Rolling Deformation Behavior, Microstructure and Properties of Beta-gamma Ti-Al-V-Y Alloy

Author:Zhang Yu

Supervisor:chen yu yong


Degree Year:2018





TiAl-based alloys with low density and excellent high-temperature properties are considered promising light high-temperature structural materials for aerospace applications,such as inlet flaps,nozzle sidewalls for turbine engines,and thermal protection systems for scramjets.Hot pack-rolling processing,as the most practical hot-working method for TiAl alloy sheet production,will probably be required for the fabrication of various TiAl alloy structural components with complex shapes.However,the high tensile stress,severe heat loss,and high strain rates associated with hot-rolling,as well as the limited hot deformability of these alloys,prevent widespread use of the corresponding TiAl alloy sheets.Recently,a novelβ-γTiAl alloy with excellent deformability has gained significant attention.This alloy is characterized by a high volume fraction of theβphase,obtained through the addition ofβ-stabilizing elements.Theβphase,with abundant independent slip systems on the grain boundaries,plays a significant role as a lubricant during thermomechanical processing.In this paper,we study the novel Beta-gamma Ti-43Al-9V-0.2Y alloy,optimise the hot-rolling processing parameters,and investigate the deformation mehcanism,dynamic recrystallization behavior,microstructure and mehcanical properties systematically.Based on the grain orientation spread(GOS)method,the deformation mechanism and dynamic-recrystallization behavior atfter hot-rolling at 1200℃were analyzed.After hot-rolling,theβ/γlamellar microstructure of the as-forged TiAl alloy was completely converted into a homogeneous duplex microstructure with an averageγgrain size of 10.5μm and dynamic recrystallization(DRX)of 65%.The high angle grain boundary(HAGB)and low angle grain boundary(LAGB)fractions increases and decreases,respectively,after hot-rolling.The DRX of theγphase in the current TiAl alloy sheet results from the occurrence of both discontinuous dynamic recrystallization(DDRX)and continuous dynamic recrystallization(CDRX)during hot-rolling and plays a key role in microstructure refinement.Furthermore,theθ=89°±3°<100>misorientation angle occurs in the DDRXed grains.The dislocations,sub-grain boundaries,and GNBs within the deformed grains are generated by the high strain rates and high stress associated with the hot-rolling process.These strain rates and stress provide a continuous driving force for the transformation of LAGBs into HAGBs,in preparation for the CDRX process.When the tensile-test temperature is increased to750℃,the sheet exhibits an excellent elongation(53%).The brittle–ductile transition temperature of the TiAl alloy sheet lies between 700℃and 750℃.According to the DSC curve of Ti-43Al-9V-0.2Y alloy,the temperature ofα(α2)+β+γphase region range between 1109℃and 1226℃.According to the grain refining principle,the effects of the diffirent hot-rolling temperatures within theα(α2)+β+γternary-phase region on the microstructure,dynamic recrystallization,texture evolution and mechanical properties were investigated systematically.During the processing of theβ-γTiAl alloy,massive fineγlaths with nano twin structures nucleate from theβ-phase ribbon rolled at low temperature.These fine laths,which are generated(with a certain OR)via theβ→γphase transformation,coarsen with increasing rolling temperature.Lower-temperature and higher-temperature rolling were conducive for dynamic-recrystallization nucleation and growth,respectively,of theγgrains.The fraction of dynamically recrystallized grains increased significantly with increasing rolling temperature.The texture of theγphase evolves significantly with the rolling temperature.Sharp recrystallization texture components,including the Cube and Goss where the crystallographic<001>directions are parallel to the RD,dominate the texture formed at 1100°C.With increasing temperature,the texture is gradually transformed into random deformation components(including orientations with<100>,<01—0>,and<11—0>).The tensile properties of the hot-rolled Ti-43Al-9V-0.2Y alloys were determined at room temperature(RT)and high temperature.The strength of the alloy sheets increases with increasing rolling temperatures of up to 1200°C.For the alloy sheet rolled at1100°C,the low angle grain boundaries,sharp<001>-oriented texture components and mechanical twins in pre-existing nano-scaledγlaths lead to improvements in the room-temperature strength.Maximum ultimate tensile strength(UTS)value of 945 MPa at RT are obtained for the sheet rolled at 1100°C.At 750℃,the alloy sheet rolled at lower temperature(1100°C)also exhibits higher strength(UTS:550 MPa).The TiAl alloy sheet hot-rolled also exhibits an excellent high-temperature ductility with 80%at750℃.The previously higher densities of dislocations and sub-structures,(as pre-stored dislocation defects)during lower-temperature rolling,also promote recrystallization nucleation during tensile deformation.DRX behavior is beneficial for both stress relief during the relatively late stages of tensile deformation,and elongation improvement via the prevention or delay of the crack extension.The small equiaxed DRXedγgrains exhibit better grain-rotation and grain-boundary sliding abilities,which are conducive to elongation improvement.The lamellar microstructures generated during and after the final hot-rolling pass at an initial rolling temperature of 1260°C in(α+γ)phase region were analyzed systematically.This work found that multi-type lamellar structures(such asβ/γ,α2/γ,APD andα2/γmixed lamellar structures)were newly formed after hot-rolling of the alloy.The morphology and nucleation-growth details of the lamellar structures,as well as the phase transformation mechanisms of fineβ/γandα2/γlamellar structures are discussed.Owing to the temperature drop and reheat associated with multi-pass rolling,relatively fine irregularβ/γand regularα2/γlamellar microstructures(average size:25μm)were generated by theβ→γandα+γ→α2+γ/γT phase transformations.These two lamellar structures have different formation mechanisms.The results of electron backscatter diffraction and transmission electron microscopy revealed that the nucleation and growth of(i)β/γand(ii)α2/γlamellar structures are induced mainly by(i)stress and(ii)a combination of the stress and temperature drop during hot-rolling,respectively.A nano-scaled antiphase domain(APD)microstructure was also generated via hot-pack rolling in the(α+γ)phase region.The formation mechanism of the antiphase boundary(APB)and the relationship between nano-scaled APD and mechanical properties are investigated systematically.Owing to the temperature drop and reheat associated with multi-pass rolling,heterogeneous and regular APD-mixed lamellar structures formed with the occurrence ofα→β+γandα→α2+γphase transformations.The APB between two nano-scaled APD structures(average size:15nm)is identified as a type of atomic shift,which is induced by the disorder-order transformation in theα/α2 phase.Moreover,compared with the corresponding as-forged alloy and other hot-workingβ-stabilized TiAl alloys,the current alloy sheet exhibited higher room-temperature strength(826 MPa)with a good ductility(1.4%).The improvement in the tensile properties is attributed mainly to the duplex microstructure consisting of fine nano-scaled APD/lamella mixed structures.The microstructure and superplasticity of Ti-43Al-9V-0.2Y(at.%)alloy sheet hot-rolled at 1100℃has been investigated systematically.A bimodalγgrain-distribution microstructure,with abundant nano-scale to submicron-scale laths embedded insideβmatrix,exhibits an impressive superplasticity.This inhomogeneous microstructure exhibits excellent superplaticity at a relatively low temperature of 800℃(strain rate sensitivity factor:m=0.3)and leads to a high enlongation(362%)at relatively low temperature of 900°C.The dynamic recrystallization behavior and grain boundary slip is the main deformation mechanism of superplasticity in TiAl alloy sheet.