Influence of Cu Content on Structure, Thermal Stability and Magnetic Properties in Fe_72-Ni₈Nb₄CuSi₂B₁₄ Alloys

The effect of substitution of Fe by Cu on the crystal structure and magnetic properties of Fe_72-Ni₈Nb₄CuSi₂B₁₄ alloys (x = 0.6, 1.1, 1.6 at.%) in the form of ribbons was investigated. The chemical composition of the materials was established on the basis of the calculated minima of thermodynamic parameters: Gibbs free energy of amorphous phase formation ΔG (minimum at 0.6 at.% of Cu) and Gibbs free energy of mixing ΔG (minimum at 1.6 at.% of Cu). The characteristic crystallization temperatures T₁ and T₁ of the alpha-iron phase together with the activation energy E for the as-spun samples were determined by differential scanning calorimetry (DSC) with a heating rate of 10-100 °C/min. In order to determine the optimal soft magnetic properties, the wound cores were subjected to a controlled isothermal annealing process in the temperature range of 340-640 °C for 20 min. Coercivity Hc, saturation induction Bs and core power losses at B = 1 T and frequency f = 50 Hz P were determined for all samples. Moreover, for the samples with the lowest Hc and P, the magnetic losses were determined in a wider frequency range 50 Hz-400 kHz. The real and imaginary parts of the magnetic permeability µ', µ″ along with the cut-off frequency were determined for the samples annealed at 360, 460, and 560 °C. The best soft magnetic properties (i.e., the lowest value of Hc and P) were observed for samples annealed at 460 °C, with Hc = 4.88-5.69 A/m, Bs = 1.18-1.24 T, P = 0.072-0.084 W/kg, µ' = 8350-10,630 and cutoff frequency at 8-9.3 × 10⁴ Hz. The structural study of as-spun and annealed ribbons was carried out using X-ray diffraction (XRD) and a transmission electron microscope (TEM).