Competitive Growth And Microstructure Evolution In Undercooling Melting Of Ni-Sn Alloys

Eutectic alloys have been well investigated and attracted much attention due to their excellent casting properties and unique growth characteristics. With the development of the rapid solidification technology, significant progress has been made in studies of the eutectic alloys. But, the theory on free eutectic growth in undercooled melts is not yet verified experimentally, and the current opinins on the cmpetitive growth mechanism between single-phase and eutectic growth are still controversial. Additionally, the coupled zone is determined largely by investigating the solidification microstructure of undercooled melts. Very often eutectic solidification is affected by cooling conditions, impurities and melt flow. How these factors alter the competitive growth and the coupled zone in undercooled melts is unclear. In recent years, the high-speed camera and high magnetic field techniques were well commercialized, which both provide advanced means for accurate measurement of crytal growth velocities in undercooled melts and allow for deep insight into those issues.In the present thesis, eutectic and near-eutectic Ni-Sn alloys were taken as model alloys. The alloys were undercooled using the glass fluxing method in combination with overheating cycles under high uniform and gradient magnetic fields. The process of rapid solidification of undercooled melts was in-situ observed using a high speed camera and a single-color pyrometer. Crystal growth velocities in undercooled melts were simulated and calculated using a software based on POV Ray 3.7. The microstructure of the solidified samples was examined by optical microscopy (OM) and scanning electron microscopy (SEM) in the back-scattering mode. Grain orientations were resolved using the electron back-scattering diffraction (EBSD) technique. The latest theory on free eutectic growth, the LZ model, was validated by fitting it to the experiment data, and the competition between single-phase and eutectic growth was investigated. The competitive growth mechanism was determined. The coupled zone in the undercooled melts was drawn based on the above results, and its reliability was examined experimentally. Additionally, the crystal growth velocities, the competive growth between different morphologies, and the coupled zone were also investigated by controlling fluid flow using the uniform and gradient magnetic fields and by adding a third element to the model alloys.Microstructure studies revealed that two types of anomalous eutectic, fine-grained and coarse-grained, are formed in undercooled Ni-Sn alloys. The dendritic morphologies of primary phase at different solidification stages were preserved in rapidly quenched samples. The examination of the microstructure morphology (grain size and distribution) and the EBSD characterization revealed that the anomalous eutectic can be formed by partial remelting (or complete remelting) of the different primary phases. The fine-grained anomalous eutectic is formed by partial remelting of eutectic dendrites and subsequent recrystallization of the Ni3Sn matrix from the remaining liquid, whereas the coarse-grained anomalous eutectic is formed by partial remelting of the primary dendrite phase from the interdendritic liquid.In-situ observations showed that the crystal growth velocity has a sudden rise, a sudden decrease, and a sudden rise following a sudden decrease with increasing undercoolings of eutectic Ni81.3Sn18.7, off-eutectic Ni80Sn20 and Ni83Sn17 compositions, respectively. The accuracy of the free eutectic growth model was verified based on the present data. The modeling of the measured data revealed that the competitive growth in the undercooled Ni-Sn liquid is controlled by the highest interface temperature criterion. The critical undercoolings were determined to be 76 K and 143 K, to be 134 K, and to be 73 K and 202 K for Ni83Sn17, Ni81.3Sn18.7 and Ni80Sn20 alloys, respectively. The same model was applied to calculate the kinetics of crystal growth in other compositions ranging between Ni85Sn15 and Ni76Sn24. The critical undercoolings, namely the boundaries of the coupled zone, were calculated. The calculated coupled zone shows a good agreement with the studies of solidified microstructure of undercooled samples. Seven types of anomalous eutectic morphologies were predicted in terms of the morphology and possible combinations of anomalous eutectic precursors, and were observed in solidified samples.The applied magnetic field has different effects on the growth velocity for the undercooled melts. The growth velocities of Ni83S17 hypoeutectic are decreased, but the growth velocities of Ni81.3Sn18.7 eutectic and Ni80Sn20 hypereutectic alloys were decreased first and then increased again with increasing intensity of the uniform magnetic fields. The maximum influence was obserfved at magnetic fields of 6 T,3 T and 2 T for Ni83Sn17 hypoeutectic, Ni81.3Sn18.7 eutectic and Ni80Sn20 hypereutectic, respectively. In the gradient magnetic fields, the growth velocities of Ni83Sn17 hypoeutectic are decreased obviously. The growth velocities of Ni81.3Sn18.7 eutectic alloys are comparable to those mesaued in the uniform magnetic fields. The eutectic growth velocities in the NisoSn2o hypereutectic alloys are close to those measured without the magnetic fields, which however are significantly larger than those measured under the uniform magnetic field of 2 T. The eutectic growth velocity changing with the magnetic field shows that the thermoelectromagnetic convection (TEMC) is driven by the magnetic fields at eutectic growth frontier. The growth velocities of ?-Ni dendrites and Ni3Sn dendrites aren't significant affected under magnetic fields or gradient magnetic fields. At the eutectic point, Solute diffusion changes which caused by external factors have no significant effect on the eutectic growth velocities. But the eutectic growth is affected both by solute diffusion and by thermal diffusion in near-eutectic compositions. The further the bulk composition deviates from the equilibrium eutectic composition, the stronger the effect is. When the melt convection just changes the solute diffusion and thermal diffusion along the liquid/eutectic interface, it hardly affects the competitive growth mechanism and the morphology of couple zone. However, when the melt convection accelerates solute diffusion in the normal direction, it will accelerate the coupled eutectic growth and enlarge the couple zone.The addition of a small amount of Co or Ge does not have any significant effects on dendritic growth velocities of primary ?-Ni in undercooled Ni-Sn eutectic alloys, but it has different effects on the eutectic growth velocities. The addition of Co increases the eutectic growth velocities, but the addition of Ge decreases the eutectic growth velocities. The addition of Co ro Ge does not affect the solidification morphology of the Ni-Sn eutectic alloys (no new phase), and does not affect the critical undercooling for the transition of the growth mode either. The imposition of a uniform magnetic field of 2 T (damping of melt convection) has a larger effect on the eutectic growth velocities than that of the addition of Co or Ge. The lower boundaries of the coupled zone are hardly affected by the external environment such as melt convection and a small amount of elemental additions. However, the upper boundaries of the coupled zone can be lifted if external conditions favor eutectic growth except for the case governed by a change of thermal diffusion, and vice versa. The effect of the addition of Co or Ge on the coupled zone is much less significant than that of melt convection.