Structural transition of metastable Co particles under stress

The metastability of Co fine particles with sub-micrometer and micrometer scale was investigated by cooling from high temperature and by applying external stress. The structural transition features of the Co particles were characterized by scanning electron microscope, X-ray diffractometer and vibrating sample magnetometer. It is found that during the cooling from high temperature the Co fine particles within in a wide size range keep their high temperature structure (FCC) to room temperature or cryogenic temperature. However, these metastable Co particles are very susceptible to the applied stress, and under the external stress these metastable Co particles easily transform to stable HCP structure at room temperature upon martensitic transformation induced by stress. The size effect on metastability of Co particles was discussed based on the influence of the surface tension on the martensitic transformation.