Plasticity and Strength of FCC Metals with a Nanotwinned Submicrograined Structure

The plasticizing and hardening effects associated with the existence of nanotwins with a density 1/lambda (where lambda is the average size (thickness) of nanotwin lamellae) in a submicrograined structure of fcc metals have been theoretically discussed in the framework of the dislocation-kinetics approach. The strength of the nanotwinned submicrocrystalline structure, which is increased as compared to the initial submicron structure, is determined, as in the case of nanograin boundaries, by the action of nanotwin boundaries as sources and barriers for moving dislocations that provide the normal Hall-Petch effect for the flow stress sigma similar to lambda(-1/2). The inverse Hall-Petch effect sigma similar to lambda(p) (where p > 0), as in the case of nanograin boundaries, is associated with the dislocation absorption by the twin boundaries. The related increased strain-rate sensitivity of the flow stresses is responsible for the significant increase in the uniform strain (from 2-3 to 8-9% in the case of nanotwinned copper) during tension of the specimens with nanotwinned submicrograined structures with retaining a high strength of the material.