Thermodynamic Modeling Of Al-Cu-Fe-Mg-Mn-Si System And Phase Transition Sequence Prediction During Its Solidification

The phase diagram and thermodynamic information of materials is the basis for kinetic and even microstructure evolution simulation. Cu, Fe, Mg, Mn and Si are the main alloying elements or impurity elements in commercial aluminum alloys. In order to improve the properties of the current Al alloys or even design novel high-performance Al alloys, establishment of a precise thermodynamic database including Al, Cu, Fe, Mg, Mn, Si is the prerequisite.The overall idea of this thesis is as follows:First, based on well-established thermodynamic descriptions of the constituent binary systems, as well as the experimental data of the ternary systems obtained in the present work and available in the literature, thermodynamic parameters for the four constituent ternary systems, i.e., Cu-Fe-Si, Cu-Mg-Si, Fe-Mg-Si and Cu-Fe-Mg, were optimized using the CALPHAD method, from which the Cu-Fe-Mg-Si thermodynamic database can be preliminarily obtained. After that, the Cu-Fe-Mg-Si thermodynamic database was combined with thermodynamic parameters of other relevant binary, ternary and quaternary systems published in our research group to establish the Al-Cu-Fe-Mg-Mn-Si thermodynamic database. Later, based on this6-component thermodynamic database, phase diagram calculations in a series of important systems in commercial aluminum alloys was conducted and compared with the experimental data. Finally, the phase transition sequence in a series of commercial aluminum alloys during solidification was predicted using Scheil-Gulliver model.The main work in this thesis consists of six parts and is concisely described in the following:(1) Based on a critical assessment of the literature data, the phase equilibria of the Cu-Fe-Si system over the whole composition range was investigated by using a combination of X-ray analysis (XRD), scanning electron microscopy with energy dispersive X-ray analysis (SEM/EDX), electro probe microanalysis (EPMA) and differential thermal analysis (DTA). The isothermal section at750℃of the Cu-Fe-Si system was determined, and new phase transition temperatures along vertical sections at30and70at.%Cu were measured. A thermodynamic modeling for the Cu-Fe-Si system was then conducted by considering the reliable experimental data from the literature and the present work. A set of self-consistent thermodynamic parameters which is found to be more reasonable than the previous assessments was obtained. The assessed thermodynamic parameters were also successfully applied to two technical cases in material design:the selection of alloying elements for the control of the formation of surface fissures in iron, and the prediction of the formation of Core/Shell type alloys. The criterion for the formation of the liquid miscibility gap in the ternary system was also summarized in this work.(2)13ternary Cu-Mg-Si alloys system were prepared by means of the powder metallurgy method. Phase equilibria at500and700℃of Cu-Mg-Si system were determined using XRD analysis. The existence of3ternary compounds in this system was verified:Sigma (Cu16Mg6Si7), Tau (Cu3Mg2Si) and Laves_C15((Cu0.8Si0.2)2(Mg0.88Cu0.12)). A thermodynamic modeling for the Cu-Mg-Si system was conducted on the basis of the experimental data obtained in this work and those critically reviewed from the literature. The complex phase relationship between Laves phase and other phases has been successfully modeled in this work. In addition, most of the experimental data can be reproduced by the presently obtained thermodynamic parameters.(3) All of the phase diagram and thermodynamic data of the Fe-Mg-Si system available in the literature were critically reviewed and assessed using thermodynamic models for the Gibbs energies of individual phases. A set of self-consistent thermodynamic description for the Fe-Mg-Si system was thus obtained. The liquidus projection and reaction scheme for the entire Fe-Mg-Si system are calculated using the thermodynamic parameters obtained in this work. The monotectic invariant equilibira related to the liquid miscibility gap in the Fe-rich corner and Mg-rich corner is also described in detail.(4) The phase equilibria of Cu-Fe-Mg system at room temperature,500℃and700℃were determined with powder metallurgy method. No ternary compound was observed in this ternary system. The thermodynamic description of the Cu-Fe-Mg ternary system was obtained via direct extrapolation from the three boundary binary Cu-Fe, Cu-Mg and Fe-Mg systems. The calculated isothermal sections at room temperature,500℃and700℃are consistent with the experimental results, indicating that there is no need to introduce any ternary interactive parameters. A series of isothermal sections and several typical vertical sections were then model-predicted. The projection of the liquidus surface and the reaction scheme were also constructed.(5) The thermodynamic parameters obtained in the above ternary systems were combined with those in other constituent binary, ternary and quaternary systems published in our research group to establish a thermodynamic database for the Al-Cu-Fe-Mg-Mn-Si system. Based on this newly established Al-Cu-Fe-Mg-Mn-Si thermodynamic database, thermodynamic calculations were performed in the Al-Cu-Mg-Mn, Al-Cu-Fe-Mg, Al-Cu-Fe-Si, Al-Cu-Mg-Si, Al-Fe-Mg-Si quaternary systems and the Al-Cu-Fe-Mg-Si, Al-Cu-Fe-Mg-Mn quinary systems. The temperatures and compositions of liquidus invariant reactions in Al-rich corner for all the above systems were calculated and compared with the experimental data. The liquidus surfaces in Al-rich corner for Al-Cu-Mg-Mn, Al-Cu-Fe-Mg, Al-Cu-Fe-Si, Al-Cu-Mg-Si and Al-Fe-Mg-Si quaternary systems were constructed. The calculated results are in good agreement with the literature data. The liquidus surfaces for the Al-Cu-Fe-Mg-Si quinary system in Al-Cu, Al-Si and Al-Mg sides, as well as those for Al-Cu-Fe-Mg-Mn quinary system in Al-Cu, Al-Mg, Al-Mn side, were also presented.(6) The thermodynamic description for Al-Cu-Fe-Mg-Mn-Si system was then employed to analyze the phase transition sequence of17commercial aluminum alloys of Al-Cu, Al-Mg and Al-Si series during solidification. Solidification paths of those aluminum alloys were simulated using the lever rule and the Scheil-Gulliver model. The simulation provides a good prediction for the experimental data available in the literature.