Theoretical Study On The Elastic Properties And Their Temperature Dependence For Al₃（Cu₂）Mg₉Si₇ And Al₃X（X=Sc,RE） Intermetallic Compounds

As the representative of lightweight structrual materials, aluminum alloys, have a more broad application prospects in the aerospace and automotive manufacturing due to their properteis, including corrosion resistance, easy processing, recyclable and low cost. Although aluminum alloys have many favorable properties, improved strength at elevated temperature has been a continuing goal. With the development of technology and new materials, people have increasingly demandings on the material properties of aluminum, the development of high strength and heat resistance aluminium alloys has become two basic trends in aluminum industry.Elastic properties is the most fundamental and important mechanical properties for solid materials. It is not only to reflect the material directly applied stress resistance, but also to indirectly reflect other mechanical properties, such as ductility, brittleness, hardness, rigidity, etc. Through the study of the elastic properties of aluminum alloys, thereby understanding their internal mechanisms of strength and heat resistance, and thus provide theoretical guidance for optimizing the design of new high-strength heat-resistant aluminum alloys.First-principles calculations within the density functional theory have been carried out to study the structural, elastic, electronic and thermodynamic properties of B’ and Q phases in Al-Mg-Si(-Cu) alloys. The obtained lattice constant a is reduced while c is increased with the addition of Cu into B’phase Al3Mg9Si7. The lower formation enthalpy of Q phase Al3Cu2Mg9Si7 shows that the structural stability is improved after the addition of Cu into the B’phase. The calculated elastic constants Cij with the exception of C13 for Q phase are larger than for B’ phase. In addition, the derived bulk, shear, Young’s modulus and Debye temperature except Poisson’s ratio are also significantly increased with Cu addition, indicating that Q phase has a favorable improvement of hardness. The elastic anisotropies of the two phases are discussed in detail using several criteria, showing that the anisotropy degree of B’phase is larger than of Q phase. The electronic structures show that the two phases possess a mixed bonding character of covalent and ionic, In particular, the Cu-Si bond plays a significant role for higher stability and larger elastic moduli.The thermoelasticity and the thermal properties of L12-type Al3Sc have been studied using the first-principles calculations combined with the quasi-harmonic approximation. The obtained elastic constants and polycrystalline elastic moduli exhibit a very gentle descent trend with increasing temperature, indicating that Al3Sc has favorable elastic heat-resistant properties. Elastic anisotropy of Al3Sc is slightly enhanced with increasing temperature. The relevant important thermodynamic properties including equilibrium volume V, isothermal bulk modulus BT, thermal expansion coefficient aL, isochoric heat capacity Cv and isobaric heat capacity Cp are further derived and discussed. The calculated results show variation trends of these thermal properties are slower than Al as temperature. The moderate variation of elastic properties and the thermal properties may be attributed to the stable Al-Sc covalent bonding which is slightly weakened with the increasing temperature.Thermal properties and thermoelasticity of L12 A13RE (RE=Er, Tm, Yb, Lu) phases have been studied by first principles calculations combined with density functional perturbation theory and quasi-harmonic approximation in temperature range 0K-900K, the calculated thermal properties including equilibrium volume, isothermal bulk modulus, Gruneisen parameters for different A13RE phases exhibit analogous variation tendency and the numerical values are associated with the atomic radius of RE. The calculated thermal expansion and heat capacity for different A13RE are almost indistinguish from each other. The calculated lattice misfits of A13RE phases show similar decreasing trend with temperature, indicating better lattice coherency of A13RE phases in Al matrix. The calculated elastic constants and elastic properties exhibit gentle decreasing trend with temperatue, indicating A13RE phases can remian definite strength at high temperature and are the potential strengthening phases of Al alloys.