Rapid Solidification And Phase Separation Of Monotectic Alloys

In this paper, the rapid solidification and microstructural characteristic of Ni-Pb and Cu-Pb monotectic alloys are investigated by melt spinning method, The cooling rate is calculated theoretically by coupling the heat transfer equation and Navier-Stokes equation, the microstructural evolution of the alloys are studied, meanwhile, the electrical resistivity,tensile strength and elongation of Cu-Pb alloys are measured experimentally, and the relationships between cooling rate and alloy properties are further analyzed theoretically. It shows that:Under rapid solidification conditions, the microstructure of Ni-8%Pb hypomonotectic alloy consists of (Ni) dendrite and (Pb) in the grain boundary. Crystal growth exhibits apparent growth orientation and forms columnar crystallites, while particle diameter is large. With the decrease of particle diameter, the nucleation increases, the microstructure is characterized by uniform and fine equiaxed crystallites. With the rise of cooling rate, the microstructural morphology changes from coarse dendrite to fine equiaxed crystallites.The liquid separation of Ni-60%Pb hypermonotectic alloys is suppressed to a great degree, which results in the formation of uniform microstructure. With the rise of cooling rate, the microstructure was remarkably refined.In the rapid solidification of Cu-Pb hypermonotectic alloys, (Cu) phase grows in the form of dendrite quickly and Pb-rich phase distributes either in the grain boundary or in the crystal grain. With the rise of cooling rate, the liquid separation is suppressed, the grain size decreases, the distribution of Pb-rich phase is more uniform. There is a liquid/solid phases coexist zone with a large temperature interval below the temperature of monotectic reaction, it has no remarkable effect on the phase reparation of alloys. Under rapid solidification, Due to the high cooling rate, the short solidification time and the effect of shear stress, the floatation of (Cu) dendrite is restricted, the uniformity of microstructure increases drastically.Due to the increase of concentration of lead and cooling rate, the solid solubility of alloy phase is expanded, the microstructure is markedly refined, the amount of grain boundary increases, which intensifies the scattering of free electrons, resulting in the rise of Cu-Pb hypermonotectic alloys resistivity. Otherwise, with the rise of cooling rate, the grain size decreases and the amount of crystal defect increases, the tensile strength of alloy increase, and elongation decrease.