The effect of heat treatment and surface treatment on the crystallisation behaviour of amorphous Fe40Ni40B20

Abstract

The effects of various pre-annealing and surface polishing treatments on the crystallisation behaviour of amorphous Fe₄₀Ni₄₀B₂₀ have been investigated by a combination of differential scanning calorimetry, optical and transmission electron microscopy, and X-ray diffractometry. The results show that the crystallisation kinetics are not significantly affected by either pre-annealing or mechanical polishing alone. However, certain combinations of pre-annealing and surface polishing, typically pre-annealing at 350°C for 20 h followed by mechanical polishing with 1200 SiC, change the crystallisation process from bulk nucleation and near-spherical growth to surface nucleation and planar inward growth, with a corresponding sharp increase in the overall crystallisation rate, and a modification in the crystallisation rate law from Y = 1 − exp(− Kt³) to Y = K't. For both surface and bulk nucleation crystallisation mechanisms, transmission electron microscopy and X-ray diffractometry show a eutectic crystal structure, consisting of a fine scale lamellar mixture of orthorhombic (Fe, Ni)₃B and f.c.c. (Fe, Ni). The increase in the overall crystallisation rate after pre-annealing and surface polishing is caused by a much higher number of nuclei at the surface compared to the bulk, with little change in the eutectic crystal growth rate. At 390°C, the nucleation density is $\text{≈7 ×}\text{10}{}^{13}\text{m}{}^3$ for bulk nucleation and $\text{≈10}{}^{13}\text{m}{}^2$ for surface nucleation, while the eutectic crystal growth rate is ≈5 μm/s in both cases. The growth rate constants obey the Arrhenius law over the temperature range 381–396°C with an activation energy of ≈3.0 eV/atom. Surface nucleation of the crystallisation process in amorphous Fe₄₀Ni₄₀B₂₀ can be stimulated either by mechanical deformation of B-rich amorphous particles in the phase-separated, relaxed amorphous alloy structure, or by Ni enrichment at the amorphous alloy surface.