The structure and its dependence on the magnetic properties of Ni ₅Co_XCu_95-X alloys produced by mechanical alloying and subsequent annealing

Lower energy-ball milling was used to prepare magnetic granular Ni ₅Co_XCu_95-X alloys produced by mechanical alloying through a milling process and subsequent annealing process, have been investigated. The pure copper shows high electrical conductivity and malleability, however the Cu-Co system in the thermodynamic equilibrium is non-soluble below 500°C. Nevertheless, mechanical alloyed particles of Cu with 5-7%Co and 5%Ni can be subjected to annealing at 500°C or consolidation-sintering treatments to obtain composite materials thereby improving their mechanical and magnetic properties suitable for electronic devices. The ultrafine Co and (Co,Ni) particles reduced and dispersed in the copper powder matrix with milling times of 20 to 60 h and thus affected the magnetic properties of the as-milled Ni₅Co_XCu _95-X powder obtained from this nonequilibrium phases synthesis. The magnetic properties of the supersaturated solid solutions are strongly dependent on the interactions among the magnetic particles and the nanometric size of these particles. The morphology, structure and size of as-milled and sintered powders were characterized by SEM, HRTEM and XRD techniques. The results show that the microstructure, hardness and magnetic properties of the granular Ni₅Co_XCu_95-X alloy have strong dependence of milling time. The continuous decrement of M_s as a function of milling time is a consequence to the variation of phase in the composition with formation of CoNi particle and the partial change of fcc-Co to hcp-Co. Super-paramagnetic behavior is observed in both as-milled and annealed powders, with a maximum H_c of 250-260 Oe obtained for 7%Co after 60h of milling. The effect of Nickel on the Ni₅Co_XCu _95-X can be explained as Ni content inhibit the two-solid (Cu-Co) phases segregation of the alloys when annealed at high temperature, leading to a grained structure with precipitated Co particles in homogeneous Cu-Ni strengthened solid solution matrix.