| Au-20Sn eutectic alloy and Au-Ni alloy are widely applied in electronic and electric industry, electronic information industry and aerospace precision electronic manufacturing industry. The eutectic microstructure with coarse primary ζ-AusSn dendrite can be obtained by conventional solidification of Au-20Sn alloy. The complex behavior of phase transition exist in Au-9Ni alloy. The technology of suction casting and injection casting and highly undercooled method, as a technology of rapid solidification, can change the microstructure of alloy, such as, extension of solid solubility, grain refinement, the changes of phase composition and phase precipitation sequence and so on. The changes of microstructure can improve the mechanical properties. Therefore, the microstructure obtained by the technology of suction casting, injection casting and highly undercooled method are different from the microstructure obtained by conventional solidification. These technologies may be effective path to improve the mechanical of alloy and broaden the application prospect of alloy.The rapidly solidified Au-20Sn eutectic alloys were prepared by four different solidification pathways, such as, graphite mold conventional casting, graphite mold injection casting, copper mold injection casting, and water-cooled copper mold suction casting. Bulk Au-9Ni alloy were undercooled to different undercoolings by the glass fluxing technique combining with cyclical superheating and a series of samples with different undercoolings were solidified. The change of temperature in solidification was monitored by the infrared pyrometer. A optical microscope and a scanning electron microscope were used to observe the microstructure of alloy. On the basis of the classical nucleation theory, transient nucleation theory and BCT model, the mechanism of the evolution of primary phase in the sub-rapidly solidified Au-20Sn alloy was comprehensively analyzed. On the basis of the classical nucleation theory, BCT model, Karma model and dendrite remelting model, the solidification structure of undercooled Au-9Ni alloy was comprehensively analyzed. Eventually, the main research achievements are as follows.The effect of cooling rates on the morphology of the primary phase of sub-rapidly solidified Au-20Sn alloys with four different cooling rates were studied.①The results show that the phase selection is related to cooling rate. When the cooling rate is in range of2.4*10-9.0*103K/min, the AusSn phase is the primary phase. When the cooling rate reach to6.0*104K/min, the AuSn phase become the primary phase.②The theory calculated results adopted by the classical nucleation theory, transient nucleation theory and BCT model further confirmed that the phase selection in sub-rapidly solidified Au-20Sn alloy is mainly dependent on the undercooling (cooling rate), which are consistent with the experimental investigations.The influence of different denucleating glass on the purification effect of Au-9Ni alloy were investigated, using glass fluxing combining with cyclical superheating. When B2O3are used as a purifying agent, the physical adsorption is the only purification mechanism. The undercooling obtained by B2O3is small and unstable. When Na2SiO3+Na2B4O7+B2O3are used as a purifying agent, the physical adsorption is also the only purification mechanism. The obtained undercooling is also small. While using Na-Si-Ca+B2O3a purifying agent, the combined effect of physical adsorption and chemical purification are the purification mechanism, which improve the undercooling ability of Au-9Ni melt.The analysis of microstructure and composition were used to discuss the formation mechanism of microstructure of Au-9Ni alloy during the undercooled solidification, in combination with the calculation of the classical nucleation, BCT model, Karma model and dendrite remelting model. When the undercooling is smaller than20K, the dendrite is stable. At the same time, solidification process mainly controlled by solute diffusion conduces that crystal growth velocity is very slow, which results in dendrite coarsening. When the undercooling is in the range of20K-70K, dendrite remelting and re crystallization of dendritic fragments results in the formation of the first solidified grain microstructure. As undercooling increases, especially in range of70K-130K, dendrite growth controlled by thermal diffusion and rapid crystal growth velocity results in the fine dendrites. At the same time, due to declining remelting, the evolution of morphology of the high-undercooling dendrite is as follows, the granular dendrite, equiaxed dendrite and directional dendrite. |
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