Heating rate dependence of coercivity and microstructure of Fe–B–P–Cu nanocrystalline soft magnetic materials

Highlights: • P-rich ribbon has smaller heating rate dependence of coercivity than B-rich ribbon. • Nano-analyses of Fe–B–P–Cu ribbons are investigated using TEM and APT. • Alloy compositions of phases determined by APT fairly agree with calculated values. • Number of Cu clusters that can nucleate α-Fe is larger in the P-rich alloy. • P-rich interface suggests P diffusion in amorphous phase control growth of α-Fe. -- Abstract: The nanocrystalline structure and soft magnetic properties of melt-spun Fe–B–P–Cu ribbons are largely influenced by heating rates for crystallization of amorphous precursors. In this study, we investigated the structure and soft magnetic properties of Fe_84.8B_4.9P_9.5Cu_0.8 (P-rich) and Fe_84.8B_10.9P_3.5Cu_0.8 (B-rich) melt-spun ribbons crystallized at two different heating rates, 0.67 K/s and 6.7 K/s, using transmission electron microscopy (TEM) and atom probe tomography (APT). The P-rich ribbon shows smaller coercivity regardless of the heating rates, while the B-rich ribbon shows large heating rate dependence of the coercivity. APT analyses have revealed that the size of Cu clusters in the P-rich nanocrystalline ribbon is larger than that in the B-rich nanocrystalline ribbon while their number densities are nearly the same. Also, the high heating rate led to a larger size and higher Cu concentration of the Cu clusters in both samples, indicating that the Cu clusters are effective as nuclei for α-Fe only when they are larger than a critical size. The solute partitioning behaviors between α-Fe and residual amorphous phase determined by APT analyses are consistent with the tie-lines between α-Fe and liquid phase in a calculated Fe–P–B ternary phase diagram. P was found to segregate at amorphous/α-Fe interface, suggesting the grain growth is controlled by the volume diffusion of P in the amorphous phase during their crystallization process.