Anisotropy of surface oxide softening of niobium single crystals

Abstract

Oriented single crystals of high purity niobium have been prepared with square cross sections such that edge dislocation lines were parallel to one pair of faces (E faces) and screw dislocation lines were parallel to the other pair (S faces). Anodic oxidation of the crystals caused a reduction of the critical resolved shear stress and the occurrence of serrated flow in the manner reported previously (Sethi and Gibala 1975). Selective oxidation of only the E faces resulted in the same reduced flow stress as when all faces were coated, whereas oxidation of only the S faces produced little change in flow stress. These results, together with other mechanical tests and complementary slip trace and etch-pit studies, illustrate that under applied stress the oxide coatings introduce large densities of mobile edge dislocations which are responsible for the enhanced plasticity.