Gradient plasticity in gradient nanocrystalline metals: Extra toughness from dislocation migration

Gradient nanocrystalline (GNC) metals exhibit an unprecedented combination of high strength and high ductility. In this study, GNC copper was obtained using ultrasonic nanocrystalline surface modification (UNSM), and its plasticity mechanism was investigated using tensile tests. It was found that UNSM treatment can grant copper superior yield strength and ductility which is beyond the capacity of conventional cold-working. Moreover, the UNSM-treated copper has a reduced strain-hardening capacity which does not lead to early necking and deteriorated ductility. The strain energy analysis shows that the yield strength of GNC material can be estimated using the root mean square of yield strengths of the coarse-grained (CG) layer and the gradient structure (GS) layer. The strain energy analysis also predicted a strong interaction between GS layer and CG layer during tensile deformation. Furthermore, a dislocation-based constitutive model was applied to understand the interaction between CG layer and GS layer. The migration of dislocations from the CG layer to the GS layer was proposed to explain the interaction. The migration of dislocations induces strain softening and, thus, releases internal strain energy. The proposed mechanism is supported by our experimental observations: the reduction in strainhardening capacity is positive-related to the depression in strain localization in UNSM-treated GNC copper. The improved ductility is attributed to the depressed strain localization and the refreshed capacity for dislocation accumulation, both resulted from dislocation migration from the CG layer to the GS layer.