Research On The Microstructure Evolution And Property Of The Multi-component Ti₂AlNb Based Alloy

Light weight,high efficiency and low cost are the main objectives of the development of aerospace structural materials.Ti₂AlNb alloy has become one of the most potential lightweight structural materials for aerospace engines due to its excellent comprehensive mechanical properties.However,Ti₂AlNb alloy can not meet the requirements of high-performance engine structural material design because of its room temperature fracture toughness,high temperature oxidation resistance and creep properties,which greatly limits its application.Therefore,how to develop the next generation Ti₂AlNb based alloys through material composition design and structure control has become an urgent problem to be solved.In recent years,there have been a lot of reports on the alloying of Ti₂AlNb-based alloys with the addition of the fourth element Mo,Zr,V and Si,but mainly concentrated in the simple system of Ti₂AlNb-X alloys.For this reason,a variety of Ti₂AlNb-based alloys containing Mo,Zr,V and Si were designed to optimize the comprehensive properties of the alloys.Based on the excellent oxidation resistance,a new multi-component Ti-22Al-25Nb-1Mo-1Zr-1V-0.2Si(at.%)alloy was selected.The oxidation resistance of the alloy and the effect of composition on the oxidation behavior of the alloy were discussed.Then,based on the dynamic material model,the hot working diagram of the new alloy was constructed to guide the formulation of hot working process.Finally,three typical microstructures were obtained based on different hot working parameters and heat treatment process.The tensile at room temperature and high temperature and creep properties were analyzed and tested.The main results are as follows:(1)A Ti₂AlNb-based alloy with Ti-22Al-25Nb-1Mo-1V-1Zr-0.2Si was designed and optimized.The isothermal oxidation behavior of Ti₂AlNb-based alloy at 650-850?was systematically studied.It was found that the weight gain of Ti₂AlNb-based alloy oxidized at 850?for 100 h was only 0.41 mg/cm^-2,showing excellent high temperature oxidation resistance.The oxidation weight gain,oxide film structure and element distribution of Ti-22Al-25Nb-1Mo-1V-1Zr-0.2Si alloy were systematically compared with those of engineering Ti-22Al-25Nb alloy under the same oxidation conditions.The results show that the oxides of Ti-22Al-25Nb alloy are mainly composed of Ti O₂ at650-750?,and the oxides of Ti-22Al-25Nb alloy at 850?are Ti O₂ and AlNbO₄.The oxidation products of multi-component Ti-22Al-25Nb-1Mo-1V-1Zr-0.2Si alloy mainly consist of Ti O₂,Al₂O₃ and Nb O.The mechanism of high temperature oxidation of two alloys was revealed,and the effects of Mo,V,Zr and Si addition on oxidation resistance were analyzed.(2)The true stress-true strain curves of multi-component Ti-22Al-25Nb-1Mo-1V-1Zr-0.2Si alloy were obtained by thermal simulation compression test.Based on the true stress-strain datas obtained by thermal simulation of compression deformation,the relationship among peak stress,deformation temperature and strain rate were analyzed.Based on the dynamic material model,the constitutive equation and hot working diagram of the alloy were constructed.The unstable deformation zone(deformation temperature 980-1050?,strain rate about 1 s^-1-10 s^-1)with strain of 0.2 and 0.6 is determined.The dissipation efficiency?is between 0.24-0.48,and?value increases with the increase of strain and the decrease of strain rate.(3)The effects of different microstructures on the tensile and creep properties of the alloy at room temperature and high temperature were investigated by controlling the microstructures of the alloy by different hot working processes.It is found that the lamellar structure obtained by air cooling in B2 single phase zone has high tensile strength and excellent creep resistance.The tensile strength at room temperature is 1188MPa,the tensile strength at high temperature is 950 MPa,and the creep strain at 650?/150 MPa is only 0.12%under 100 h loading.The equiaxed structure obtained by air cooling in O+B2 phase region has the characteristics of high plasticity,low creep resistance and strength.The elongation at room temperature is 9.0%,the elongation at high temperature is 36%,and the creep strain is more than 0.2%at 650?/150 MPa loading for 100 h.The ambient temperature,high temperature tensile and creep properties of the duplex structure obtained by air cooling after forging in the three-phase zone of?₂+O+B2 are between the lamellar and the equiaxed.(4)The effect of heat treatment process on the microstructure evolution and properties of the alloy was investigated by controlling the microstructures of the alloy with different heat treatment processes.It is found that with the increase of solution temperature,the size of primary?₂ phase decreases,the volume fraction decreases,the distribution of?₂/O particles becomes uniform,and the volume fraction of B2 phase increases.The tensile strength increases at room temperature and high temperature,but the plasticity decreases.With the increase of aging temperature,the volume fraction of acicular O phase decreases,but the thickness and size of O phase increases.The change of strength and plasticity of the alloy is contrary to that of solution treatment,while the creep property of the alloy decreases at first and then increases.In addition,the mechanism of dynamic transformation from?₂ phase to O phase during high temperature tension is described,and the dynamic decomposition model of?₂ phase is constructed.