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 solidification principles of these materials still lack quantitative description. In this work, the solute partition behavior and its influence on the solidification process of multi-component alloys are studied based on CALPHAD. A complete thermodynamic model for the accurate calculation of the partition coefficients in solidification is described. The model is applied to Al-Si-Mg ternary alloys, and the predicted partition coefficients are compared with some former experimental data. Good agreement between the calculation results and the experimental data demonstrates the validity of the present model. The variation of solute partition coefficients of both Si and Mg is studied in dendritic solidification of Al-Si-Mg alloys. It is found that the partition coefficient changes greatly during solidification. By coupling CALPHAD with a micro-scale solidification model, the predicted solidification path for different composition and cooling rates and the off-eutectic fraction of Al-Si-Mg alloys agrees well with the experimental results. The conventional theory of constrained dendrite growth for binary alloys is extended to multi-component alloys based on the CALPHAD method with consideration of the solute interactions, through which the primary dendrite spacing is estimated.