An investigation of bulk nanocrystalline copper fabricated via severe plastic deformation and nanoparticle consolidation

Abstract

Ultrafine grained (UFG) and nanocrystalline materials have attracted considerable interest because of their unique mechanical properties as compared with coarse grained conventional materials. The fabrication of relatively large amounts of these materials still remains a challenge, and a thorough understanding of the relationship between microstructure and mechanical properties is lacking. The objective of this study was to investigate the mechanical properties of UFG and nanocrystalline copper obtained respectively by a top down approach of severe plastic deformation of wrought copper and a bottom up approach of consolidation of copper nanoparticles using equal channel angular extrusion (ECAE). A critical assessment and correlation of the mechanical behavior of ECAE processed materials to the microstructure was established through the determination of the effect of strain level and strain path on the evolution of strength, ductility and yield anisotropy in UFG oxygen free high conductivity copper in correlation with grain size, grain morphology and texture. ECAE was shown to be a viable method to fabricate relatively large nanocrystalline consolidates with excellent mechanical properties. Tensile strengths as high as 790 MPa and fracture strain of 7 % were achieved for consolidated 130nm copper powder. The effects of extrusion route, number of passes and extrusion rate on consolidation performance were evaluated. The relatively large strain observed was attributed to the bimodal grain size distribution and accommodation by large grains. The formation of bimodal grain size distribution also explains the simultaneous increase in strength and ductility of ECAE processed wrought Cu with number of passes. Texture alone cannot explain the mechanical anisotropy in UFG wrought copper but we showed that grain morphology has a strong impact and competes with texture and grain refinement in controlling the resulting yield strength. Tension-compression asymmetry was observed in UFG wrought copper. This asymmetry is not always in favor of compression as reported in literature, and is also influenced by grain morphology through the interaction of dislocations with grain boundaries. Different prestrains in tension and compression should be experimented to have a better understanding of the encountered anisotropy in Bauschinger parameter in relation with the observed tension-compression asymmetry.