Experimental And Computational Investigation On The Thermodynamic And Elastic Properties Of Key Binary And Ternary Systems In Al Alloys

It is well known that an efficient way to develop new materials is the integration of materials design and materials synthesis, while the key factor for materials design is the prediction of material properties. The prediction of the properties for materials has been developed to be a systematic, comprehensive and theoretical science, which is also the key factor to significantly improve the properties of traditional and advanced materials. Accurate descriptions of thermodynamic and elastic properties are important information for materials design. However, there is a lack of systematic description on the thermodynamic and elastic properties in Al-based alloy systems.The object of the present dissertation is about multi-component commercial Al alloy system, aiming at establishing the basis of a systematic thermodynamic and elastic properties database by integrating data from key experiments, CALPHAD (CALculation of PHAse Diagram) and first-principles calculations. The overall idea of the present work is:A hybrid approach of key experiments, first-principles calculations and CALPHAD was firstly employed to establish the accurate thermodynamic databases of the Al-Mg-Mn and Cu-Si-Zn systems, and first-principles calculations was then utilized to predict the thermodynamic properties of Al-contained compounds in the multi-component alloys at both0K and finite temperatures, and the elastic properties of the Al-contained compounds and the effects of alloying elements to the elastic properties of Al systematically. A scientific integration of the key experiments, CALPHAD method and first-principles calculations was established in the present work to accurately describe the thermodynamic and elastic properties Al alloys, and can serve as the guidance for accurate description of the thermodynamic and elastic properties for other alloy systems. The major content of the present dissertation is summarized as follows:(1) The melting behavior of λ-Al4Mn is contradictory in the literature due to the nucleation barrier. The phase equilibria in the Al-rich side of the Al-Mn system have been reinvestigated by precisely designed key experiments, the melting behavior of λ-Al4Mn was correctly elucidated for the first time, and two invariant reactions associated with λ-Al4Mn were observed. The model Al12Mn4(Al,Mn)10previously used for Al8Mn5was modified to be Al12Mn5(Al,Mn)9based on crystal structure data. In addition, the high-temperature form of Al11Mn4was included in the present assessment. Based on the present experimental data and reliable literature data, a new set of thermodynamic description for the Al-Mn system and Al-Mg-Mn system was obtained in the present work. Comprehensive comparisons show that the experimental data are well accounted for by the present descriptions and significant improvements were found when compared with previous assessments.(2) The application of the Al thermodynamic databases is restricted by the model inconsistence of the y phase. In an effort to provide a compatible thermodynamic description of the Cu-Si-Zn system for the multi-component Al-based thermodynamic database, the Cu-Zn binary system was remodeled using the CALPHAD approach with a new sublattice model Zn4(Cu,Zn)1(Cu,Zn)8for the y-Cu5Zn8phase. In addition, the isothermal section of the Cu-Si-Zn ternary system at600℃was experimentally determined by preparing fifteen alloys with their composition selection guided by computational predictions. At600℃, no ternary compounds were observed, and five three-phase equilibria were well determined. In particular, the longstanding controversy regarding the four three-phase equilibria in the Cu-rich corner involving the phases a,(3, γ-Cu5Zn8, and K-Cu7Si was resolved experimentally in the present work. Subsequently, a thermodynamic description of the Cu-Si-Zn system was obtained over the studied temperature range and the entire composition range based on the presently modeled Cu-Zn system and the experimental data from the literature and the present measurements.(3) A systematic first-principles calculations of energy vs. volume (E-V) equations of state (EOS), enthalpy of formation and single crystal elastic stiffness constants (cij's) has been performed for Al-based binary and ternary compounds. The calculated enthalpies of formation, and cij's of these compounds were compared with the available experimental data in the literature. In addition, elastic properties of polycrystalline aggregates including bulk modulus (B), shear modulus (G), Young's modulus (E), B/G ratio, and anisotropy ratio were also determined and compared with the experimental and theoretical results available in the literature. All the compounds studied in the present work are mechanical stable based on Born's criteria. The systematic predictions of elastic properties and enthalpies of formation for Al compounds provide helpful insight into the understanding and design of Al-based alloys.(4) The finite-temperature thermodynamic properties for the technologically important Al compounds have been studied based on first-principles calculations. The thermodynamic properties were predicted in terms of the quasiharmonic approach by considering both the lattice vibrational and thermal electronic contributions. When possible, the predicted properties were compared with data from experiments and thermodynamic modeling, and a good agreement is found. It is also found the Neumann-Kopp law, which is commonly used to estimate the Cp for compounds in CALPHAD method, is not valiad for some of the investigated compounds. The predicted thermodynamic properties herein provide robust foundation for thermodynamic modeling of Al systems studied herein.(5) The effects of fifteen alloying elements on elastic properties of Al have been investigated using first-principles calculations. A good agreement is obtained between calculated and available experimental data. Lattice parameters of the studied Al alloys are found to be dependent on atomic radii of solute atoms. The elastic properties of polycrystalline aggregates including bulk modulus, shear modulus, Young's modulus, and the B/G ratio are also determined based on the calculated elastic constants. It is found that the bulk modulus of Al alloys decreases with increasing volume due to the addition of alloying elements and they also related to the total molar volume (Vm) and electron density (nAl31X)with the relationship of nAl31X=1.0594+0.0207(?)...