Abstract：Two kinds of micrometer sized original particles were annealed in different conditions to prepare Co particles with different crystal structures and different particle sizes. The evolution of the crystal structures and morphologies of particles was analyzed by X-ray diffraction apparatus and scanning electron microscope. It is indicated that almost single FCC phase can be obtained if proper annealing conditions are involved during the annealing of Co particles. There is a critical size of phase stability, below which HCP-FCC transition of Co particles does not occur during cooling and high temperature phase can stably exist at room temperature. The critical size is affected by the state of surface/interface, i.e. lower surface/interface energy gives rise to a smaller critical size. In the view of the suppression of martensitic transformation by extra pressure, the effect of the particle size and the state of surface/interface on the phase stability of Co particles is discussed.
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