Structural Evolution And Solidification Behavior Of Highly Undercooled Nickel-Lead Monotectic Alloy Melts

By employing the complex method of molten glass purification andsuperheating cycling, structural evolutions of undercooled Ni-2%Pb single-phase, Ni-25%Pb hypomonotectic, Ni-31.44%Pb monotectic and Ni-40%Pb hypermonotectic alloys were investigated systematically. The following results were concluded.Within the achieved undercooling range 10 to 280K, the solidification structures of Ni-2%Pb single-phase alloy underwent three changes at 22K, 88K and 187K, respectively. With the increase of undercooling, common dendrites, the first granular grains, undercooled dendrites and the second granular grains came out one after the other. The granulation mechanism of the first granular grain is owing to dendrite remelting and recrystallization, and the second is owing to dendrite breaking-up and recrystallization. With the increasing of undercooling, thermal undercooling replaces constitutional undercooling and acts as the main contribution in the dendrite growth.When melts were undercooled less than 50K, the solidification microstructures of Ni-31.44%Pb monotectic alloy were composed of coarse dendrites grains of nickel based solid solution and interdendritic lead phase. However, when the undercooling increased to the range 70 to 232K, the arms of the coarse dendrite grains were refined. Moreover, uniformly precipitated fine lead particles resulted from primary supersaturated solid solution were observed. It was found when the undercooling exceeded 242K, refined granular grain with homogeneously distributed fine lead particles on it formed, which is confirmed owing to the dendrite breaking-up and subsequently recrystallization.IllWhen the undercooling was less than 55K, the microstructure of Ni-40%Pb hypermonotectic alloy consists of coarse dendritic grain with interdendritic lead lumps. Increasing undercooling to the range 100 to 198K, the macrosegregation was serious. When the undercooling was up to 292K, granular grains formed. Analytical result showed that the degree of macrosegregation was related to the solid/liquid phase transition velocity and the residual liquid fraction during the rapid solidification stage. The granulation mechanism of the granular grains is owing to the dendrite breaking-up and recrystallization.The phase selection of high temperature melts was calculated by adopting steady state and transient state nucleation theories, respectively. The calculation results of the steady state nucleation theory shown that the undercooled Ni-31.44%Pb monotectic melts solidify in the same mode of the undercooled single-phase alloy melts, i.e., undercooled Ni-31.44%Pb monotectic alloy solidifies in the form of dendrite essentially during the stage of rapid solidification and after recalescence, the interdrendritic residual melts solidify in the equilibrium mode. The calculation results of the transient state nucleation theory indicated that the Ni-40%Pb hypermonotectic melts solidified with phase-separation during the small undercooling range, and when the undercooling increases to middle undercooling range, the Ni-40%Pb alloy melts solidified in the same mode as that of Ni-31.44%Pb monotectic melts, that is, growing in the form of dendrites during the stage of rapid solidification.