The Microstructures And Properties Of Rapidly Solidified Copper-Lead Monotectic Alloys From Highly Undercooled Melts

The high undercooling and rapid solidification of Cu-Pb monotectic alloys in highly undercooled melts were studied by employing the complex method of molten glass purification and repeating superheating. The melt temperature and cooling curves of Cu-Pb alloys were measured by the infrared thermoscope. The competitive nucleation ofα(Cu) phase and L2 phase and solidification behavior in Cu-20%Pb hypomonotectic alloy were analyzed by means of classical nucleation theory along with transient nucleation theory. The microstructures evolution of Cu-20%Pb hypomonotectic, Cu-34.15%Pb monotectic and Cu-40%Pb hypermonotectic alloys were observed by optical microscope. Microstructure evolution mechanism of Cu-20%Pb hypomonotectic alloy was analyzed by BCT-LKT dendrite growth model and theories on thermodynamics and dynamics of dendrite growth. The hardness, friction and tribology performance, thermal expansion coefficient of Cu-20%Pb hypomonotectc alloys were tested, and their dependence on the undercooling were also discussed. The microstructure and properties of Ni-20%Pb hypomonotectic alloy solidified in undercooled melt were also studied in order to compare with those of Cu-20%Pb hypomonotectic alloy. The investigative results indicated that:There are two recalescences in the solidification of Cu-20%Pb hypomonotectic alloy melt undercooled below 238K. It can be proved by the classical nucleation theory that isomorphous transformation occurs earlier than monotectic reaction andα(Cu) nucleates firstly during the solidification of undercooled Cu-20%Pb hypomonotectic alloy melt. The first recalescence is caused by nucleation and growth ofα(Cu) dendritic and the second one is attributed to nucleation and growth of L2 phase and growth ofα(Cu) dendritic.The microstructure of Cu-20%Pb hypomonotectic alloy with no undercooling is composed of the coarse dendrite grains and interdendrite Pb phases. The coarse dendrite grains andinterdendritic Pb phases are refined as the undercooling increasing. It can be concluded that the mechanism of dendrite refinement with undercooling less than 85K is dendrite remelting, whereas dendrite remelting and dendrite fragmentation process are responsible for refinement with undercooling more than 170K. There are monotectic cellular structures in the solidification microstructures of Cu-34.15%Pb monotectic alloy and cracks around monotectic cellular structure that appears with undercooling less than 130K. The delamination of the different phase occurs in Cu-40%Pb hypermonotectic alloys with undercooling more than 50K.The hardness of Cu-20%Pb hypomonotectic alloy can be increased and its uniformity can be improved by high undercooling solidification. The hardness of the alloy is increased from 34.9 kgf/mm2 with no undercooling to 42.5 kgf/mm2 with undercooling of 238K. The wear resistance is well at small undercooling and the antifriction property is well at middle undercooling. The antifriction property and wear resistance are all well with undercoling above 143K. The wear mechanism varies from adhesive wear, abrasive wear and surface fatigue to adhesive wear and slight abrasive wear when undercooling increase from zero to a large value.The heat expansion performance of the alloy can be improved by high undercooling solidification. For example, the average linear thermal expansion coefficient decreases from 20.5×10-6/℃with no undercooling to 9.8×10-6/℃with undercooling of 238K. The microstructure evolution of Ni-20%Pb hypomonotectic alloy is similar to that of Cu-20%Pb hypomonotectic alloy. Though the hardness uniformity and the heat expansion performance of the former are better than that of the latter, the tribology performance of the former is poorer than that of the latter.