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

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.