| Based on the ordered orthorhombic phase,the Ti2 AlNb alloy exhibits great advantages over other traditional titanium-aluminium alloy in high temperature performance such as creep resistance and fatigue resistanc.In consequence,it has drawn more and more attention in aerospace field.Despite the advantages above,the alloy also suffer deficiencies,such as the brittleness as well as the poor processability in room temperature,restricting its application and development.One way to solve the problem is to investigate its processability in elevated temperature.Therefore,it is important to focused on inquiring the suitable processing parameters as well as the mechanism of phase transformation of the Ti2 AlNb alloy in high temperature.Meanwhile,the thermal deformation process is an effective way to improve the microstructure and eliminate internal defects.For the Ti2 AlNb alloy,the isothermal compression can help promoting lamellar globularization and dynamic recrystallization,which is beneficial to mechanical properties.Thus,it is imperative for us to predict the microstructure in elevated temperature.In this work,by heating the Ti-22Al-25 Nb alloy to various temperatures for different times the precipitation and coarsening behaviors of the three phases in Ti-22Al-25 Nb alloy are explored.Meanwhile,the processing map was also established for obtaining optimum processing windows and achieving desired microstructures during hot working.Furthermore,a kinetics model based on Avrami type equation was conducted to demonstrate the evolution of DRX during the isothermal deformation process.The conclusions can be summarized as follows:(1)During the aging process,the temperature as well as the isothermal time have a strong influence on the microstructure of the alloy.In the O+B2+?2 region,higher temperature usually contribute to smaller grain size and boost precipitation of ?2 phase.However,the behavior tend to be contrary in the O+ ?2 region.(2)The microstructure and phase composition of Ti-22Al-25 Nb alloy are affected by quenching methods and cooling rate at 1030°C.During air cooling process,the fraction of ?2 phase was lager than that of water quench or oil quench.While cooling down,?2 phase is precipitated before O phase.Slower cooling rate may also result in larger grain size and higher area fraction of ?2 phase and O phase.(3)For Ti-22Al-25 Nb alloy,the temperature and the strain rate have a strong influence on the true stress-true strain curve during isothermal compression process.The low temperature as well as the high strain rate usually contributes to a high stress peak.At high temperature and low strain rate,the curve tends to exhibits a steady-state flow after continuous decreasing.Based on the true stress-true strain curve,the constitutive equation and the processing map was obtained.It is found that high power dissipation were mainly distributed in the middle of the map,where the temperature ranges from 980-1060°C and strain rate ranges from 0.1-0.001s-1.Through microstructure observation,the high power dissipation was found to be caused by DRX and lamellar globularization,while the instability regions,which we should avoid during hot deformation,tend to be accompanied by adiabatic shear bands.(4)Based on the Avrami type equation,the kinetics model of dynamic recrystallization was established.The strain rate,the temperature,the power dissipation and the DRX curve have some relationships.Slow strain rate,high temperature as well as high power dissipation usually means fast DRX evolution process. |
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