Fe-Co-B Soft Magnetic Ribbons: Crystallization Process, Microstructure and Coercivity

In this work, a detailed microstructural investigation of as-melt-spun and heat-treated Fe67Co20B13 ribbons was performed. The as-melt-spun ribbon was predominantly amorphous at room temperature. Subsequent heating demonstrated an amorphous to crystalline alpha-(Fe,Co) phase transition at 403 degrees C. In situ transmission electron microscopy observations, carried out at the temperature range of 25-500 degrees C and with the heating rate of 200 degrees C/min, showed that the first crystallized nuclei appeared at a temperature close to 370 degrees C. With a further increase of temperature, the volume of alpha-(Fe,Co) crystallites considerably increased. Moreover, the results showed that a heating rate of 200 degrees C/min provides for a fine and homogenous microstructure with the alpha-(Fe,Co) crystallites size three times smaller than when the ribbon is heated at 20 degrees C/min. The next step of this research concerned the influence of both the annealing time and temperature on the microstructure and coercivity of the ribbons. It was shown that annealing at 485 degrees C for a shorter time (2 s) led to materials with homogenous distribution of alpha-(Fe,Co) crystallites with a mean size of 30 nm dispersed in the residual amorphous matrix. This was reflected in the coercivity (20.5 A/m), which significantly depended on the volume fraction of crystallites, their size, and distribution.