New insight for mechanical properties of metals processed by severe plastic deformation

The steady state is expected when applying severe plastic deformation (SPD) techniques to process metals. In this study, we propose a method to study the steady state regarding the microstructural features and the mechanical properties, which are verified by the experimental results. More importantly, the initial state of as-prepared metal depending mainly on the processing conditions is critical for the subsequent constitutive relations. Within the framework of dislocation-based crystal plasticity, it is revealed that the dislocation density and flow stress will not change during further deformation when the initial dislocation density is equal to the steady-state value under the loading conditions. In addition, if the initial dislocation density is larger or smaller than the steady-state value, then the dislocation density and flow stress will decrease or increase with increasing strain, and tend to approach the steady-state value. These mechanisms provide not only a physical interpretation for various experimental observations such as near-perfect elastoplasticity, strain hardening and strain softening, but also insightful guidance for investigating the strengthening mechanisms in gradient nano-grained metal produced by SPD.