Rapid solidification of undercooled monotectic alloy melts

Abstract

Structural evolution of bulk undercooled Ni–11.56 at.% Pb monotectic alloy was investigated systematically by using molten glass denucleating combined with superheating cycling. Within the achieved undercooling range 10–286 K, the solidification structures were classified into three categories. When the undercooling was less than 50 K, the structures consisted of coarse dendrites and interdendritic lead phase. With the undercooling increasing into the range of 70–232 K, the dendrite clusters were refined and fine lead particles separated out from the supersaturated primary dendrite arms because of solute trapping. When the undercooling exceeded 242 K, the granular grains formed and fine lead particles homogeneously distributed in the whole sample. The phase selection of high temperature melts was analyzed by adopting steady state nucleation theory. The calculated results shown that undercooled Ni–11.56 at.% Pb monotectic alloy melts solidified in the form of α(Ni) dendrites essentially during the stage of rapid solidification and after recalescence, the interdendritic residual melts solidified in the equilibrium mode. Based on the observation of the solidification structures and the calculated results with BCT dendritic growth model, it was confirmed that the granulation mechanism of the granular grains was owing to the primary dendritic disintegration and subsequent recrystallization.