EFFECT OF IMPURITIES ON AMORPHOUS RIBBON FORMING AND CRYSTALLIZATION CHARACTERISTICS IN THE FE-B-SI SYSTEM

The effect of manganese and carbon impurities on ribbon forming charac teristics and crystallization behavior in Fe80.8 - xMnxB13.6Si5.6 - yC y has been studied. The source of these impurities was carbothermic fe rroboron, iron powder or steel. Alloy 1 (x = 0.1, y = 0.2) gave good q uality ductile ribbons of 35 +/- 2 mu m thickness, but Alloy 2 (x = 0. 5, y = 2), with higher manganese, gave brittle ribbons. Differential s canning calorimetric studies on amorphous ribbons at different heating rates showed two-step crystallization for both alloys, i.e first alph a-Fe crystallizes, followed by tetragonal Fe3B at higher temperature. These phases were confirmed by X-ray diffraction and transmission elec tron microscopy. The effect of manganese content on peak temperatures was studied. Higher manganese content (0.5 a/o) increased the peak tem peratures for the crystallization of alpha-Fe at all heating rates. Th is 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 ef fect as observed in alpha-Fe. The activation energies for alpha-Fe cry stallization were not affected by a higher manganese content. For the crystallization of Fe3B 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 t he same (approximate to 2.5) in both alloys for the first-stage crysta llization. The average Avrami exponent was the same for the second sta ge of crystallization. The local Avrami exponent, which showed nonunif ormity, indicates that multiple mechanisms for crystallization are ope rational.