Effect of Mechanical Milling and Cold Pressing on Co Powder

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 Å 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 =3.80 Å. However, due to presence of (100) and (210) peaks, this phase is thought to be FCT with lattice parameters ==3.80 and =3.07 Å. 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 shrunk from 3.80 to 3.75 Å. However, during annealing of the CP milled Co powder at 750°C, the FCT to FCC transition occurred, yielding the FCC phase with =3.51 Å.