Investigation Of Mechanical Properties Of Typical Long Period Phases In Ti-Al And Mg-Zn-Y Alloys

Ti-Al alloys possess outstanding properties including high strength, high elastic modui, light weight, low density, good thermal conductivity, high melting point, excellent resistance to oxidation and corrosion. As one of the most promising high-performance light-weight structural materials, Ti-Al alloys have been widely applied in aerospace, aircraft, high speed trains, automobile, etc. However, their practical applications are still limited because of their extreme brittleness and poor ductility at room temperature. As excellent light-weight structural materials, Mg alloys have been widely applied in the aerospace, transports and electronics mainly due to their low densities and highest strength-to-weight ratio. However, their use has been limited because of lower tensile strength and inferior ductility. The recently developed Mg-Zn-Y system seems to be particularly promising in improving the mechanical behavior of Mg-based alloys. Specially, the Mg97ZniY2(at.%) alloy developed by rapidly solidified powder metallurgy (RS P/M) exhibits extremely high thermal stability at elevated temperatures and excellent mechanical properites with tensile yield strength of610Mpa and elongation of5%at room temperature. It has been found that there are a larger number of novel long period structures (LPSs) in Ti-Al alloys and Mg-Zn-Y alloys, and the mechanical properties have been greatly improved due to these LPSs.First principles calculations have been carried out to investigate the lattice parameters, formation enthalpy, elastic constants, bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, elastic anisotropy, tensile stress strain relations and shear stress strain relations of typical long period phases in Ti-Al alloys and Mg-Zn-Y alloys based on density function theory in this paper. The charge density distribution, density of states, interaction of atoms and chemical bonding were analyzed in detail to reveal the underling relation of mechanical properties and electronic structures and micro-structures in electronic and atomic level, and the relevant mechanism was further clarified.Based on density function theory, the structural stabilities, elastic and electronic properties of typical LPSs Al5Ti3, h-Al2Ti and r-Al2Ti in Al-rich TiAl alloys together with γ-TiAl were studied by first-principles calculations. The obtained lattice parameters by relaxation of crystalline cells are in good agreement with the experimental data. The calculated formation enthalpies show that r-Al2Ti has the highest structure stability from energetic point of view, and then followed by h-Al2Ti, Al5Ti3and γ-TiAl. The elastic constants were calculated, suggesting that these structures are mechanically stable. Bulk modulus B, shear modulus G, Young’s modulus E and Poison’s ratio v of polycrystalline materials were derived from the elastic constants. By several criteria, elastic anisotropies were analyzed, showing that these structures possess different degree of anisotropies. The electronic density of states and charge density distribution indicate that due to strong hybridization between Al-2p and Ti-3d, there is a strong directional bonding between Ti and Al atoms.First-principles calculations were performed to systematically investigate the structural, mechanical and electronic properties of seven one dimensional long period structures (1D-LPSs) of Al3Ti, and the influence of antiphase boundary period parameter M’ was analyzed and discussed in detail. With increase of M’,the calculated lattice parameter a shows a decreasing tendency, whereas c*, c*/a and V0show the opposite variation tendency. Under small strains, the calculated elastic constants C11is decreased, while opposite variation tendency can be found for C12, C44and C66. M’ has little influence on bulk modulus B for these1D-LPSs, whereas shear modulus G and Young’s modulus E are increased, but the Poison’s ratio v and ratio B/G are decreased. Furthermore, Ga is increased while Go/Ga is decreased. Oppositely, Ea is decreased while Eo/Ea is increased. Three-dimensional directional representation of elastic moduli and the plane projections show that the elastic anisotropy is enlarged. At larger strains, the tensile stress-strains were further studied. The results show that except for<32>, the uniaxial tensile deformation under [100] direction is much easier than [001] direction. The critical tensile strainsε[100] indicate that the ductility along [100] direction decreases as M’increase. The electronic structures show that the bonding exhibit complex mixing of metallic and covalent character. With increase of M’,the metallic bonding is weakened which leads to poor ductility, and the covalent bonding is enhanced which results in larger stiffness. The electronic structures also indicate that as the strain increase the stability of the structure is lowered.The stress-strain relationships for different shear processes of Llo TiAl have been investigated from first principles calculations, and the peak shear strengths in four slip systems were obtained. It is found that the ideal shear strength of L10TiAl occurs in the <112]{111} direction. Via analyzing the structural unit cell, bond length and charge density, the deformation modes under shear were elaborately discussed. It is indicated that the interaction between atoms is gradually weakened accompanying with depletion of charge density as the strain increase. The density of states was studied in detail and shown that strong hybridization exists between Ti3d and Al1p and the structure stability would be lowered with increase of the strain.Theoretical investigations of the effect of Y and Zn atom substitution on elastic and tensile properties of6H-type ABCBCB LPSO structure in Mg97Zn1Y2alloy have been performed. Under small strains, the elastic properties including elastic constants and elastic modulus were investigated, and the influence of Y and Zn substitution were discussed in detail. Elastic anisotropies were analyzed by several methods, and the results show that the anisotropy in compression is almost negligible, whereas the anisotropy in shear is relatively large. Furthermore, the shear anisotropy becomes larger with Zn substitution than Y substitution. Under larger strains, the tensile stress-strain relations were calculated. The results show that at small strains anisotropy of Young’s modulus for Mg95Zn is larger than that for Mg95Y, whereas at lager strains anisotropy of peak tensile stress for MggsZn is smaller than that for Mg95Y. The ideal tensile strengths for both Mg95Y and Mg95Zn phases occur in<1120> direction, and the ideal tensile strength is increased with single Zn atom substitution. The electronic structures indicate that the Mg-Y and Mg-Zn bonds exhibit covalent feature due to hybridization, so the interactions of Mg with Y and Zn are enhanced, and the electronic structures further reveal that the directional bonding of Mg-Y and Mg-Zn would lead to large anisotropy of tensile stress-strain relations. As the strain increases, directional bonding is gradually weakened, and the structure stability is also lowered.