Effect of mechanical milling and cold pressing on co power

Abstract

Cold pressing (CP) of the amorphous-like Co powder suppressed most of the XRD peaks, in particular the peak along (100) plane. The DSC curve of unmilled CP Co powder has shown a distinct sharp exothermic peak at 615◦C. Upon annealing at 700◦C, only the FCC phase with lattice parameter of 3.51 °A was detected by XRD. Our results implied that the exotherm at 615◦C corresponds to compaction-pressure-assisted HCP to FCC first-order phase transition. The XRD analysis of 30 h milled powder revealed for the first time the FCC phase with a = 3.80 A°. However, due to presence of (100) and (210) peaks, this phase is thought to be FCT with lattice parameters a = b = 3.80 and c = 3.07 A°. Consequently, the high-energy milling carried out in the current work induced for the first time HCP to FCT transition in Co. Upon CP of milled powder, the lattice parameter a shrunk from 3.80 to 3.75 A°. However, during annealing of the CP milled Co powder at 750◦C, the FCT to FCC transition occurred, yielding the FCC phase with a = 3.51 A°.