Origin of misorientations for fcc→hcp transformation in pure cobalt and its in-situ validation

The origin of the misorientations after fcc (face-centered cubic) to hcp (hexagonal close-packed) transformation in pure cobalt was elucidated by utilizing the electron backscatter diffraction (EBSD) technique and transformation crystallographic models. It is found the Shoji−Nishiyama orientation relationship during fcc→hcp transformation leads to four hcp variants, characterized by a common misorientation angle of 70.5° with respect to the direction, which is the predominant misorientation observed. Other statistically significant misorientation angles between hcp grains, including 32°, 36°, 38°, 60°, 71° and 86°−91°, are also identified. These newly observed misorientation angles are linked to the microstructure of the fcc matrix at elevated temperatures, with twin structures in the fcc matrix being the primary cause. Furthermore, a novel method is proposed for estimating the fraction of twins in the fcc grains based on misorientation angles between hcp variants, which is found to be consistent with experimental observations. In-situ EBSD observations validate the possible origin of fcc twins from the hcp→fcc transformation.