Examination of surface effects on low-temperature deformation of niobium

Abstract

Effects of surface orientation and surface oxide films on low-temperature deformation have been examined in thin niobium single-crystal sheets (∼200 μm thick) with two tensile axis orientations. All specimens were outgassed in UHV (residual resistivity ratio ∼2500) and some of them were coated with anodic oxide films (∼65 nm thick). Tensile tests were carried out at 77 K and at 4 × 10⁻⁴ s⁻¹. No effect of surface orientation on deformation was observed. The presence of surface oxide films caused a softening effect and nearly eliminated the strong dependence on tensile axis orientation reported previously. The observation substantiates the contention that the orientation dependence is caused by the difference in screw dislocation glide mode between the two tensile axis orientations and is not induced by the thin sheet shape of the specimens. Observations made in partially coated specimens indicate that the surface film softening is caused by a massive generation of edge dislocations at the coated surface and that the mobility of these edge dislocations is significantly higher than that of the screw dislocations.