Thermocalc-based Numerical Simulation Of The Microsegregation And The Solidification Paths Of Multicomponent Alloys

Microsegregation is a prevalent phenomenon in a solidification process. The research on microsegregation is very important to understand the solidification behavior and predicate the solidification structures and properties. Most of the engineering alloys are multicomponent and multiphase. Therefore, microsegregation modeling for multicomponent and multiphase alloys, and predicting their solidification behaviors and structures have important scientific and engineering significances. The thermodynamic data supplied by the phase diagram is necessary in calculating the microsegregation. But the phase diagrams of multicomponent alloys are very scarce.In order to enhance the computational efficiency and simplify the modeling,most of the micro solidification models for multicomponent and multiphase alloys established so far, supposed completely mixing in the liquid phase,that is,only the solid-phase diffusion is considered while assuming a sufficiently large liquid diffusion. A unified micro-/macro-segregation model was proposed by the present research group also Based on this assumption. In the present thesis, a numerical simulation technique will be adopted to investigate the solutes redistributions and diffusion behaviors in both the growing solid-phase and interdendritic liquid. The major accomplished work includes:A coupling algorithm with Thermo-Calc for the solidification paths of multicomponent and multiphase alloys is established, by modifying our research group's previous numerical model, which is for a long-scale directional solidification of binary alloy, available for the microsegregation and solidification path calculations for multicomponent and multiphase alloys.The corresponding calculation procedure is written using FORTRAN. Computations for the solidification paths of Al-3.66%Cu-6.46%Si .Al-26.9%Cu-2.07%Si and Al-11.35%Cu-21.73%Mg are performed and the results are compared with that by the unified micro-/macro-segregation model.An implicit differential scheme-based numerical model accounting for the solutal diffusions in both the dendrite-length-scaled solid and liquid phases, appeared in a recent international journal, for a unified multicomponent/multiphase micro-/macro-segregation computer modeling was unscrambled. The corresponding algorithm, again Thermo-Calc/Database-linked, and the computational program were built using FORTRAN. The initially testing calculation results were obtained.Using the ThermoCalc software and COST2 database, the spatial structures for the boundaries around theα-Al region in Al-Cu-Mg ternary phase-diagram system were investigated, and a mass of solidification thermodynamic data for the liquidus surface, and that for fcc-(Al), S, TAU and AlCu_Theta solid-phases between 720K and 755K were calculated. From these data, the each corresponding boundary surface can be image-visibly reconstructed, and the corresponding fitting surfaces and their sectioning (monovariant) curves are to be obtained for highly efficient computations with a ThermoCalc-coupled, unified multicomponent micro-/macro-segregation model.Experiments are conducted with two different compositions in each the Al-rich corner of Al-Cu-Si and Al-Cu-Mg systems. On the basis of analyses on the measured cooling curves and the solidification microstructures observed, the corresponding solidification paths are determined. The average microstructural dimensions and the solidification rates are measured. Using these experimental data and the presently developed computer codes, the solidification paths for the each multicomponent and multiphase sample are calculated. The computational results are comparable to the experiments.