Observation of twins in polycrystalline cobalt containing face-center-cubic and hexagonal-close-packed phases

► We have observed the microstructure of deformed and annealed Co. ► Annealing twins of Co can be formed not only in HCP phase, but also in FCC phase. ► A high fraction of special misorientation, 71.4∘/〈12¯10〉, is identified in HCP Co. ► These twins in HCP Co are phase transformation twins.