Density Functional Theory Studies On Thermodynamic Properties Of TiAl

TiAl alloy is widely used in aerospace,industry for its advantages of light weight,thermal stability,high specific stiffness.However,the poor ductility and unstable mechanical properties under high temperature limit its practical application.This dissertation systematically investigate the mechanical and thermodynamic properties of X(X=Zr,Nb,Mo)doping for TiAl alloy by using density functional theory(DFT)and density functional perturbation theory(DFPT).Explore the effect way for improving the thermodynamic properties of TiAl.The formation energies of Ti8Al7X and Ti7Al8X(X= Zr,Nb,Mo)system are calculated negative,indicating that they are stable.The energy difference of Ti8Al7X and Ti7Al8X suggest that Zr,Nb,Mo all tend to occupy Ti-site,and with the atomic number increases the ability of Zr,Nb,Mo occupy the position of Ti become weaker.The more the axial ratio c/a close to 1,the better the ductility is.The calculated axial ratio show that X occupy the Al-site can improve the ductility of TiAl,underless X occupy Ti-site has little influence on the ductility.The electronic structure of Ti8Al7X and Ti7Al8X shows that X locating at the Al-site can weaken the directionality of Al-Al,Al-Ti covalent bond,consequently improve the ductility of TiAl.Zr doping weakens the bonding strength,make the TiAl alloy become soft.While Nb and Mo strengthen the,enhance the compression resistace and the strength of alloy.On the basis of density functional perturbation theory(DFPT),we calculate phonon dispersions,obtain the free energy the equilibrium lattice constant at different temperatures,and derive the thermodynamic properties.The calculating results found that Zr substiting Ti can increase the stability and strength under high temperature,so Ti7Al8Zr system has a good ability to resist creep;TiAl-Nb has improves the strength of TiAl but unstable under high temperature;Mo doping enhanced the intensity under 500K,but decrease the thermostable above 500K significantly.