Grain refinement process in a cold-rolled polycrystalline cobalt

The microstructural evolution of a cold-rolled polycrystalline cobalt (Co) was investigated via electron backscatter diffraction, electron channeling contrast imaging and transmission electron microscope. High fraction of pre-existed phase transformation related boundaries (PBs) with misorientation of similar to 71 degrees/ < 11<(2)over bar>0>, together with deformation induced twinning and phase transformation, refine the grain size from 20 mu m in the un-deformed Co to nano scale before fracture. The grain refinement arises from the interplays among PBs, twins, and face centered cubic (FCC) plates, which is fundamentally different from grain refinement mechanisms reported in other hexagonal close-packed (HCP) materials.