Crystallization kinetics of Fe–B–Si metallic glasses

Abstract

The crystallization processes for the amorphous metallic alloys Fe 78B 13Si 9 and Fe 81B 13.5Si 3.5C 2 (at.%) were investigated using X-ray diffraction measurements performed in situ during Joule heating, with simultaneous monitoring of the electrical resistance. We determined the main crystalline phases formed during heating, and correlated these results to the electrical resistance variations. As the annealing current is increased, the resistance shows an initial decrease, followed by a decrease to a minimum due to nucleation and growth of α-Fe and FeB phases. In situ diffraction measurements also showed a metastable γ-Fe phase, which occurs at higher temperatures. The exothermal peaks observed in the differential scanning calorimetry (DSC) corroborate the results. We have performed DSC measurements with several heating rates for each alloy, from which we determined the Avrami exponents and crystallization activation energies. The obtained activation energies (326±5 and 329±5 kJ/mol for Fe 78B 13Si 9; 346±13, 356±11 and 558 ± 20 kJ/mol for Fe 81B 13.5Si 3.5C 2) are comparable to reported energies for amorphous iron alloys. The Avrami exponents for the Fe 81B 13.5Si 3.5C 2 alloy ( n 1=1.3±0.1, n 2=1.5±0.1, n 3=1.3±0.2) are consistent with diffusion controlled growth of particles of appreciable initial volume (for the first and third reactions) and 3-d growth of small particles with nucleation rate close to zero (for the second reaction). The n 1=1.6±0.2 and n 2=2.0±0.1 obtained for Fe 78B 13Si 9 reveal diffusion controlled growth of small particles with decreasing nucleation rates.