A robust phenomenological model based on a modified size-dependent Voce-type constitutive equation is proposed to describe the dependence of strength on twin thickness, for nanotwinned copper (nt-Cu) polycrystals, in agreement with experiments and related atomistic simulations. A gradient plasticity argument is employed to determine the critical nanotwin thickness where the transition from Hall–Petch (HP) hardening to inverse Hall–Petch (IHP) softening occurs. Strain rate and temperature effects are also discussed. The proposed constitutive equation may be used for engineering design purposes by controlling the interplay between grain size and twin thickness.

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