The metastability and martensitic transformation of Co particles

Co particles with different sizes and different crystal structures were prepared by annealing of loose particles as well as sintering of compact Co particles in vacuum. The allotropic transition characteristics of Co particles were investigated to determine the effect of particle size and surface/interface energy on the metastability of Co particles. It was found that there existed a critical size below which the high-temperature phase (fcc) could be achieved at room temperature. The dimension of the critical size of particles was dependent on the surface/interface energy, i.e., lower surface/interface energy gave rise to smaller critical size. In the view point of the suppression of martensitic transformation by extra pressure resulting from the surface/interface energy, the metastability of Co particles was discussed.