Strain-induced grain refinement of cobalt during surface mechanical attrition treatment Original Research Article

The microstructural evolution during surface mechanical attrition treatment of cobalt (a mixture of hexagonal close packed (hcp) and face-centered cubic (fcc) phases) was investigated, in order to reveal the mechanism of grain refinement and strain accommodation. The microstructure was systematically characterized by both cross-sectional and planar-view transmission electron microscopy. In the hcp phase, the process of grain refinement, accompanied by an increase in strain imposed in the surface layer, involved: (1) the onset of image deformation twinning, (2) the operation of image prismatic and image (0 0 0 1) basal slip, leading to the formation of low-angle dislocation boundaries, and (3) the successive subdivision of grains to a finer and finer scale, resulting in the formation of highly misoriented nanocrystalline grains. Moreover, the formation of nanocrystallites at the grain boundary and triple junction was also observed to occur concurrently with straining. By contrast, the fcc phase accommodated strain in a sequence as follows: (1) slip of dislocations by forming intersecting planar arrays of dislocations, (2) {1 1 1} deformation twinning, and (3) the γ(fcc) → ε(hcp) martensitic phase transformation. The mechanism of grain refinement was interpreted in terms of the structural subdivision of grains together with dynamic recrystallization occurring in the hcp phase and the γ → ε martensitic transformation in the fcc phase as well.