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Abstract A gradient-structured Cu sample composed of grain-size gradient surface layer (GSL) and homogeneous coarse-grained (CG) core was fabricated by surface severe plastic deformation. Microhardness measurements revealed that both tension-induced softening in the top nanograined surface layer and hardening in the subsurface layer of integrated gradient sample were far more pronounced than that of a freestanding GSL, i.e. extraordinary softening and hardening occurred in gradient sample. The synergetic deformation between GSL and CG core produced accumulation of geometrically necessary dislocations and thereby back stress strengthening, resulting in extra flow stress and hardening. The extraordinary softening was attributed to the extra flow stress-assisted grain coarsening in nanograined layer. Furthermore, strain measurements proved that the CG matrix plays critical roles in stabilizing the grain coarsening and homogenizing the strain distribution in nanograined layer, which were favorable for the uniform ductility. [ABSTRACT FROM AUTHOR]

The low strength of pure Ti significantly limits its biomedical applications, although its biocompatibility is the best. Here, by controlled annealing and cryogenic drawing after rolling, we synthesize a set of pure Ti combining the strength of ultrafine-grained Ti and the ductility of coarse-grained Ti. The microstructure is characterized with fine grains (a few microns), abundant twins, and high dislocation density, which enable both short dislocation-slip path for strengthening and enough defects accumulation room for work hardening. Twins provide strengthening due to the subdivision effect of twin lamella on grains and dislocations accumulation near the twin boundaries. High flow stress helps to activate FCC phase transformation at large-strain stage. The excellent ductility is originated from the sustained work hardening enabled by the initial texture, fine grains, the interaction among dislocations, FCC lamellae, and twins. [ABSTRACT FROM AUTHOR]