| With the rapid development of the aerospace industry,there are increasingly urgent demand for lightweight high-temperature,high-strength and high-toughness structural materials.It is found there are abundant choices to control the composition,structure and performance of the alloys in Ti-AI-Nb ternary system.Ti2AINb-based alloys with a dominated orthorhombic(O)phase have higher specific strength,room-temperature plasticity,fracture toughness,creep resistance,better oxidation resistance and non-magnetic properties,and so on,and that is one of the most promising high-performance structural materials for the aerospace applications.In this thesis,the structural stability,high-temperature elastic properties and thermodynamic properties of Ti2AlNb-based O phase were systematically studied by combining with the first-principles calculations based on the density functional theory(DFT)and quasi-harmonic approximation(QHA).The relationship between the elastic mechanical properties and electronic structure was revealed.The thermodynamic database for end-members of Ti2AlNb 0 phase was calculated and constructed by calculation of phase diagrams(CALPHAD)to study the relationship between the ordering behavior of the alloying elements on sublattice and the temperature,and the influence of the ordering behavior on the properties of the Ti2AlNb O phase were investigated finally.The main results were stated in the following.The elastic mechanical properties of the full-ordered Ti2AlNb 0 phase decrease with increasing temperature from 0 K to 1300 K.And BIG values indicate that the Ti2AlNb O phase is an intrinsic ductile material at different temperatures.The Ti2AlNb O phase is anisotropic,which is close to the isotropy with the increase of temperature.The predicted results agree with the available litereatures.Based on the electronic structure analysis,it is found that the decrease of the elastic properties with the increase of temperature is mainly due to the weakening of the hybridization and non-localization of the electronic states.The end-member thermodynamic database was constructed by using the enthalpy of formation and the Gibbs free energy of formation of the end-member compounds,respectively.CALPHAD was used to calculate the relationship between the ordering behavior of the alloying elements and temperature in the Ti2AINb-based O phase with different compositions.When the temperature is lower than 800 K,the results show that the ordering behavior is almost the same for the two kind of thermodynamic databases,respectively.When the temperature is higher than 800K,however,the site occupancy of alloy elements tend to be disordered based on Gibbs free energy of formation of the end-member thermodynamic database,which agree better with the available literatures.So the end-member thermodynamic database concerned the relationship of the Gibbs free energy of formation and temperature was constructed to describe the ordering behavior of alloying elements,which is more reasonable and accurate.The site occupying fractions of elements in sublattice were calculated based on the end-member thermodynamic database concerned the relationships between the Gibbs free energy of formation and temperature.The Special Quasirandom Structure(SQS)was used to calculate the properties of Ti2AINb-based O phase based on the site occupying fractions,and the influence of the ordering behavior on the elastic properties was discussed The results show that the Young's modulus and the shear modulus decrease with the decrease of the ordering behavior,while the bulk modulus changes less.That is due to the reduction of the ordering behavior increases the lattice distortion,while affects the volume of the alloy phase slightly.The investigated approach and quantitative results for O phase are essential fundamental for the designing and controlling the composition,heat treatment,phase structure and mechanical properties of Ti2AlNb-based alloy. |
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