Kinetics of glass transition, crystallization and soft magnetic properties of the Fe76.5Nb3B20Cu0.5 glassy alloys

Differential scanning calorimetry was used to investigate the non-isothermal crystallization kinetics of the Fe76.5Nb3B20Cu0.5 glassy alloys. The nanocrystallization products and soft magnetic properties as a function of annealing temperatures were also studied by using X-ray diffractometer and vibrating sample magnetometer. The activation energies for Eg, Ex, and Ep are equal to 499 ± 3, 558 ± 7, and 693 ± 2 kJ mol−1 by Kissinger method, respectively, and 512 ± 3, 571 ± 9, and 715 ± 2 kJ mol−1 estimated by Ozawa method, respectively. The results indicate that the glass transition process is easier than the nucleation and grain growth process, while the grain growth process is much difficult than the nucleation process. The local Avrami exponent n(x) varies with the crystallized volume fraction x, indicating that the crystallization process has different crystallization mechanisms. The n(x) ranging from 1.0 to 1.2 for 0.05 < x < 0.2 represents the growth of particles of appreciable initial volume. The 0.5 < n(x) < 1.0 for x ranging from 0.2 to 0.8 stands for the growth of a large number of the pre-existing nuclei. Isothermal annealing experiment indicates that the initial nanocrystallization product is α-Fe phase, and the final nanocrystallization products are composed of α-Fe, Fe2B and Fe3B phases. With increasing annealing temperature, both the saturation magnetization Ms and coercivity Hc are enhanced. High temperature annealing is found to weaken their soft magnetic properties, which is ascribed to the precipitation of boride.