion.