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

Crystallization processes for the amorphous metallic alloys Fe-Si-B-Ni and Fe-B-Zr-Cu were investigated, in-situ during Joule-heating by X-rays diffraction technique 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. With increasing annealing current, the resistance initially decreased due to stress relaxation, followed by a drop to a minimum value due to massive nucleation and growth of alpha-Fe nannocrystals. Further annealing causes the formation of small fractions of Fe-B, B2Zr or ZrO2, while the resistance increased due to temperature enhancement. In-situ XRD measurements allowed the identification of metastable phases, as the gamma-Fe phase which Occurs at high temperatures. Exothermal peaks observed in the differential scanning calorimetry (DSC) for each alloy corroborate the finings. We also have performed DSC measurements at several heating rates, which allowed 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-Si-B-Ni) are comparable to the 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.