Unequal effect of thermodynamics and kinetics on glass forming ability of Cu–Zr alloys

The glass forming ability (GFA) of Cu–Zr alloys has been still ambiguous, due to incomplete or lacking thermophysical properties of Cu–Zr liquids in supercooled and stable states, although tremendous effort has been devoted. We provide here the comprehensive thermophysical properties of Cu–Zr liquids, such as undercoolability, density, viscosity, fusion enthalpy, temperature–time-transformation (TTT) diagram, and crystal–liquid interfacial free energy. Three compositions, Cu64Zr36, Cu56Zr44, and Cu50Zr50, show distinctive anomalies in undercoolability, nose time in TTT, and crystal–liquid interfacial free energy, but not in density and viscosity in supercooled and stable liquid states. The anomalies reflect that the GFA is dominantly governed by thermodynamics rather than kinetics in these bulk metallic glasses (BMGs). In addition, we find that positions of nose temperatures in the TTT curves are below 1/2 (Tg + Tl), which implies unequal contribution of thermodynamics and kinetics. We discuss that empirical GFA parameters cannot explain the glass formation of Cu–Zr alloys due to the unequal contribution, and the Turnbull GFA criterion (Trg = Tg/Tl) is valid for the equal contribution of the two effects. The present experimental findings shed light on the ongoing debate about the GFA criterion of Cu–Zr BMGs.