Thermodynamic approach to glass-forming ability of water-quenched Pd-P-based and Pt₆₀Ni₁₅P₂₅ bulk metallic glasses

Despite its importance, a thermodynamic approach to determining the glass-forming ability (GFA) of bulk metallic glass (BMG) remains a goal to be achieved. We examined the GFA of water-quenched Pd-P-based and Pt ₆₀Ni₁₅P₂₅ BMG's in which their molten alloys were sufficiently treated with a dehydrated B₂O₃ flux prior to and during quenching to room temperature. This allowed us to envisage the applicability of the classical steady-state homogeneous nucleation theory because the suppression of heterogeneous nucleation worked effectively. GFA was examined by comparing the critical cooling rate Rch for glass formation with the maximum diameter d_max of glass. To calculate Rch, the homogeneous nucleation rate I_ss(T), and the growth rate u_c(T) were estimated as functions of the undercooling temperature of molten alloys. Then, the free energy difference ΔG_L-x(T) between the liquid and crystalline phases, and the viscosity η(T) of the liquid were experimentally determined while the surface energy σ_sL(T) at the liquid-nucleus interface was estimated by calculation. The d_max of rod BMG's correlated strongly to Rch through the relation Rch dmax-3/10 mm3 Ks^-1.