Dynamic Modeling Of The Al-Cr-Si, Mn-Ni-Si,Cr-Ni-Ti, Al-Fe-Mg-Ni-Si And Al-Cu-Fe-Mg-Ni Systems In Multicomponent Al Alloys

Abstract:Al alloys are widely applied in aeronautics, astronautics and automobiles due to their excellent comprehensive properites. Cu, Fe, Mg, Mn, Ni, Si, Cr and Ti are the important alloying elements or additives in the Al alloys. Knowledge of phase diagrams and thermodynamic properties of related systems is the theoretical basis to understand the performance of Al alloys. In order to promote the properties, improve the production process and develop new Al alloys, it is essential to establish an accurate thermodynamic database of the multicomponent Al alloys.Five key systems (Al-Cr-Si, Mn-Ni-Si, Cr-Ni-Ti, Al-Fe-Mg-Ni-Si and Al-Cu-Fe-Mg-Ni) in the multicomponent commercial Al alloys are selected as research objectives in the present thesis. A hybrid approach of key experiments, first-principles calculations and CALPHAD (CALculation of PHAse Diagram) is employed to establish the accurate thermodynamic databases of these key systems. The major research achievements of the present work are:(1) The available experimental data of the Al-Cr-Si system are first critically evaluated. The enthalpies of formation of the τi (Al13Cr4Si4) and τ2(Al9Cr3Si) phases at0K are computed via the first-principles calculations. The Al-Cr-Si system is re-optimized based on the present first-principles results and the previous descriptions of the Al-Cr, Al-Si and Cr-Si systems as well as the experimental data from the literature. Finally, an optimal set of thermodynamic parameters of the Al-Cr-Si system is obtained. In addition, the liquidus projection and reaction scheme of this system are also constructed in the present work.(2) The isothermal section of the Mn-Ni-Si system at1000℃is determined by means of XRD and SEM/EDX. The enthalpy of formation for the τ4(MnNiSi) phase at0K is computed via the first-principles calculations. Based on the present obtained experimental data and first-principles results as well as the experimental data from the literature, the thermodynamic modeling of the Mn-Ni-Si system is performed. One single function is used to describe the Gibbs energies of both the ordered and disordered phases including L12and Fcc_A1as well as Bcc_B2and Bcc_A2. A set of self-consistent thermodynamic parameters is obtained. In addition, the liquidus projection and reaction scheme of the Mn-Ni-Si system are also constructed.(3) A thermodynamic assessment of the ternary Cr-Ni-Ti system together with a refined binary Ni-Ti sub-system is made using the CALPHAD method. The ordered-disordered transition between Bcc A2and Bcc_B2phases is described using a four-sublattice model for the first time. An optimal set of thermodynamic parameters for the Cr-Ni-Ti system is obtained by considering the experimental data from the literature. Comparisons between the calculated and measured phase diagrams indicate that all of the reliable experimental information is satisfactorily accounted for by the present modeling.(4) The Al-rich corner of the Al-Fe-Si system is re-optimized and the parameters of the Al9FeNi phase in Al-Fe-Ni system are modified in present work. Combining the thermodynamic parameters of these two systems and those of other ternary systems from literature, three quaternary systems (Al-Fe-Mg-Si, Al-Cu-Fe-Mg and Al-Cu-Mg-Ni) are extrapolated and three quaternary systems (Al-Fe-Ni-Si, Al-Fe-Mg-Si and Al-Cu-Fe-Ni) are modeled. Then, a thermodynamic database of two quinary systems (Al-Fe-Mg-Ni-Si and Al-Cu-Fe-Mg-Ni) is constructed. The established database is used to describe the solidification behaviors of6063alloy (Al-0.39Si-0.20Fe-0.43Mg, in wt.%) and2618alloy (Al-2.24Cu-1.42Mg-0.9Fe-0.9Ni, in wt.%) under equilibrium and Gulliver-Scheil non-equilibrium conditions. The reliability of the obtained thermodynamic database is verified by the good agreement between calculation and experiment.