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It was demonstrated that liquid phase separation by nucleation and growth (LNG) occurs in undercooled molten Pd40.5Ni40.5P19 for undercoolings ΔT ≤ 60 K (ΔT = Tl − T where Tl is the liquidus and T is the kinetic crystallization temperature), and liquid state spinodal decomposition (LSD) occurs for ΔT ≥ 100. For 60 ≤ ΔT ≤ 100 K, it is the transition regime from LNG to LSD. A ternary phase diagram is introduced to summarize the reactions occurred in undercooled molten Pd40.5Ni40.5P19. Finally, it is suggested that LSD has an important impact on glass-forming ability of metallic alloys.

When molten Pd40.5Ni40.5P19 is undercooled way below its liquidus Tl, liquid state spinodal decomposition is observed. Crystallization of this system is particularly interesting because it has plenty of interfaces. The microstructure of an as-crystallized specimen can be divided into four regions, namely, A: random spinodal; B: aligned and elongated spinodal; C: coarsened spinodal and island structure; and D: rod structure and ternary eutectics. The origin of these different microstructures is discussed.

It was demonstrated that molten Pd40.5Ni40.5P19 undergoes liquid state phase separation in the undercooling regime δT = T1 − T where T1 is the liquidus of Pd40.5Ni40.5P19 and T is the kinetic crystallization temperature. Liquid state phase separation by nucleation and growth takes place for δT ≤ 60 K while that by spinodal decomposition occurs for δT ≥ 100 K. Microstructural analysis of the undercooled specimen obtained in the undercooling regime of 60 ≤ δT ≤ 100 K indicates that it is the transition regime. Finally, it was found that when undercooled molten Pd40.5Ni40.5P19 undergoes liquid state spinodal decomposition, it first decomposes into two liquid networks, which is finally replaced by a system of three liquid networks.

The specific volumes of molten Pd40Ni40P20, Pd77Cu6.5Si16.5, Pd82Si18, and Ni80P20, which are all glass formers, and were measured near the liquidus. It was found that the ratio α/Tg, where α is the thermal expansion coefficient and Tg is the glass transition temperature, remains fairly constant for all these systems. The scaling follows from the fact that with a larger thermal expansion coefficient, the free volume of the molten system would be squeezed out faster resulting in a larger Tg. This scaling behavior also allows us to roughly predict the glass transition temperature of a glass forming system. The glass forming ability can also be predicted by a parameter defined as α/T1.