Electronic Structure And Mechanical Properties Of Cubic Al-based Intermetallics

Al-based intermetallic compounds have long been known to possess attractive chemical, physical, electrical, magnetic and mechanical properties that are often superior to those of ordinary metals. Therefore, they have wide application, especially, in the aerospace field, microelectronic, motorized vehicles and domestic industry. But their brittleness at room temperature has severely hampered their applications. Recent theoretical investigations have revealed that the mechanical behavior, in particular the intrinsic brittleness of intermetallics should be attributed originally to the nature of their chemical bonds or their electronic structure, although various factors may cause different embrittlement cases. However, only few researches for deeply understanding the electronic structures and mechanical properties of Al-based intermetallic compounds through the electronic structure information were introduced because of the restrict of the experimental condition. Therefore, it is crucial to investigate the electronic structures in detail in order to understand the brittleness of the alloys completely.It is well known that first principles calculations based on density functional theory is a capable way to caculate the properties of solid and surface, and predict many material qualities and their tendency of variations from microscopic view. Firstly, we carry out a systematic investigation of electronic structure and mechanical properties of typical cubic structure of Al-based intermetallic compounds (AlCu₃, AlCu₂Zr and AlZr₃) by first principles calculations. Secondly, we report the relationship between the bulk and surface properties of ternary Al-based intermetallic compounds (AlCu₂Ti, AlCu₂Mn, AlCu₂Zr and AlCu₂Hf). The main contents of our work are:(1) First-principles calculations were performed to study on alloying stability, electronic structure and mechanical properties of Al-based intermetallic compounds (AlCu₃, AlCu₂Zr and AlZr₃). The calculated results show that the lattice parameters obtained after full relaxation of crystalline cells are consistent with experimental data. The negative value of formation energies and the cohesive energies show that these Al-based intermetallics have strong alloying ability and structural stability. AlZr₃ phase has the highest structural stability, AlCu₂Zr has an intermediate structural stability, while AlCu₃ has a lowest structural stability. In addition, the elastic constants Ci j of these Al-based intermetallics were calculated, and the elastic modulus (bulk modulus B , shear modulus G , Young's modulus E , Poisson's ratioνand anisotropy constant A ) of these phases were derived from the elastic constants. The mechanical properties of these intermetallics are further discussed.(2) First-principles calculations were also performed to study on alloying stability, mechanical properties, electronic structure and surface properties of AlCu₂X (X=Ti, Mn, Zr, Hf) intermetallics within the generalized gradient approximation. The formation energy and cohesive energy were calculated and used to study the stability of AlCu₂Ti, AlCu₂Mn, AlCu₂Zr and AlCu₂Hf intermetallics. The results showed that AlCu₂Zr phase have the strongest alloying ability and structural stability in these four intermetallics. According to the calculated density of states (DOS) of these intermetallics, it is found that the highest structural stability of AlCu₂Zr is attributed to the lower value of the DOS at the Fermi level, i.e. N(EF). Three independent elastic constants ( C₁₁, C₁₂ and C 44) as well as mechanical paraneters such as bulk modulus B , shear modulus G and Poisson's ratioνof these four intermetallics have been calculated. In addition, the electron work functions (EWF) and surface energies of (100) and (110) surfaces of AlCu₂X intermetallics were also investigated. Based on the above research, we further to analysis and discuss the bulk and surface properties of these materials, and found out the intrinsic relation of mechanical properties and electronic structure.