Non-equilibrium solidification of undercooled Ni-31.44%Pb monotectic alloy melts

By using the method of molten glass denucleating combined with superheating cycling, solidification behavior of the bulk undercooled Ni 31.44% Pb monotectic alloy melts was systematically investigated. The results indicated that the undercooled monotectic alloy solidifies in form of dendrite essentially during the stage of rapid solidification and after recalescence, the residual melts between the dendrites solidify in the equilibrium mode. Within the achieved undercooling range, the solidification structures are classified into three categories. When the undercooling is less than 50?K, the structures are composed of coarse dendrites and interdendritic lead phase. With the undercooling increasing into the range of 70 ～ 232?K, the dendrite clusters are refined and fine lead particles separate out from the supersaturated primary dendrite arms because of solute trapping. When the undercooling exceeds 242?K, the granular grains form and fine lead particles homogeneously distribute in the whole sample. Based on the observation of the solidification structures and the calculated results with BCT model, it is found that the granulation mechanism of the granular grains is owing to the primary dendrite disintegration and recrystallization.