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
