Deformation mechanisms during severe plastic deformation of a Cu-Ag composite

A Cu-37 at%Ag composite was produced by high-pressure torsion processing of elemental Cu and Ag powders at room temperature. The initial micrometer-sized powder particles were compressed directly in the high-pressure torsion tool and subsequently deformed to different strain levels. The microstructural evolution was studied in detail by scanning and transmission electron microscopy and synchrotron X-Ray measurements, and related to the mechanical properties by microhardness and nanoindentation measurements. The HPT process led to an alignment of Cu and Ag into a lamellar composite microstructure. With increasing applied strain the Cu and Ag lamellae were continuously thinned and simultaneously an ultrafine-grained microstructure was formed in the separate Cu and Ag lamellae. When the lamella spacing reached values lower than the respective grain sizes inside the lamellae, a further lamella thinning occurred causing a significant hardness increase of the composite. At lamella spacings below 50 nm deformation started to localize in 150-300 nm broad shear bands, which surprisingly exhibited no softening. Instead, the steady formation of new shear bands aided to transform the lamellar structure into a nanocrystalline equi-axed microstructure and additionally rotated the lamellar matrix towards the shear plane. This process led to an additional refinement of the alloy and a hardness increase until a constant hardness level was obtained. Combined analyses by synchrotron X-ray and transmission electron microscopy measurements revealed that, after reaching the saturation microhardness level, mechanical mixing of Cu and Ag occurred in the shear bands, which can be attributed to the enormous strains accommodated in the shear bands. Due to the localized deformation by shear bands, structural and chemical homogenization of the alloy was not achieved even at very high applied strains. The final microstructure was composed of nanocrystalline single-phase supersaturated regions embedded in a residual nano-lamellar matrix. (C) 2016 Elsevier B.V. All rights reserved.