Influence of the milling time on mechanical and magnetic properties of Cu₉₀Co₅Ni₅ alloy obtained by mechanical alloying

The structure, mechanical and magnetic properties of Cu₉₀Co ₅Ni₅ alloys produced by mechanical alloying and subsequent cold consolidation and sintering behavior, have been investigated. A system of small Co and Ni magnetic particles embedded in the non-magnetic copper matrix were prepared through a mechanical milling process by using a planetary ball mill under argon atmosphere for 20 to 60 h. The morphology and particles size, phase formation and chemical composition of the alloyed powder samples for each milling time were characterized by scanning electron microscope and powder X-ray diffraction techniques, respectively. After milling for 60 h, a supersaturated solid solution with coercive field Hc with maximum value of 235Oe was obtained. The continuous decreasing trend of saturation magnetization (M_s) with increasing of milling time can be explained by the reduction of copper oxide by (CoNi) oxide formation, confirmed by powder XRD patterns. The XRD analyses of the as-milled samples revealed that the Bragg peaks of FCC-Co changed partially to HCP-Co on increasing the milling time. Cu₉₀Co₅Ni ₅ powders cold consolidated and sintered at 650°C for 1h segregated mainly into two-phases of mixed (fcc,hc)-Co and fcc-CuNi. After sintering, the mechanical properties for 60h milling reached its optimum, 26HV in hardness corresponding to 250MPa as compressive strength. TEM microanalysis of sintered alloys revealed Co cluster of 2 to 5 nm in size separated each one by 10 to 20 nm in size. The variation of magnetic properties and its dependence on structural-precipitation change with milling time are discussed.