Surface crystallization of Fe-based amorphous alloys induced by Ti impurity and resulting degradation of soft magnetic properties

Fe-based amorphous alloys are renowned for their exceptional soft magnetic properties, yet their performance is highly susceptible to impurity-induced surface crystallization. This study systematically investigated the surface crystallization mechanism in a representative Fe83Si2B12P3 (at.%) amorphous alloy and the resulting degradation of soft magnetic properties. The surface microstructure of the ribbons prepared from industrial raw materials (IRM) and high-purity raw materials (PRM) at different cooling rates was first characterized. Surface crystallization is observed in IRM ribbons, characterized by < 001 > -oriented α-Fe dendritic grains confined exclusively to the ribbon's free surface. These α-Fe dendrites grow on the free surface and into the interior of the ribbons, with grain size increasing as the cooling rate decreases. Subsequently, Mn, Al, Ti, S impurities were individually added to PRM and Ti is identified as the key impurity triggering surface crystallization. Combining with the calculations of thermodynamics and crystal lattice disregistry, the results indicate that trace amounts of Ti impurity preferentially oxidize on the ribbon's free surface to form TiO2 during melt-spinning. These TiO2 inclusions then serve as potent heterogeneous nucleation sites for α-Fe due to the low lattice mismatch at the α-Fe(100)//TiO2(110) interface, leading to the surface crystallization of the IRM and PRM + Ti ribbons. This significantly increases the coercivity (Hc) and core loss (P) due to magnetic domain pinning by the coarse dendrites. These findings underscore the critical importance of controlling Ti impurity in raw materials, which is the key to suppressing surface crystallization and ensuring the production of high performance Fe-based amorphous alloys.