Effect of impurities on amorphous ribbon forming and crystallization characteristics in the Fe-B-Si system

Abstract

The effect of manganese and carbon impurities on ribbon forming characteristics and crystallization behavior in Fe 80.8− x Mn x B 13.6Si 5.6− y C y , has been studied. The source of these impurities was carbothermic ferroboron, iron powder or steel. Alloy 1 ( x = 0.1, y = 0.2) gave good quality ductile ribbons of 35 ± 2 μm thickness, but Alloy 2 ( x = 0.5, y = 2), with higher manganese, gave brittle ribbons. Differential scanning calorimetric studies on amorphous ribbons at different heating rates showed two step crystallization for both alloys, i.e first α-Fe crystallizes, followed by tetragonal Fe 3B at higher temperature. These phases were confirmed by X-ray diffraction and transmission electron microscopy. The effect of manganese content on peak temperatures was studied. Higher manganese content (0.5 a/o) increased the peak temperatures for the crystallization of α-Fe at all heating rates. This trend was not observed for the second peak at heating rates of 0–20 K/min. However, heating rates exceeding 20 K/min caused the same effect as observed in α-Fe. The activation energies for α-Fe crystallization were not affected by a higher manganese content. For the crystallization of Fe 3B the activation energy was 299 kJ/mol for Alloy 1, which is lower than for 2605 SC or Alloy 2. The average and local Avrami exponents determined by isothermal annealing experiments were the same (≈2.5) in both alloys for the first-stage crystallization. The average Avrami exponent was the same for the second stage of crystallization. The local Avrami exponent, which showed nonuniformity, indicates that multiple mechanisms for crystallization are operational.