Comparisons between homogeneous boundaries and heterophase interfaces in plastic deformation: Nanostructured Cu micropillars vs. nanolayered Cu-based micropillars

Both the homogeneous boundaries and the heterophase interfaces play important roles in crystalline plasticity as they often serve as obstacles for dislocation motion, as well as dislocation sources/sinks. In this work, microcompression tests were carefully performed to explicitly identify the relevant plasticity mechanisms of nanostructured micropillars with five distinct nanostructures: (i) Cu nanotwinned multicrystalline micropillars; (ii) Cu nanocrystalline multicrystalline micropillars; (iii) Cu/X (X = Cr, Zr) nanotwinned nanolayered micropillars; (iv) Cu/X nanocrystalline nanolayered micropillars; and (v) Cu/Cu–Zr crystalline/amorphous nanolayered micropillars. By characterizing their stress–strain response and evolution of strain-rate sensitivity (SRS) with strain, our findings elucidate the effects of homogeneous boundaries, heterophase interfaces and their coupling effects on the plastic yield, and reveal the fundamentally different roles these perform in the rate-limiting process of nanomaterials. In sharp contrast to the normal strain-dependent SRS in nanostructured Cu micropillars with homogeneous boundaries that monotonically decreases with increasing strain, nanolayered Cu-based micropillars with heterophase interfaces exhibit inverse strain-dependent SRS that monotonically increases with increasing strain. These expected (normal) and unexpected (inverse) SRSs are quantitatively explained by a dislocation model in terms of the strain-related dislocation mean free path. These findings provide valuable insights into our understanding of the fundamental roles that homogeneous boundaries and heterophase interfaces play in plastic deformation.