Thermodynamic Description Of Multi-component Multi-phase Alloys And Its Application To The Solidification Process

Metallic materials in engineering applications are mostly multi-component and multi-phase alloys. The application of solidification modeling to practical technology is closely linked to our ability to model the microstructure development in multi-component alloys. In this work, the solute partition behavior and its influence on the solidification process of multi-component alloys were studied in detail based on the Calphad technology.A complete thermodynamic model for the accurate calculation of the partition coefficients in solidification process was described. The model was applied to Al-Cu binary alloy and Al-Si-Mg ternary alloy, and the predicted partition coefficients were compared with some former experimental data. Good agreement between the calculation results and the experimental data demonstrates the validity of the present thermodynamic model for the prediction of the partition coefficients in solidification process.The Levenberg-Marquardt method, the best algorithm to obtain the least-square solution of non-linear equations, was applied to calculate the stable phase equilibria. It was used for the prediction of solidification of ternary Al-Si-Mg system. The calculated phase equilibria agree well with the experimental results.The variation of solute partition coefficient was studied in dendritic solidification process of Al-Si-Mg alloys. It was found that the partition coefficient changed greatly during solidification process, which should have important influence on the formation of the microsegregation and the precipitation of various phases. The solidification path and eutectic fractions were predicted by employing the binary partition coefficients and Calphad technology separately. The results showed that great errors had been introduced by assuming the partition coefficient as a constant. By coupling Calphad method with microscale solidification model, the predicted solidification path and the eutectic fraction of Al-Si-Mg alloys agree well with the experimental results.The effect of cooling rates on the solidification process of Al-2.06 wt% Si-1.58 wt% Mg alloy was experimentally investigated. It was found that the solidification sequences were L->Li+Fcc_Al->Li+Fcc_Al+Si-±Li+Fcc_Al+Si+Mg2Si under low cooling rate and L-+Li+Fcc_Al-*Li+Fcc_Al+Mg2Si-*Li+Fcc_AlJrSi+Mg2Si under high cooling rate, respectively.A method to predict the solid-liquid interface stability during unidirectional solidification was developed by coupling M-S model with Calphad method. The method was applied to Al-0.38wt% Zn and Al-0.34wt% Si-0.14wt% Mg alloys, and the predicted results were compared with some former experimental data. The good agreement between the calculation results and the experimental data demonstrates the superiority of the present method to the classical model based on constant partition coefficient assumptions.The conventional theory of constrained dendrite growth for binary alloys was extended to multi -component alloys based on the Calphad method with considerations of the solute interactions in each phase. The variable solute partition coefficients and liquidus slopes under different tip undercooling were calculated in detail for a series of Al-Si-Mg alloys. The influence of variable partition coefficients on the kinetics of dendrite growth was demonstrated quantitatively. The primary dendrite spacing, the most important microstructure scale, was predicted in several Al-Si-Mg alloys. By comparing the results with the experimental results of former researchers, the present method was proved to be a superior method for the prediction of primary dendrite arm spacing.By using the concept of solute diffusion layer thickness, the back diffusion flux was easily obtained without the time consuming finite difference scheme. The application of the simplified model to Al-Cu-Mg alloys was studied in detail. The good agreement between the experimental results and the calculated values shows that this simplified model is suitable for the prediction of dendrite arm coarsening during solidification of multi-component alloys.Taking into account the effect of solute interactions on both phase equilibria and diffusion behaviors in each phase, the conventional theory of constrained dendritegrowth for binary alloys was extended to multicomponent alloys. The variable solute partition coefficients and the diffusion matrix were obtained based on the thermodynamic databases and the diffusion mobility coefficients during the dendrite growth process in multi-component alloys. The calculated data were used to evaluate the influence of multi-component diffusion on the kinetics of dendrite solidification of Cu-Sn-Zn ternary alloys.The solute redistribution during the rapid solidification of multi-component alloys was theoretically studied based on thermodynamic analyses. Transportation processes of the two solutes were taken into account to determine the compositions on the both sides of the growth interface. The analysis of the interface diffusion process reveals that the solute partition at the growth interface depends on the diffusion coefficient, growth rate, the solute partition ratios in the two binary sub-systems. After the analysis of the diffusion processes in the bulk liquid and solid, the solute distribution profiles in the directionally solidified samples were obtained. From the results, the path for the interface composition variation was calculated. It was found that the path was dependent on the diffusion coefficients of the solutes in the liquid.