Non-isothermal crystallization kinetics and soft magnetic properties of the Fe67Nb5B28 metallic glasses

Differential scanning calorimetry (DSC) was used to investigate the thermal behavior and non-isothermal crystallization kinetics of the Fe67Nb5B28 metallic glasses prepared by melt-spinning method. DSC traces exhibit that the crystallization takes place through a single exothermic reaction, and it processes a good thermal stability in thermodynamics. The activation energies for nucleation and grain growth processes were calculated to be 536 +/- 22 and 559 +/- 20 kJ mol(-1) by Kissinger equation, respectively, and 551 +/- 24 and 574 +/- 20 kJ mol(-1) by Ozawa equation, respectively. It means that the grain growth process is more difficult than the nucleation process. The variation of local Avrami exponent n(x) with crystallized fraction x demonstrates that the crystallization mechanism varies at different stages. The n(x) is larger than 2.5 at the initial stage of 0 < x < 0.3, implying a mechanism of diffusion-controlled three-dimensional growth with increasing nucleation rate. The n(x) decreases from 2.5 to 1.5 in the range of 0.3 < x < 0.65, suggesting that the crystallization belongs to three-dimensional nucleation and grain growth with decreasing nucleation rate. And n(x) lies between 1.0 and 1.5 in the range of 0.65 < x < 0.95, indicating that the crystallization corresponds to the growth of particles with an appreciable initial volume. Low-temperature annealing corresponds to the precipitation of alpha-Fe, Fe2B, and Fe23B6 phases, and further annealing leads to the formation of alpha-Fe, Fe2B, and FeNbB phases. The magnetic properties in relation to microstructure change of the Fe67Nb5B28 metallic glasses are discussed.