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

The microstructure of a polycrystalline Co annealed at 873 K, followed by furnace cooling to room temperature, was studied in this paper. It was found that a small amount of high temperature (above 690 K) face centered cubic (FCC) phase is retained after the slow cooling, and there is a Shoji-Nishiyama orientation relationship between the low temperature hexagonal close packed (HCP) and the retained FCC phases. Annealing twins were observed not only in FCC phase but also in HCP phase. A high fraction of special misorientation, 71.4 ∘ / 〈 1 2 ¯ 10 〉 , was identified in the HCP phase, which was correlated to the phase transformation twins.