Viscosity of the supercooled liquid and crystallization kinetic analysis in the Fe74B20Nb2Hf2Si2 amorphous alloy

The non-isothermal thermal behavior of Fe74B20Nb2Hf2Si2 bulk metallic glass (BMG) was studied utilizing various thermal analytical methods. The obtained results showed that the viscosity behavior of this alloy in the supercooled region was strong due to the high kinetic fragility parameter (D* = 49.5) and low fragility parameter (m = 20.2). Also, the apparent activation energy (Ep) values for crystallization process were calculated in the range of 188.7–394.6 kJ/mol using the Gao-Wang method. Additionally, the Avrami's exponent (n) values increased from 2 to 4 with the progress of the crystallization process. The two-parameter Šesták-Berggren (SB) model was used to demonstrate the autocatalytic behavior of crystallization. Furthermore, the kinetic analysis revealed that all crystallization stages were controlled by the interface. The phase analysis showed that the Fe23B6, α-Fe, Fe2(Hf, Nb), and Fe2B phases were formed during the crystallization process. As obtained by the microstructural characterization, it was founded that the low dimensional growth (m = 2) is due to the complex structure of the main phase of Fe23B6 in the first crystallization stage. Moreover, the high dimensional growth (m = 3) during the second and third crystallization stages attributed to the Fe2B phase as the heterogeneous nucleation, as well as a significant increase in the volume fraction of the α-Fe phase, and the preferential orientation of the various crystalline phases. Hence, the values of Avrami's exponent (n) in the second and third crystallization stages were calculated equal to 3 and 4, respectively.