The effect of Cu addition on the microstructure and low-temperature magnetization of nanocrystalline Fe_87−xZr₇B₆Cu_x and Fe_81−xZr₇B₁₂Cu_x alloys

The microstructure and low-temperature magnetization of nanocrystalline Fe_87−xZr₇B₆Cu_x and Fe_81−xZr₇B₁₂Cu_x alloys (x = 0,1 and 2 at.%) have been studied using a combination of differential scanning calorimetry, x-ray diffraction analysis, transmission electron microscopy and superconducting quantum interference device magnetometry. The results show that the substitution of 1 at.% copper for iron in both low- and high-boron-content alloys has significant effects on the microstructure and magnetic properties, while increasing the copper content to 2 at.% does not cause any further significant change in the microstructure and magnetic properties. For the low-boron-content alloys, the addition of 1 at.% copper promotes a uniform distribution of α-Fe grains, decreases the grain size without changing the phase structure, and decreases the exchange stiffness constant from 180±6 to 122±2 meV Å². For the high-boron-content alloys with the addition of 1 at.% copper, the constitutional crystalline phase changes from a metastable phase with the cubic `Fe₁₂Si₂ZrB' structure to the equilibrium phase α-Fe. For the nanocrystalline Fe₈₁Zr₇B₁₂ alloy with the metastable phase after being heated to 893 K at 20 K min⁻¹, the saturation magnetization σ_s(0) = 115±1 emu g⁻¹, and the exchange stiffness constant D = 86±2 meV Å², confirming that the metastable phase is a weaker ferromagnetic phase.