Three-Stage Hardening in Tantalum Deformed in Compression

High-purity tantalum single crystals were deformed in compression at various temperatures and strain rates. The shape of the stress/strain curve is strongly dependent on orientation, but, for certain orientations, the stress/strain curve could be divided into three stages for tests conducted over a narrow range of temperatures and strain rates. The work-hardening rates for the various stages were determined and compared with those obtained by other workers. Strain-rate-change and temperature-change experiments were evaluated to yield values of activation energy and activation volume. The values obtained were comparable with those obtained in other investigations on polycrystalline tantalum or on single crystals where three-stage hardening was not observed.