Research On High Temperature Deformation Behavior Of TiAl Alloys With Lamellar Microstructure

TiAl based alloys have excellent high temperature strength, creep resistance, oxidation resistance, flame retardant, high elastic modulus and low density. These characteristics make them show remarkable growth in the future in aerospace industry. However the inherent casting shrinkage, shrinkage cavity and segregation defects restrict their applications in high-risk parts. The results show that thermoplastic processing can greatly improve the mechanical properties of these alloys, and widen the scope for use of the materials. However, the thermoplastic processing performance of TiAl based alloys is very poor. It is necessary to carry out detailed study on thermoplastic deformation behavior. In this paper, isothermal forging on four different microstructures of TiAl-based alloys were carried out. The purpose of this study is to achieve TiAl based alloys with good thermal processing and mechanical properties by controlling the microstructure and choosing proper thermal processing parameter.In this paper, hot deformation behaviors of four different morphology of lamellar TiAl-based alloys are investigated by isothermal physical simulation. SEM characterization is used to analysis microstructure evolution and TEM characterization is used to analysis deformation and softening mechanism. Surface observation, combined with the results of SEM and TEM analysis, is used to analysis the apparent plasticity of the material. According to the dynamic material model, the thermal processing map is made. Deformation mechanism is analyzed by the change of internal power dissipation rate. It will be helpful to obtain TiAl-based alloys which have good processing properties and mechanical properties.Ti-43Al-5Nb-0.03Y alloy with coarse lamellar microstructure have poor hot deformability. The volume fraction of dynamic recrystallizedγgrains and its size increase with increasing temperature and decreasing strainrate. Complete recrystallized microstructure is observed above 1200℃. Dislocation slip and twinning in dynamic recrystallizedγgrains are the main plastic deformation mechanism in this alloy. Dynamic recrystallization ofγphase is the main softening mechanism.The relationship between strainrate, temperature and stress can be expressed by Arrehenius equation. The activation energy of this alloy is 457.1 kJ?mol^-1 inα₂+γregion. The appropriate processing region is decided byη>65% according to the deformation map and processing map of this alloyTi-43Al-5Nb-0.03Y alloy with refined lamellar microstructure have limited hot deformability. The volume fraction of dynamic recrystallizedγgrains and its size increase with increasing temperature and decreasing strainrate. Complete recrystallized microstructure can be observed above 1150℃. Dislocation slip and twinning in dynamic recrystallizedγgrains are the main plastic deformation mechanism in this alloy. Dynamic recrystallization ofγphase is the main softening mechanism. The relationship between strainrate, temperature and stress can be expressed by Arrehenius equation. The activation energy of this alloy is 593.7 kJ?mol^-1 inα₂+γregion. The appropriate processing region is decided byη>46% according to the deformation map and processing map of this alloy.Ti-42Al-9V-0.3Y alloy with refined lamellar microstructure has excellent hot deformability. The volume fraction ofβgrains increased with increasing temperature and decreasing strainrate. Stripγphase precipitates inβmatrix and complete recrystallized microstructure can be observed above 1100℃. Dislocation slip inβmatrix andβ/γlamellae are the main plastic deformation mechanism in this alloy. Dynamic recovery ofβphase and dynamic recrystallization ofγphase are the main softening mechanism. The relationship between strainrate, temperature and stress can be expressed by Arrehenius equation. The activation energy of this alloy is 531.7 kJ?mol^-1. The appropriate processing region is decided byη>45% according to the deformation map and processing map of this alloy.Ti-45Al-5Nb-0.8Mo-0.3Y alloy with refined lamellar microstructure has good hot deformability. The volume fraction ofβgrains decreased with increasing temperature and decreasing strainrate. Complete recrystallized microstructure can be observed above 1100℃. Dislocation slip in recrystallizedγgrains are the main plastic deformation mechanism in this alloy. Dynamic recovery ofβphase and dynamic recrystallization ofγphase are the main softening mechanism. The relationship between strainrate, temperature and stress can be expressed by Arrehenius equation. The activation energy of this alloy is 395.5 kJ?mol^-1. The appropriate processing region is decided byη>43% according to the deformation map and processing map of this alloy.Introduce of a certain amount ofβphase into TiAl based alloys can effectively accelerated decomposition of lamellar structure. Then the completed globalization temperature can be reduced. Dynamic recovery ofβphase and dynamic recrystallization ofγphase are the main softening mechanism. So the flow stress was reduced. So the introduce of a certain amount ofβphase into TiAl based alloys is a feasible method for achieving TiAl based alloy which has excellent deformability and good mechanical properties.