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

Journal of alloys and compounds 695, 2285 - 2294 (2017). doi:10.1016/j.jallcom.2016.11.085
A Cu-37 at%Ag composite was produced by high-pressure torsion processing of elemental Cu and Agpowders at room temperature. The initial micrometer-sized powder particles were compressed directlyin the high-pressure torsion tool and subsequently deformed to different strain levels. The microstructuralevolution was studied in detail by scanning and transmission electron microscopy and synchrotronX-Ray measurements, and related to the mechanical properties by microhardness and nanoindentationmeasurements. 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 andsimultaneously 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, afurther lamella thinning occurred causing a significant hardness increase of the composite. At lamellaspacings below 50 nm deformation started to localize in 150e300 nm broad shear bands, which surprisinglyexhibited no softening. Instead, the steady formation of new shear bands aided to transform thelamellar structure into a nanocrystalline equi-axed microstructure and additionally rotated the lamellarmatrix towards the shear plane. This process led to an additional refinement of the alloy and a hardnessincrease until a constant hardness level was obtained. Combined analyses by synchrotron X-ray andtransmission electron microscopy measurements revealed that, after reaching the saturation microhardnesslevel, mechanical mixing of Cu and Ag occurred in the shear bands, which can be attributed tothe enormous strains accommodated in the shear bands. Due to the localized deformation by shearbands, structural and chemical homogenization of the alloy was not achieved even at very high appliedstrains. The final microstructure was composed of nanocrystalline single-phase supersaturated regionsembedded in a residual nano-lamellar matrix.
Published by Elsevier, Lausanne