Microstructural evolution of a polycrystalline cobalt during tensile deformation

The microstructural evolution of cobalt during tensile deformation was investigated via electron back scatter diffraction and transmission electron microscope. The undeformed sample has an almost pure hexagonal closed-packed structure, and there exist a large number of special boundaries with a $70.5°/<11\bar{2}0>$ misorientation. Twinning dominates the tensile deformation process, especially the $\left\{10\bar{1}2\right\}$ twins. When the strain reaches 15%, the volume fraction of the twins reaches up to 61%. Meanwhile, $\left\{10\bar{1}2\right\}-\left\{10\bar{1}2\right\}$ double twinning and even $\left\{10\bar{1}2\right\}-\left\{10\bar{1}2\right\}-\left\{10\bar{1}2\right\}$ triple twinning are triggered, which work together with primary twins and martensite phase transformation to refine the grains. In addition, two different formation modes of special boundaries with misorientation of $39°/<11\bar{2}0>$ are observed. One is formed by partial dislocations slipping, and the other is formed by $\left\{10\bar{1}1\right\}-\left\{10\bar{1}2\right\}$ double twinning.