The deformation of niobium single crystals at temperatures between 77 and 4.2 K

Abstract

The plastic deformation of niobium single crystals, purified by ultra-high-vacuum annealing, has been investigated in tension at temperatures between 77. and 4.2 K. The strongly orientation-dependent behaviour has been demonstrated by studying two tensile axis orientations, C (χ=l°, λ = 47°) and H (χ = 30°, λ = 45°). C specimens deformed readily even at 4.2 K. H specimens yielded at a high stress (two to three times that for C) with a large load drop. The deformation of C specimens was dominated by the anomalous slip, while the (101) [111] primary slip system was predominantly observed in H specimens. The load drop of H specimens was not due to twinning, but rather to the formation of shear bands. It has been demonstrated that the difference observed between C and H is caused by the difference in the operating slip systems, i.e. the anomalous slip in C specimens and the primary slip in H specimens, instead of an extrinsic cause. The effect of electron irradiation was examined at 30 K. Irradiation causes a hardening effect in C specimens, while softening was observed in H specimens. These hardening/softening effects of electron irradiation are attributed to the properties of the operating slip systems, the anomalous slip in C and the primary slip in H. A model of low-temperature deformation of b.c.c. metals based on these observations can rationalize the contradicting interstitial effects reported earlier for niobium and iron.