Evaluation of the amorphous-forming ability of Ni-Si-B ternary alloys using the CALPHAD approach

The amorphous-forming ability of Ni-Si-B ternary alloys has been evaluated by coupling the Davies-Uhlmann kinetic approach with the CALPHAD method. For the computation, the time-temperature-transformation (TTT) diagram, which gives the time necessary for the formation of the detectable amount of crystal during transformation, was obtained at a finite temperature. The critical cooling rate for amorphization could be defined as the minimum cooling speed that does not intersect the TTT curve and, hence, these critical cooling rates enable us to evaluate the glass-forming ability of Ni-Si-B ternary alloys. The driving force for the crystallization of the crystalline phase was derived, on the basis of the thermodynamic functions of each phase formulated by the present study. The calculated results showed good agreement with the experimental data on the compositional range of amorphization in this alloy system. According to our calculations, at a given alloy composition, it was found that the phase other than the primary crystalline phase controls amorphization. This finding is closely related to the smaller entropy of fusion for the phase that controls amorphization versus that of the primary crystalline phase.