Surface oxidation of the Fe based amorphous ribbon annealed at temperatures below the glass transition temperature

The Fe based amorphous ribbons with the nominal compositions Fe_84.2-xC_11.0Si_xB_3.9P_0.9(x = 4.5 and 13.5 at.%) were oxidized at low temperatures of 300 and 400 °C (i.e., well below the crystallization temperature) for 24 h in the air atmosphere. Significantly different oxidation kinetics between surfaces of the wheel side and the air side were observed, respectively. A continuous amorphous Si oxide layer with a few nanometers in thickness was formed on the air side surface of as-spun ribbon and effectively protected the ribbon from oxidation during annealing. However, discontinuity of the amorphous Si oxide layer was observed near casting defects on the wheel side surface. Moreover, due to low cooling rate near the casting defect regions, nanocrystals can be formed resulting in enhanced oxidation kinetics of the wheel side surface. As the annealing temperature is increased to 400 °C, the air side surface exhibited deteriorated oxidation resistance attributed to partial nanocrystallization of α-Fe. Nanocrystals of α-Fe formed on the surface acted as effective nucleation sites for the formation of iron oxides. Increasing annealing temperature leads to the increased thickness of both amorphous and crystalline oxide layers. However, effects of surface casting defects on the oxidation resistance can be significantly reduced by increasing the Si content.