Researches On Processing Optimization And Microstructure-property Relationship Of TC21 Titanium Alloy

To adapt to the durability/damage tolerance design of the aircraft structure,as the key material in aviation field,titanium alloys are gradually moving towards high damage tolerance.Based on the metallurgical principle,damage tolerant titanium alloys,such as TC4-DT and TC21 alloy,have been developed in China to meet the damaged aircraft structures’ requirements for the static strength and fatigue properties.Mechanical properties of titanium alloys are determined by the microstructure,and they are very sensitive to processing parameters.The new damage tolerance design principles bring forward rigorous demands for the thermo-mechanical processing of titanium alloys.Exploring microstructure-property relationship and optimizing processing parameters of the damage tolerant titanium alloy are of great significance for improving the alloy’ performance and extending the service life of the aircraft.In this paper,the forming property and mechanical properties of TC21 titanium alloy were researched systematically.Based on isothermal hot compression tests,flow behaviors and microstructure evolution during the high temperature deformation were studied,and the constitutive equation was established.Processing parameters were optimized by processing maps and forging and heat treatment experiments.The relationships between tensile properties,fracture toughness and microstructural parameters were quantitatively described,and the main microstructural parameters influencing the alloy’s mechanical properties were determined.Strengthening and toughening mechanisms of titanium alloys were revealed.The main research work was as follows:The influence of the soaking temperature before the forging on microstructures of TC21 alloy with equiaxed starting microstructure was studied.It was found that the microstructure consisted of equiaxed α phase,α plates and residual β phase at temperature below 900℃,the microstructure was mainly made up of equiaxed α phase and equiaxed β grains when the temperature was 900℃ and the β transus,while the microstructure was composed of equiaxed β grains above the β transus.During the deformation in two phase field,equiaxe α phase had little plastic deformation.The globularization of α plates was the main form of the microstructure evolution below 900℃,while the dynamic recrystallization of β grains above 900℃.In single β field,dynamic recrystallization of β grains occurred.With the increase of the temperature and the drop of strain rate,the DRX volume fraction and the grain size of DRX grains increased.The globularization behaviors of TC21 alloy with lamellar initial microstructure deformed in two phase field were researched.It was found that the globularization kinetics meet Avrami equation,and as the temperature increased and the strain rate declined,the globularization volume fraction increased.The globularization process of the lamellar microstrucuture mainly included sub-boundary formation and boundary splitting.The sub-boundaries were formed by dynamic recovery and the shear and bend of α plates,while the boundary splitting was realized through the connections of thermal etching grooves.There were significant characteristics of dynamic softening in the flow curves of TC21 alloy,and with the temperature increasing and the strain rate decreasing,the flow stress reduced.Discontinuous yielding appeared in single β field and the ranges of high tempereature-high strain rate and low tempereature-moderate strain rate in two phase field.The constitutive equation based on the hyperbolic sine type Arrhenius function was established,and it could accurately predict the flow stress of TC21 alloy.The processing maps of TC21 alloy was constructed based on the dynamic material model,and the safe processing domains were determined: 840℃~1040℃,0.01s-1~1s-1 and 820℃~1040℃,0.001s-1~0.01s-1.The safe processing domains were characterized by dynamic recovery,dynamic recrystallization,the globularization of α plates and superplasticity.The instability regions mainly focused on the range of high strain rates,and they were characterized by localized plastic flow in two phase field and adiabatic shear bands in single β field.The optimizing experiments of forging and heat treatment parameters were conducted to improve the mechanical properties of TC21 alloy.The optimized heat treatment was solution treatment and aging: 900℃/1h,AC+590℃/4h,AC.After general forging at 980℃,the mechanical properties of this alloy met the technical conditions,but the forging temperature range is narrow.When the near isothermal forging with die temperature of 940℃ and strain rate of 0.01s-1 was adopted,the mechanical properties of this alloy also met the technical conditions,and the forging temperature range was extended to 940℃~990℃.Under those optimized processing parameters,the microstructure was composed of pan-like β grains with similar grain sizes,in which was the crisscross coarse α plates and sometimes a handful of equiaxed α phase.The quantitative relationships between mechanical properties and microstructural parameters of TC21 alloy were established.It was found that increasing the content of equiaxed α phase and residual β matrix would enhance the alloy’s strength,the increase of equiaxed α phase was helpful to improve the alloy’s plasticity,and the increasing in content and thickness of crisscross α plates could improve the alloy’s fracture toughness.The strengthening and toughening mechanisms of titanium alloys were revealed.With increasing the content of equiaxed α phase and residual β matrix,the incoherent and semi-coherent boundary increased,and the resistance of the dislocation motion increased,and finally the alloy was strengthened.The effective slip length in equiaxed or bi-modal microstructure was the equivalent diameter of equiaxed α phase due to the incoherent α/β phase boundary,through which it was difficult for dislocations to pass.The effective slip length in lamellar microstructure was the equivalent diameter of β grains or α colonies because the dislocations could slip in β grains or α colonies due to the semi-coherent phase boundary of α plates and secondary α platelets.The increasing equiaxed α phase limitted the size of the β grains or α colonies,which reduced the effective slip length,and finally enhance the alloy’s plasticity.The α phase was tough.If the crack encountered α phase during the propagation,the energy consumed by the plastic zone of the crack tip would increase.So the increasing of α phase could enhance the fracture toughness.during the propagation,the crack could bypass the equiaxed α phase,but had to pass through the crisscross α plates,therefore,crisscross α plates were helpful to improve the fracture toughness.With thickening of α plates,the energy consumed by the plastic zone of the crack tip in α plates also increased,which also improved the fracture toughness.