Influence of Cu Content on Structure, Thermal Stability and Magnetic Properties in FeNiNbCuSiB Alloys.

The effect of substitution of Fe by Cu on the crystal structure and magnetic properties of FeNiNbCuSiB alloys ( = 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 Δ (minimum at 0.6 at.% of Cu) and Gibbs free energy of mixing Δ (minimum at 1.6 at.% of Cu). The characteristic crystallization temperatures and of the alpha-iron phase together with the activation energy 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 , saturation induction and core power losses at B = 1 T and frequency f = 50 Hz were determined for all samples. Moreover, for the samples with the lowest and , the magnetic losses were determined in a wider frequency range 50 Hz-400 kHz. The real and imaginary parts of the magnetic permeability ', ″ 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 and ) were observed for samples annealed at 460 °C, with = 4.88-5.69 A/m, = 1.18-1.24 T, = 0.072-0.084 W/kg, ' = 8350-10,630 and cutoff frequency at 8-9.3 × 10 Hz. The structural study of as-spun and annealed ribbons was carried out using X-ray diffraction (XRD) and a transmission electron microscope (TEM).