Abstract: Phase transition of cobalt induced by mechanical milling and high pressure and its transition mechanisms were studied. α-Co with the hcp. structure completely transformed to β-Co with the fcc. structure after milling for 60 h due to the stacking fault and local temperature resulted from impact and shear force of steel balls during the milling process. However, the reverse transition from β-Co to α-Co was not observed, even milling up to 120 h.The reverse transition was studied under high pressure, and it was found that pressure can accelerate the transition from β-Co to α-Co, which is due to that this transformation is a diffusionless one and its thermodynamic barrier decreases by pressuring.

Key words: phase transition, Co, high pressure, mechanical milling