Microstructure and mechanical properties of bulk highly faulted fcc/hcp nanostructured cobalt microstructures

Abstract

Nanostructured cobalt powders with an average particle size of 50nm were synthesized using a polyol method and subsequently consolidated by spark plasma sintering (SPS). SPS experiments performed at 650°C with sintering times ranging from 5 to 45min under a pressure of 100MPa, yielded to dense bulk nanostructured cobalt (relative density greater than 97%). X-ray diffraction patterns of the as-prepared powders showed only a face centered cubic (fcc) crystalline phase, whereas the consolidated samples exhibited a mixture of both fcc and hexagonal close packed (hcp) phases. Transmission electron microscopy observations revealed a lamellar substructure with a high density of nanotwins and stacking faults in every grain of the sintered samples. Room temperature compression tests, carried out at a strain rate of 10−3s−1, yielded to highest strain to fracture values of up to 5% for sample of holding time of 15min, which exhibited a yield strength of 1440MPa, an ultimate strength as high as 1740MPa and a Young's modulus of 205GPa. The modulus of elasticity obtained from the nanoindentation tests, ranges from 181 to 218GPa. The lowest modulus value of 181GPa was obtained for the sample with the highest sintering time (45min), which could be related to mass density loss as a consequence of trapped gases releasing.