Microstructure and mechanical properties of ultrafine grained Cu-0.8Â wt%C alloy with a bimodal microstructure produced by powder metallurgy techniques

Bulk ultrafine grained Cu-0.8 wt%C alloy samples were fabricated by spark plasma sintering (SPS) of a nanocrystalline Cu-0.8 wt%C alloy powder prepared by high energy mechanical milling. The SPS temperature was 800 °C. The SPSed samples exhibited a bimodal microstructure consisting of ultrafine and coarse Cu grains (average grain sizes: ~95 nm and ~1 µm respectively) and a yield strength of 483 MPa, but underwent a premature fracture at a stress of 511 MPa with a low elongation to fracture of only 0.3%. It is established that the premature fracture and low tensile ductility are caused by the high flow stress associated with grain boundary strengthening and reaching the strength of IPBs before reaching its maximum. Hot extrusion of the SPSed sample at 800 °C caused substantial growth of the ultrafine Cu grains and enhancement of the strength of IPBs, leading to a clearly lower tensile yield strength of 353 MPa, but mature fracture with an ultimate tensile strength of 428 MPa and a significantly improved elongation to fracture of 6.5%.