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

Abstract: By employing the complex method of molten glass purification and superheating cycling, structural evolution and solidification behavior of undercooled Cu-20%Pb hypomonotectic, Cu-34.15%Pb monotectic and Cu-40%Pb hypermonotectic alloys were investigated systematically. The following results were concluded.By using this method, high undercooling of Cu-20%Pb hypomonotectic, Cu-34.15%Pb monotectic and Cu-40%Pb hypermonotectic alloys up to 208, 204 and 208K were obtained respectively.During solidification it was found that only once recalescence occurred for Cu-34.15%Pb monotectic and Cu-40%Pb hypermonotectic alloys melts with low undercoolings, and that twice recalesences were observed for those melts with high undercoolings; It was found that twice recalescence occurred for Cu-20%Pb hypomonotectic alloy and the second recalescence was lesser with low undercoolings, and that only once recalesence was observed for those melts with high undercoolings.The solidification microstructures of Cu-34.15%Pb monotectic alloy were composed of coarse dendrites grains of copper based solid solution and interdendritic lead phase. However, when the undercooling increased, the arms of the coarse dendrite grains were refined. Increasing undercooling to the range 123 to 162K, hot-cracks occurred on the samples after solidification Obvious delaminations were found between 174K and 204K undercooling range. When melts were undercooled less than 98K, The solidification microstructures of Cu-20%Pb hypomonotectic alloy were composed of coarse dendrites grains of copper based solid solution, interdendritic lead phase and blocks of lead . The arms of the coarse dendrite grains were refined and interdendritic lead phase became smaller with the undercooling increasing. And hot-cracks were also found in the Cu-20%Pb hypomonotectic alloys when the undercooling exceeding 133K. The solidification microstructures of copper phase of Cu-40%Pb hypermonotectic alloy became obviously dendritic when the undercoolings were less than 11 OK and dendrites grains were not more and more obvious when the undercoolings were more than 11 OK. Delaminations also foundin the Cu-40%Pb hypermonotectic alloys when the undercooling exceeding 152K.The author considered that L2 phase of nucleated firstly basis of cooling curves and solidification microstructures and later a(Cu) phase nucleated; The time of two phases nuceating was close when the undercooling was less. The first phase of Cu-20%Pb hypomonotectic alloy was (Cu) phase, and (Cu) phase growing and monotectic action brought once recalesence respectively.Dendrites grains were not more and more obvious in the Cu-20%Pb hypomonotectic alloy under the moderate undercoolings, because the more thin place of dendrites grains were melted and broke up when the undercoolings increase. Hot-cracks occurred on the samples because of pulling between crystal lattice. Obvious delaminations were found in the Cu-34.15%Pb monotectic and Cu-40%Pb hypermonotectic alloys by reason of hit and consociation of the lead phase.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 magnitude of enthalpy of liquid changing into liquid H_L was extraordinary important for the competitive nucleation of (Cu) phase and L2 phase in Cu-34.15%Pb monotectic alloys' reaction. Basing on L2 phase nucleating firstly, the probable range of the magnitude of H_L gained preponderated over 5 x 10~8J-m"3. By the calculation of the transient state nucleation theories the probable range of the magnitude of H_L gained exceeded 1 x 10~9 J m-3...