Crystallization kinetics of Fe-B based amorphous alloys studied in-situ using X-rays diffraction and differential scanning calorimetry

Abstract

The crystallization processes for the amorphous metallic alloys Fe74B17Si2Ni4Mo3 and Fe86B6Zr7Cu1 (at. %) were investigated using X-rays diffraction measurements performed in-situ during Joule-heating, with simultaneous monitoring of the electrical resistance. We determined the main structural transitions and crystalline phases formed during heating, and correlated these results to the observed resistance variations. As the annealing current is increased, the resistance shows an initial decrease due to stress relaxation, followed by a drop to a minimum value due to massive nucleation and growth of a-Fe nanocrystals. Further annealing causes the formation of small fractions of Fe-B, B2Zr or ZrO2, while the resistance increases due to temperature enhancement. In situ XRD measurements allowed the identification of metastable phases, as the g-Fe phase which occurs at high temperatures. The exothermal peaks observed in the differential scanning calorimetry (DSC) for each alloy corroborate the results. We also have performed DSC measurements with several heating rates, which allowed the determination of the Avrami exponent and crystallization activation energy for each alloy. The obtained activation energies (362 and 301 kJ/mol for Fe-B-Zr-Cu; 323 kJ/mol for Fe-B-Si-Ni-Mo) are comparable to reported values for amorphous iron alloys, while the Avrami exponent values (n = 1.0 or n = 1.2) are consistent with diffusion controlled crystallization processes with nucleation rates close to zero.