Abstract
A combination of thermogravimetry, scanning and transmission electron microscopy, electron probe microanalysis and differential scanning calorimetry has been used to investigate the oxidation kinetics, and oxide morphology, structure and composition in amorphous and crystalline Fe78Si9B13 alloys. Kinetic data indicate that the oxidation reactions of both amorphous and crystalline Fe78Si9B13 obey a parabolic rate law over the temperature range 300 to 450°C with activation energies of 120 and 86 kJ/mol respectively, indicating that grain boundary diffusion is probably the rate controlling process. The parabolic rate constant for oxidation of crystalline Fe78Si9B13 is consistently higher than for amorphous Fe78Si9B13 over the temperature range 300–450°C, so that the amorphous alloy always shows a better oxidation resistance. Electron microscopy and electron probe microanalysis show that the oxide scales formed on both amorphous and crystalline Fe78Si9B13 consist of SiO2, Fe3O4 and Fe2O3, but the detailed microstructure and compositions are different. The oxide scale formed on amorphous Fe78Si9B13 contains more SiO2 and has a small particle size, while the oxide scale formed on crystalline Fe78Si9B13 contains more Fe3O4 and consists of larger particles. The difference in oxide growth between amorphous and crystalline Fe78Si9B13 is caused by the difference in alloy microstructure.
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