Microstructural effects and large microhardness in cobalt processed by high pressure torsion consolidation of ball milled powders

The effects of plastic deformation on Co microstructure after a high pressure torsion (HPT) process on unmilled and ball milled powders have been studied and compared to those induced simply by ball milling (BM). X-ray diffraction analyses reveal that both processing routes generate large amounts of stacking faults in Co, which result in a mixture of hcp and fcc phases. However, significant differences in the amount and type of faults accumulated are encountered between BM and HPT. The combination of both processing routes induces a heavily distorted microstructure in Co. In particular, BM+HPT brings about a drastic reduction in crystallite size. Remarkably, high values of microhardness (in excess of H V=7.3 GPa) are obtained in foils prepared by long-term milling followed by HPT. It is proposed that, in addition to the crystallite size refinement, the large amounts of twin faults, together with the two-phase mixture, also contribute to enhance the microhardness.