Mechanical alloying of Fe-Ni based nanostructured magnetic materials

Alloys with the composition Fe₄₀Ni₃₈B ₁₈Mo₄, Fe₄₉Ni₄₆Mo₅ and Fe₄₂Ni₄₀B₁₈ were processed from elemental powders by mechanical alloying and the microstructure was studied by differential scanning calorimetry (DSC), X-ray diffractometer (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. Nanocrystalline fcc solid solution was achieved as a result of mechanical alloying in all three alloy compositions and the grain size reduced to nanoscale but lattice strain was introduced. Molybdenum was found to affect the products of mechanical alloying, specifically, the Fe₃B phase formed in the Fe₄₂Ni₄₀B₁₈ alloy while no boride phase formed in the Fe₄₀Ni₃₈B₁₈Mo₄ alloy. SEM studies indicated that the presence of boron was found to make the milling process easier. Elemental mapping by SEM as well as XRD results showed that molybdenum does not dissolve easily in the Fe-Ni solid solution produced by milling. The DSC results suggested that an amorphous structure together with nanocrystals was obtained in the Fe₄₀Ni₃₈B ₁₈Mo₄ and Fe₄₂Ni₄₀B₁₈ alloys. A two-stage crystallization process was found in the Fe ₄₀Ni₃₈B₁₈Mo₄ and Fe ₄₂Ni₄₀B₁₈ alloys, the presence of boron was found to make amorphization easier. TEM investigations were consistent with these XRD and DSC results. Heat-treated samples of the Fe₄₀Ni ₃₈B₁₈Mo₄ and Fe₄₂Ni ₄₀B₁₈ alloys milled for 100 h showed that molybdenum inhibited the grain growth. The saturation magnetization of the heat-treated Fe₄₀Ni₃₈B₁₈Mo₄ alloy milled for 100 h was stable, coercivity was reduced; on the other hand, the M_s of heat-treated Fe₄₂Ni₄₀B₁₈ alloy milled for 100 h decreased and the H_c increased. This difference in magnetic behavior is due to the alloying addition of molybdenum which affected the microstructural evolution during heat treatment, specifically by inhibition of the increase in grain size.