Broad crystallization window and high thermal stability of FeCoNiBCuSiNb nanocrystalline alloy with good magnetic softness

Doping with large-sized atoms plays a critical role in stabilizing the thermal processing window and regulating microstructural evolution. In this work, the effects of Nb doping on the thermal stability, crystallization window, and magnetic properties of (Fe 0.8 Co 0.2 ) 84-x Ni 2 B 12.5 Cu 1 Si 0.5 Nb x (x = 0, 1, 2, 3, denoted as Nb0, Nb1, Nb2, and Nb3) alloys were investigated. Compared with the Nb0 alloy, 3 at.% Nb doping markedly elevates both the first and second crystallization onset temperatures, yielding an exceptionally broad crystallization window (Δ T x ) of 207 °C. Thermodynamic analysis reveals that Nb3 alloy possesses high nucleation and growth energy barriers, which strongly facilitate the excellent thermal stability of the amorphous matrix. As a result, the Nb3 alloy subjected to annealing at 500 °C for 30 min exhibits a stable and uniform microstructure with sizes of α-Fe(Co) nanograins ranging from 20 to 44 nm and an average diameter of 29.3 nm. This microstructure ultimately yields a relatively low coercivity of 17.1 A/m while maintaining a saturation magnetization of 1.58 T. This combination of high thermal stability, a large Δ T x , and good magnetic performance underscores its strong potential for industrial scalability.