Interatomic bonding, elastic properties, and ideal strength of transition metal aluminides: A case study for Al₃(V,Ti)

On the basis of ab initio density-functional calculations we have analyzed the character of the interatomic bonding in the intermetallic compounds Al₃(V,Ti) with the D0₂₂ and L1₂ structures. In all structures we found an enhanced charge density along the Al-(V,Ti) bonds, a characteristic feature of covalent bonding. The bonds in Al₃V with the D0₂₂ structure are more saturated and stronger than the corresponding bonds in Al₃Ti. High symmetry of the transition metal sites in the L1₂ structure leads to higher metalicity of alloys assuming this structure. The bond strength is quantitatively examined by tensile deformations. The ideal strength of Al₃V and Al₃Ti under uniaxial tensile deformation was found to be significantly higher than that of both fcc Al and bcc V. We investigated also the changes of the interatomic bonding in Al₃V during tensile deformation. We found that the covalent interplanar Al-V bonds disappear before reaching the maximal stress. The weakening of the bonding between the atomic planes during the deformation is accompanied by a strengthening of in-plane bonding and an enhanced covalent character of the intraplanar bonds. Interplanar bonding becomes more metallic under tensile deformation.