Electronic Structure And Mechanical Properties Of Nb-doped TiAl Intermetallic Compounds

This dissertation focuses on the investigations of the electronic structure and mechanical properties of ?-TiAl,orthorhombic TiAl2 and the TiAl3 intermetallic with DO22 structure,in which the Al or Ti atoms are partly and uniformly substituted by the Nb atoms,by using the first-principles calculation method based on the density functional theory.The effect of Nb doping on the ductility of y-TiAl,orthorhombic TiAl2 and the TiAl3 compounds will be revealed by analyzing the electronic structure and mechanical properties of Nb-doped TiAl systems.It will provide some theoretically guidances and advices for the experimentally implementation.The results suggest that:(1)The structural symmetry of ?-TiAl,orthorhombic TiAl2 and the TiA13 intermetallic with DO22 structure has been changed after the A1 or Ti atoms are partly and uniformly substituted by the Nb atoms.(2)The results of the formation energy of the doped systems show that Nb atoms are more likely to occupy the sites of Ti atoms to form a stable structure for y-TiAl and TiAl3,while the Nb atoms are more likely to occupy the position of Al atoms to form a stable structure for the orthorhombic TiAl2.The formation energy of the doped systems increases with the increase of doping concentration.(3)Nb doping weakens the covalent bonding between Ti atoms and A1 atoms and enhances the metal bonding between them.Each doping systems have metal conductivity,which is beneficial to decrease the room temperature brittleness of y-TiAl,orthorhombic TiAl2 and the TiAl3 intermetallic with DO22 structure and to improve the ductility of the systems.Compared with the Ti atoms substituted by Nb atoms,the ductility of the system can be more improved in which the A1 atoms are partly and uniformly substituted by the Nb atoms.The fracture strength of Nb-doped systems is both better than undoped systems,especially in the system of Ti atom replaced by Nb.(4)The density of state of the Nb-doped systems and the correspondent pure phase indicates that the density of state near Fermi energy have been increased after Al atoms being replaced by Nb atoms in the Nb-doped systems.This is an important factor for improving the ductility of y-TiAl,orthorhombic TiAl2 and the TiAl3 intermetallic with DO22 structure.