Modeling grain size dependent optimal twin spacing for achieving ultimate high strength and related high ductility in nanotwinned metals

We have developed a mechanism-based plasticity model of nanotwinned metals to investigate the effect of twin spacing on strength, ductility and work hardening rate of such materials. In particular, the unique roles of dislocation pile-up zones near twin and grain boundaries, as well as twinning partial dislocations, in strengthening and work hardening are incorporated in the model. Competition between different local failure mechanisms associated with twin lamellae and/or grain boundaries is considered in evaluating the tensile ductility of nanotwinned metals. The present study provides a quantitative continuum plasticity model capable of describing the variations in strength, ductility and work hardening rate of nanotwinned metals with the twin spacing. For nanotwinned copper a grain size of 500 nm, the model predicts a critical twin spacing for the maximum strength at 13 nm, in excellent agreement with experimental observations. The critical twin spacing is found to be linearly proportional to the grain size, which is consistent with recent molecular dynamics simulations.