Interplay of crystallization, stress relaxation and magnetic properties for FeCuNbSiB soft magnetic composites

• Nanocrystalline phase formation and stress relaxation achieved by annealing the FeCuNbSiB SMCs.
• Crystallization initiates with increased nucleation sites followed by existing crystal growth and coalescence.
• Stress relaxation completes with increased annealing temperature.
• Excellent magnetic performance obtained by coupling of nanocrystals and minimized anisotropy.

Fe73.5Cu1Nb3Si15.5B7 amorphous alloy was fabricated into nanocrystalline soft magnetic composites (SMCs). Crystallization process and the evolution of stress relaxation of the SMCs during annealing have been investigated and their effects on the magnetic properties are revealed. Crystallization of the amorphous alloy is accomplished by increment of nucleation sites with the annealing temperature ranged from 500 °C to 600 °C. Stress induced by milling and compaction relieves with increased annealing temperature and complete relaxation can be achieved at around 575 °C. Coupling of small grains (9.1 nm–12.8 nm) and stress relaxation ensures the FeCuNbSiB SMCs of relatively low coercivity, high permeability and low loss. Further raised annealing temperature above 600 °C leads to dominating growth and coalescence of the existing α-Fe(Si) grains and the precipitation of the Fe23B6 phase, which deteriorates the magnetic properties. Optimized soft magnetic properties of the SMCs such as small coercivity (4.5 Oe), large effective permeability (115) and low total core loss (200 mW/cm3 measured when Bm = 0.1 T and f = 50 kHz) can be achieved by post-annealing at 575 °C.