Plastic deformation induced strong and stable nanograined face-centered cubic Co

Face-centered cubic (FCC) Co has been confirmed to exhibit excellent magnetic and catalytic properties as well as mechanical properties. However, its lattice structure is metastable at room temperature. In the present study, by using an accumulative high-pressure torsion treatment, a strong and stable nanograined FCC Co was generated through allotropic transformation coupled with mechanical relaxation processes. As the grain size of Co was refined to about 11 nm, HCP-FCC phase transformation was activated and formation of almost 100% nanograined FCC Co. This phase transformation was through twin faults generation on every (0001) plane continuously. The nanograined FCC Co shows superior thermal stability both in grain size and lattice structure. After annealing at 973 K for 30 min, no obvious grain coarsening or reverse phase transformation was observed. High-density stacking faults and twins in the nanograined FCC Co were formed by emitting partial dislocations from grain boundaries (GBs), which may relax the GBs and improve the thermal stability. The nanograined FCC Co with an average grain size of 11 nm exhibited a hardness of 7.98 GPa, showing a good combination of high strength and excellent thermal stability.