A detailed study of the deformation of high purity niobium single crystals

Abstract

A study has been made of the stress-strain behaviour, secondary slip geometry and slip line morphology of niobium single crystals purified by annealing in ultra-high vacuum. Within favourable limits of orientation, temperature and strain rate the stress—strain curves exhibit well-defined three-stage hardening. The principal secondary slip system is not usually that bearing the highest proportion of the applied stress and this anomalous behaviour can account for the orientation dependence of overshoot. For temperatures below 250°K the stress-strain curve is non-linear and the initial hardening rate very high. The slip lines observed on crystals deformed at low temperatures belong to a {110} ⟨111⟩ system which, in extreme orientations, is only the sixth most highly stressed system. The slip appears as crystallographic lamellae, in contrast to the wavy slip observed for all other systems.