| Ni-Sn eutectic alloys were undercooled with the molten glass fluxing and cycling superheating methods. The solidification microstructure evolution with undercooling and forming mechanism of anomalous eutectic were investigated, systematicly. The pattern of anomalous eutectic and its distribution in the solidification microstructures were observed by optical microscope and SEM analysis techniques. The competitive nucleation in the undercooled melts was analyzed using classic nucleation theory (CNT) and time-dependent theory (TDT), respectively. The growth of. α(Ni) phase and Ni3Sn phase were described based on BCT/LKT model. The remelting of the primary dendrites was discussed on the dimensionless superheating model. Based on these theories, a detailed description for the formation of the anomalous eutectic was given out. Compared the regularity of solidification microstructure evolution of Ni-27.5wt%Sn, Ni-30wt%Sn hypoeutectic, Ni-32.5wt%Sn eutectic, Ni-35wt%Sn hypereutectic with the coupled growth zone which made up by the calculated metastable-liquidus, the microstructure selection map of the undercooling dependent morphologies in Ni-Sn eutectic alloys was obtained. Main conclusions of the thesis are as follows:(1) The critical undercooling (AT=65K) of anomalous eutectic formation was determined by the observation of solidification microstructures. The volume fraction of anomalous eutectic was found to increase with the increasing undercooling. Finally, the as-solidified structure was entirely occupied by the anomalous eutecticwhen △T>140K.(2) Based on the calculation results of the classic nucleation theory and time-dependent nucleation theory, it indicates that the nucleation work and theincubation periods of a(Ni) phase are less than that of NisSn phase, the nucleation rate of a(Ni) phase is higher than that of NisSn phase. So the a(Ni) phase defeat Ni3Sn phase in the competitive nucleation, and nucleate directly from the melt.(3) According to the LKT/BCT dendrite growth model, the relationship between growth velocity V and undercooling AT is determined. The growth velocity of cc(Ni) phase is beyond that of the NiaSn phase when the undercooling is higher than 50.8K. This indicates that, in highly undercooled solidification, free growth of oc(Ni) dendrite into the undercooled melts occurs firstly and the growth of NisSn phase follows.(4) The dimensionless superheating caused by recalescence due to rapid dendrite growth of a(Ni) was calculated. The dimensionless superheating will stays in a high level if the melt subjects to a undercooling above a typical value, which favors the remelting of the as-formed a(Ni) dendrites.(5) In terms of competitive nucleation and growth between cc(Ni) and Ni3Sn, and remelting of a(Ni) dendrites, the forming mechanism of anomalous eutectic in Ni-Sn eutectic was interpreted. The primary a(Ni) dendrites were remelted during the process of recalescence and surrounded by the NiaSn phase which grew slowly. The results of rapid quenching and SEM analysis indicate that the forming mechanism of anomalous eutectic is correct.(6) The regularity of solidification microstructure evolution of Ni-27.5wt%Sn^ Ni-30wt%Sn hypoeutectic, Ni-32.5wt%Sn eutectic, Ni-35wt%Sn hypereutectic were analyzed. The coupled growth zone which made up by the metastable-liquidus was calculated. Compared the experiment results with calculation results, the microstructure selection map of the undercooling dependent morphologies in Ni-Sn eutectic alloys was obtained. When the solidification process occurs in the zone above the metastable-liquidus, the microstructure of alloy consists of primary dendrite and eutectic. If the melt is solidified in the zone below the metastable-liquidus, the microstructure of the sample is totally eutectic. Theanomalous eutectic will appear in the as-solidified microstructure with the increase of undercooling. When the undercooling is above a critical value, the as-solidified microstructure is fully occupied by anomalous eutectic.
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