Theoretical Study On Mechanical And Thermodynamic Properties Of Titanium Aluminum Alloys

Among titanium aluminum alloys,γ-TiAl and α2-Ti3Al are widely used in aerospace,marine,automotive turbine manufacturing industries and other fields for their good specific strength,specific stiffness and oxidation resistance.The single-crystal γ-TiAl,single-crystal α2Ti3Al and γ-TiAl systems doped with C/O are studied by using the first-principles method.The main conclusions are as follows:1.Temperature is an important factor affecting the mechanical and thermodynamic properties of alloys.In the thesis,the mechanical and thermodynamic properties of γ-TiAl and α2-Ti3Al are studied by using the combined method of density functional perturbation theory,quasi-harmonic approximation and continuum elasticity theory at high temperature.By analyzing the phonon dispersion relations and elastic constants,it is proved that γ-TiAl and α2-Ti3Al have phase stability at high temperature.The calculations of volume thermal expansion coefficient and heat capacity at constant volume show that the heat capacity of α2-Ti3Al and volume of y-TiAl are easily affected by temperature.The results of elastic modulus indicate that a2-Ti3Al has better resistance to deformation and shear stability,while γ-TiAl has better ductility and resistance to external pressure at high temperature.The results show that γ-TiAl and α2-Ti3Al have broad application prospects at high temperature.2.C and O impurities are easily trapped in alloys and affect stability and mechanical properties of alloys.In the thesis,the effects of C and O atoms on the mechanical properties of γ-TiAl are investigated by using the Nielsen-Martin theory.The results of formation energy prove that O atoms are more likely to be trapped in γ-TiAl.The phonon dispersion relations and elastic constants show that the phase stability of γ-TiAl is not affected by C and O atoms at high temperature.The calculations of tensile strength show that C and O atoms have little effect on the theoretical tensile strength ofγ-TiAl.The results of elastic modulus and partial densities of states indicate that C and O atoms weaken the ductility of γ-TiAl,but improve its hardness and γ-TiAl still retains good resistance to external pressure at high temperature.The results prove that γ-TiAl is a superior high-temperature material.