Study On High Temperature Deformation And Microstructure Evolution Of Ti₂AlNb Alloy

Compared with the traditional nickel-based super alloy,Ti₂AlNb based alloy possess high specific strength and specific stiffness,as well as excellent high temperature creep resistance,good fracture toughness and oxidation resistance,and low coefficient of thermal expansion characteristics in the aviation.As the potential lightweight high-temperature structural materials used in 650?to 800?,the ordered orthorhombic Ti₂AlNb-based titanium intermetallic alloys have been received considerable attention in the field of intermetallic compounds.Due to the addition of a large amount of Nb element,the O phase which had the ordered orthorhombic structure could be stable as an equilibrium phase.Because of complex relationship between the main components phases of hcp?₂?Ti₃Al,DO19 structure?,bcc??disordered?/B2?ordered structure?and O?CmCm symmetry based on Ti₂AlNb?,Organizational structure and mechanical properties are sensitive to deformation process.Therefore,it is certainly important and worthy to systematical investigate how the process parameters influence on the microstructure of Ti₂AlNb-based alloy during the high temperature deformation.The high temperature mechanical behavior and recrystallization of Ti-22Al-26Nb alloy was researched by isothermal hot compression tests.The regression analysis of hyperbolic sine model was adopted to obtain the high temperature constitutive model,assisted with the Gorsman formula.The results show that it is feasible to establish the high temperature constitutive model of Ti-22Al-26Nb alloy based on the hyperbolic sine function,which can describe the high nonlinearity of the thermodynamic parameters of Ti-22Al-26Nb alloy at high temperature complex relationship.From the modified Avrami equation,it is predicted that the recrystallization volume fraction and the strain show a recrystallization kinetics growth trend,and the complex softening behavior of the alloy during high temperature deformation is revealed.Thermal characteristics of the phase transformation involved were characterized by differential scanning calorimetry by heating Ti₂AlNb pre-alloyed from room temperature to 1160?.The test was aim to provide a theoretical basis for the development of hot processing techniques to control the microstructure of alloys.The initial and as-processed specimens were subjected to microstructural characterization using Scanning Electron Microscope,X-ray diffraction and electron backscattered diffraction.Analyze the change of microstructure morphology under different conditions to systematically explain the internal microscopic topography transformation relationship of the alloy.It is found that the content of?₂ phase in the microstructure of Ti-22Al-26Nb alloy decreases with the deformation temperature.But the degree of isometric increased gradually with the increase of temperature.The magnitude of the deformation mainly affects the precipitation of the O phase and the orientation of the?₂ phase.When the amount of deformation is large to a certain extent,the ordering of the B2 phase structure precipitates an orthogonal O phase,which exists in a sheet form after the cooling process.The precipitation of O phase shows a peak at the B2 diffraction peak in the X-ray diffraction results.With the decrease of the strain rate,the peak intensity of the strongest diffraction peak is weakened.The transformation of the O phase at high temperature is not limited by the influence of temperature.The high strain rate can also promote the precipitation of O phase on the substrate.During the hot-pressing deformation at 950°C,the?₂ phase is prone to twinning of the{1012}crystal plane,in which the saturated fine platelet O phase precipitates inside the?₂ particles.It is Formed an incompletely transformed O phase alternating with a distorted?₂ phase inside the?₂phase.After hot compresed at 1050°C,the sample is almost completely equiaxed and the recrystallized B2 grains are mainly surrounded by high-angle grain boundaries.