Thermodynamic approach to glass-forming ability of water-quenched Pd-P-based andPt60Ni15P25bulk 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${}_{60}$Ni${}_{15}$P${}_{25}$ BMG's in which their molten alloys were sufficiently treated with a dehydrated ${\mathrm{B}}_{2}$O${}_{3}$ 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 ${R}_{c}^{h}$ for glass formation with the maximum diameter ${d}_{max}$ of glass. To calculate ${R}_{c}^{h}$, the homogeneous nucleation rate ${I}_{\mathrm{ss}}$($T$), and the growth rate ${u}_{c}$($T$) were estimated as functions of the undercooling temperature of molten alloys. Then, the free energy difference \ensuremath{\Delta}${G}_{L\ensuremath{-}x}$($T$) between the liquid and crystalline phases, and the viscosity $\ensuremath{\eta}(T)$ of the liquid were experimentally determined while the surface energy ${\ensuremath{\sigma}}_{\mathrm{sL}}(T)$ at the liquid-nucleus interface was estimated by calculation. The ${d}_{max}$ of rod BMG's correlated strongly to ${R}_{c}^{h}$ through the relation ${R}_{c}^{h}$ \ensuremath{\approx} ${d}_{max}^{\ensuremath{-}3}$/10 mm${}^{3}$ Ks${}^{\ensuremath{-}1}$.