Microstructure and mechanical properties of ultrafine-grained fcc/hcp cobalt processed by a bottom-up approach

Bulk Co samples having a mean grain size of similar to 300 nm were processed by hot isostatic pressing of a high purity Co nanopowder synthesized by chimie douce. The grain interior exhibited a highly faulted nanoscale lamellar microstructure comprising an intricate mixture of face-centered cubic, hexagonal close-packed phases and nanotwins. Room temperature compression tests carried out at a strain rate of similar to 2 x 10(-4) s(-1) revealed a yield stress of similar to 1 GPa, a strain to rupture of similar to 5%. During straining it was found that the hexagonal close-packed phase content increased from 55% to 65% suggesting a deformation mechanism based on stress-assisted face-centered cubic to hexagonal close-packed phase transformation. In addition, an apparent activation volume of similar to 3b(3) was computed which indicates that the deformation mechanism was controlled by dislocation nucleation from the numerous boundaries. Nonetheless, in such an intricate the overall mechanical properties are discussed in term of a complex interplay between microstructure, lattice dislocation plasticity, transformation-induced plasticity and possibly twin-induced plasticity. (C) 2009 Elsevier B.V. All rights reserved.