Characterization of deformation twinning in polycrystalline cobalt: A quantitative analysis

Electron backscattering diffraction technique was used for a statistical analysis of twin formation in deformed polycrystalline cobalt. Deformation microstructure of samples plastically strained after a monotonous tensile test was studied by scanning and transmission electron microscopy. The analysis shows that the principal twin mode is the tension {10 (1) over bar2} type. Multiple twin variants of this mode are present inside a grain. The higher occurrence corresponds to the variant with the misorientation of about 60 degrees giving the best strain accommodation. It was found that besides grains favorably oriented, which are more prone to twin, there is also twinning in some grains not favorably oriented, which points out an influence of the local state of internal stress. Grain orientation has a more pronounced impact on twinning characteristics than the grain size. Favorably oriented grains exhibit a strong and positive correlation of the twin frequency, the twin area fraction, and the presence of multi-variant twins in a grain. On the other hand, large grains are more prone to twin, exhibit several twin variants but small twin area fraction.