Experiments on the deformation of niobium single crystals. I. Stress versus strain curves and slip systems in compression and tension

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Abstract
Single crystals of niobium were purified by heating in high vacuum and then tested in tension or compression at various temperatures. Three-stage hardening curves were found at room temperature for single slip orientations, irrespective of whether the primary slip plane was of {101} or {211} type. An initial rapid hardening (stage 0) was also found for {101} slip, and was often associated with low or negative stage I hardening rates. Microstrain measurements showed that the much less pronounced stage 0 observed for {211} slip was mainly attributable to the initial build-up of plastic strain rate caused by machine softness. Another difference between {101} and {211} slip was the latent hardening on the conjugate slip plane, which caused large overshoot in tension when primary slip was on {101}, but not on {211}. At 158°K. stress versus strain curves were parabolic, except for ideal {211} single slip orientations tested in compression. At still lower temperatures, most crystals yielded by twinning (especially in compression) unless first pre-strained above 90°K.