Microstructure evolution and mechanical behavior of bulk copper obtained by consolidation of micro- and nanopowders using equal-channel angular extrusion

The consolidation of copper micro- and nanoparticles (325 mesh, 130 nm, and 100 nm) was performed using room-temperature equal-channel angular extrusion (ECAE). The effects of extrusion route, number of passes, and extrusion rate on consolidation performance were evaluated. The evolution of the microstructure and the mechanical behavior of the consolidates were investigated and related to the processing route. Possible deformation mechanisms are proposed and compared to those in ECAE-processed bulk Cu. A combined high ultimate tensile stress (470 MPa) and ductility (∼20 pct tensile fracture strain) with near-elasto-plastic behavior was observed in consolidated 325-mesh Cu powder. On the other hand, early plastic instability took place, leading to a continuous softening in flow stress of bulk ECAE-processed copper. Increases in both strength and ductility were evident with an increasing number of passes in the bulk samples, which appears to be inconsistent with grain-boundary-moderated deformation mechanisms for a microstructure with an average grain size of 300 to 500 nm. Instead, this increase is attributed to microstructural refinement and to dynamic recovery and bimodal grain-size distribution. Near-perfect elastoplasticity in consolidated 325-mesh Cu powder is explained by a combined effect of strain hardening accommodated by large grains in the bimodal structure and softening caused by recovery mechanisms. Compressive strengths as high as 760 MPa were achieved in consolidated 130-nm copper powder. Although premature failure occurred during tensile loading in 130-nm consolidated powder, the fracture strength was still about 730 MPa. The present study shows that ECAE consolidation of nanoparticles opens a new possibility for the study of mechanical behavior of bulk nanocrystalline (NC) materials, as well as offering a new class of bulk materials for practical engineering applications.