Liquid phase separation and its thermodynamic description in undercooled Ni–Pb hypermonotectic melts

Liquid–liquid phase separation inside immiscible gap of undercooled Ni–Pb system melts was investigated systematically by water quenching. The experimental results showed that liquid–liquid phase separation inside immiscible gap could not be inhibited by high undercooling and rapid solidification, and there was an obvious difference in the morphology of primary phases between Ni–20 at.%Pb and Ni–40 at.%Pb hypermonotectic melts. Thermodynamic analysis showed that there existed some composition and temperature ranges in which no direct formation of the stable solid from the undercooled melt under the monotectic temperature, and dissolving of the metastable solid into the single liquid phase above the monotectic temperature did not occurred, which was the reason of that monotectic cells and α-Ni global grains appeared in the water-quenched microstructure of Ni–20 at.%Pb hypermonotectic melts and that of Ni–40 at.%Pb melts, respectively.