Delayed nucleation in Fe40Co40P14B6 metallic glass

Metallic glass with a nominal composition of Fe40Co40P14B6 (numbers denote at.%) was prepared by the melt-spinning process. The as-quenched FeCo-based ribbons have good soft magnetic properties: the saturation magnetic polarization μ0Ms=1.45 T, a quasistatic coercive force of 4 A m−1 and a maximum differential magnetic permeability of about 110 000 at 60 Hz. Fe40Co40P14B6 glass crystallizes by a eutectic reaction resulting in the face-centered cubic (f.c.c.) solid solution and the body-centered (b.c.) tetragonal (FeCo)3(PB) compound. The crystallization temperatures, determined using differential scanning calorimetry, are approximately 60 K higher than those for Fe40Ni40P14B6. Kolmogorov–Johnson–Mehl–Avrami model has been modified to account for temperature dependencies of nucleation and crystal growth rates and has been employed to quantify the non-isothermal glass crystallization process. Two fitting parameters: the activation energy of atomic transfer across crystal/glass interface Q and crystal/glass interfacial tension σ, which govern crystal nucleation and growth, have been calculated from the set of experimental data of crystallization temperatures at different heating rates. While the estimated crystal growth rates in Fe40Co40P14B6 and Fe40Ni40P14B6 amorphous alloys are rather close, the homogeneous nucleation frequency in the FeCo-glass was found to be essentially lower than that in the FeNi-counterpart. It was concluded that a higher value of the crystal/glass interfacial tension σ is responsible for enhanced thermal stability of the Fe40Co40P14B6 glass.