Papers by Keyword: Undercooling

Abstract: The phase-field model was built by coupling with the concentration field and temperature field,The dendrite growth process of Magnesium alloy was simulated under the different anisotropic strength and different undercooling.The results show that with the enlarge of anisotropic strength, dendritic morphology change from seaweed-like to snow-like, trunk grows along the optimal direction,and the secondary dendrite arm grow along the most optimize direction as well; With undercooling increasing, the more coarse primary dendrite arm, the more developed secondary dendrite arm, dendrites around the thermal diffusion layer becomes thinner,and dendrite tip’s thermal diffusion layer is thinner than the dendrite roots,but segregation phenomenon decreases slowly. When Δ=1.0, the grain will directly generate cellular dendrite and it does’t appear segregation phenomenon

Abstract: Effects of the technological parameters (superheat, cycle times and holding time) on undercooling and structure of Ag-28.1 wt.% Cu eutectic alloy were discussed by orthogonal experiment. It shows that the influence of the superheat on undercooling is biggest, followed by cycle times, and finally holding time. Then the optimum process parameters of getting different undercooling are given in this paper. In the Ag-28.1 wt.% Cu eutectic alloy melt, cellular growth of lamellar eutectics takes place because of the large difference in composition between the two eutectic phases and the very large thermal diffusion coefficient of the liquid, and when undercooling is equal to or higher than 76 K, lamellar eutectics grow in a dendritic form during rapid solidification.

Abstract: Faceted growth of primary Al₃Ni phase in the hypereutectic Al-Ni alloy in a high magnetic field was investigated. It was found that faceted growth of primary Al₃Ni phase was enhanced in the presence of the magnetic field. However, the fibrous to granular transition of Al-Al₃Ni eutectics occurred. The undercooling of primary and eutectic phases during solidification was measured using differential thermal analysis. It was showed that the undercooling of primary phase was hardly changed but that of eutectics markedly increased in the magnetic field. According to Cahn theory of crystal growth, the critical driving force was used to satisfactorily explain the morphology transition in the magnetic field.

Abstract: In order to study the influence of undercooling, crystallization time to Al₂O₃ nanoparticles. To design a set of nanofluids crystal test system, and study the process of base fluid and Al₂O₃ - H₂O nanofluids crystallization. Results show that the base fluid added nanoparticles, degree of undercooling significantly, with the increase of mass fraction of nanoparticles and fluid crystallization time is shortened. Nanofluids phase transition temperature increased by 0.5 °C than water, reduce the energy consumption of the refrigeration unit.

Abstract: Upon non-equilibrium solidification, the intrinsic parameters, such as moving velocity, temperature, solute partition coefficient, and liquid and solid concentrations at the interface, deviate from their equilibrium characteristics, and the morphology of the as-solidified structure and the grain size are influenced by the non-equilibrium liqulid/solid transformation, which further influences the subquent solidstate transformation. Adopting molten glass purification technology combined with cycle superheating method, the microstructure evolution of Ni-11at.%Si alloy in different undercooling was investigated. It was found that, with the increase of the initial undercooling, grain refinement occurred in microstructures of undercooled Ni-11at.%Si alloy. Meanwhile, the NL model was used to discuss the two different dendrite morphologies. According to Karmas model for dendrite fragmentation, the grain refinement of undercooled Ni-11at.%Si alloy was in good agreement with the experimental data, and the grain size was reduced with the increasing ΔT. The energy-dispersive spectroscopy (EDS) measurement was applied to analyze the solid solubility of Si atom in α-Ni matrix. It was found that the solid solubility of Si atom in α-Ni matrix increased with undercooling. At the undercooling of T>220K , a complete solute trapping occurred.

Abstract: In order to study the precipitaion of Ni3Si particle in undercooled Ni-Si alloy, calorimetric analyses were carried out using non-isothermal measurements by DSC. The scanning electron microscopy (SEM) and the transmission electron microscopy (TEM) measurements were used to describe qualitatively and quantitatively the precipitate microstructures. The non-isothermal DSC thermograms exhibited one reaction peaks and it indicated that the precipitation process is an exothermic reaction. The evolution for the precipitate was obtained in the as-solidified Ni-Si alloy subjected to DT=195K, meanwhile, the precipitate size was found increased with decreased heating rate in the TEM images. The largest precipitate size was about 120nm, and the precipitates still kept spherical shape. Model prediction for the precipitation of Ni3Si particle has been performed. Good agreement with experimental data has been achieved

Abstract: The solidification behavior of undercooled Cu98Cr2 alloys was investigated under different rapid solidification methods. The results shown that whole lamellar (α+β) eutectics appeared in melt spun alloys for the sample undercooled into the coupled zone. The microstructure of arc melted Cu98Cr2 alloys consisted of primary β(Cr) particles distributes on α(Cu) matrix and (α+β) eutectics set in cellular α (Cu) phase. However, due to the different thickness and different solidification rate, the microstructure of splat quenched Cu98Cr2 alloys shown that only cellular supersaturated α solid solution occurred for thin flake and primary β(Cr) particles and (α+β) eutectics occurred for thicker flakes.

Abstract: The effects of gravity and kinetic undercooling upon the melt/crystal interface in a vertical Bridgman-Stockbarger crystal growth system by numerical simulation. Thermal transport, melt convection and kinetic undercooling are simulated by two-dimensional transient calculations. Time evolution of the centerline interface location difference is tracked. This study then compares and discusses the results among the different gravitational acceleration. We also investigate the effects of the undercooling on the interface.

Abstract: Fe-Co single-phase alloy melts with different Co contents were undercooled using fluxing method. The maximum undercooling DT = 457K (relative undercooling DT/Tm=0.259) was achieved in this work. At low undercooling (DT), single-phased microstructure was observed, but metastable bcc phase emerged in the as-solidified microstructure once DT exceeded a critical value, DTcrit. In the presence of classical nucleation theory, phase selection in the undercooled Fe-Co melt was investigated, and the theoretical calculation was coincided with the experimental result.

Abstract: The dynamic evolution of grain growth in the process of aluminum casting and the impact of different casting conditions on the grain growth were simulated by using the Cellular Automation(CA) method in this paper. The simplified binary alloy was used to simulate the growth of the grain in the undercooled melt, Finite Difference Method (FDM) combined with relative motion was used, and dynamic evolution of microstructures in the process of aluminum twin-roll casting was achieved. Visual Fortran programming language was adopted to calculate and realize the image post-processing. based on the growth of the grain in the undercooled melt, the impact of the undercooling triggered by cooling and the casting speed on aluminum strip in the casting process was simulated. The results indicate that, in the condition of a certain nucleation rate, as the cooling intensity increases, bigger the grain is, which provides a basis for optimizing the twin-roll casting process parameters.