| Ti2AlNb alloys have been favored as potential materials for aerospace application due to their excellent specific strength,good oxidation,and creep resistance.Addition of alloying element Mo to the Ti2AlNb alloys was shown to enhance the phase transformation from body-centered cubic B2 to orthorhombic O and slow down from close-packed hexagonal ?2 to B2,improving the plasticity creep resistance of the alloy.Therefore,we took the Ti-22Al-24Nb-0.5Mo alloy as the research object and used hot isostatic pressing and thermal extrusion or forging to obtain the rod or billet for as-received materials before hot simulated deformation.The stress-strain relationship,hot processing maps and microstructural evolution of the Ti-22Al-24Nb-0.5Mo alloys in the temperature of 930 ?-1050 ? and the strain rate of 0.001-1 s-1 were investigated,providing technical support for the optimization of hot working.The Ti-22Al-24Nb-0.5Mo alloy powders were prepared by ultrasonic gas atomization without crucible induction melting.The stress field and relative density distribution during the process of hot isostatic pressing were studied by ABAQUS software.The results showed that the stress value and relative density at the edge of the alloy are larger than that in the center.Combined with X-ray tomography?Micro-CT?technology,three dimensional images of the volume fraction,size and the internal defects of the alloy confirmed the simulation results.The hot deformation behavior and microstructural evolution of the extruded and forged alloys were studied.The results showed that the flow stress of both alloys increased with decreasing temperature and increasing strain rate.When the strain rate is higher than 0.1s-1,the stress-strain curves showed multiple yielding phenomena.According to the Arrhenius equation,the apparent activation energy of the extruded and forged alloys in the three-phase region??2+O+B2?is about 700 k J · mol-1,but in the two-phase region??2 +B2?,the apparent activation energy of the forged alloy?571 k J · mol-1?is higher than that of the extruded alloy?417 k J · mol-1?.According to the hot processing map,the suitable working zone of the extruded alloy in the three-phase region is in the range of temperature 950-990? and the strain rate less than 0.005s-1.In this case the highest power dissipation rate is 47%,and the microstructure is uniform and the spheroidization of O phases occurred.The suitable working zone of the extruded alloy in the two-phase region is the temperature higher than 1020? and strain rate lower than 0.005s-1,and temperature higher than 1030? and strain rate higher than 0.1 s-1,where the highest power dissipation rate is 47% and the volume fraction of dynamic recrystallization is high.The suitable working zone of the forged alloy in the two-phase region is in the range of temperature 990-1030? and strain rate lower than 0.005 s-1,and temperature higher than 1035? and strain rate of 0.005-0.1 s-1,where the highest power dissipation rate is 52%;the grains size is uniform and continuous dynamic recrystallization occurred.The instable zone of the extruded alloy is the temperature less than 980? and strain rate of 0.005-0.05 s-1,or temperature 980-1010? and strain rate more than 0.1s-1,where the volume fraction of dynamic recrystallization is low.The instable zone of the extruded alloy is the temperature below 940? and strain rate less that 0.05s-1,or temperature below 1000 ? and strain rate more than 0.05s-1,where discontinuous dynamic recrystallization occurred.When the strain rate is higher than 0.1s-1,the multiple yield phenomena of the alloy are attributed to the short-time imbalance between the dynamic softening caused by discontinuous dynamic recrystallization and the work hardening caused by the interaction of the second phases and the dislocations. |
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