Tailoring deformation-induced effects in Co powders by milling them with α–Al2O3

The evolution of the structural and magnetic properties of metal-ceramic, cermet, nanocomposite powders, consisting of Co and α–Al2O3 in different proportions, prepared by ball milling has been investigated. The overall microstructure of the system, after long-term milling, is found to be very sensitive to the amount of α–Al2O3, yielding a less refined and faulted hexagonal-close-packed (hcp)-Co structure for the sample with larger α–Al2O3 percentage. The increased presence of the ceramic counterpart also causes a delay of the face-centered-cubic (fcc) to hcp-Co stress-induced transformation during ball milling. The results seem to indicate an evolution of the role of α–Al2O3, from increasing locally the strain rate of the mechanical work for small amounts of ceramic to absorbing milling energy for large amounts of α–Al2O3. The magnetic properties correlate with the obtained microstructure, where the amount of hcp-Co and stacking faults and the isolation of the Co particles by the α–Al2O3 control the coercivity.