Abstract:The microstructures and evolution mechanism of the undercooled Ni-20%Pb(molar fraction) alloy were investigated systematically by high undercooling solidification technique. The experiment results indicate that the morphology ofα-Ni phase and the distribution of Pb element in undercooled Ni-20%Pb alloys change with the increase of undercooling. The main evolution mechanisms ofα-Ni are dendrite remelting and recrystallization. Pb phase in the microstructure
of Ni-20%Pb hypermonotectic alloy originates fromL2phase separated from the parent melt during the cooling process through immiscible gap andL′2phase formed at the temperature of monotectic transformation. The solubility of Pb element inα-Ni phase under high undercooling condition is up to 5.83% which is obviously higher than that under equilibrium solidification condition. The real reason that causes the solubility difference is distinct solute trapping.

Key words: high undercooling; Ni-Pb hypermonotectic alloy; structural evolution; dendrites remelting; recrystallization