Study On The Control Of Microstructure And Mechanical Properties Of Damage Tolerance Titanium Alloy TC4-DT Forging

With the design philosophy of airplane evolving from addressing initial static strength tonowadays durability/damage tolerance, the criterion for selecting materials has been changedaccordingly. The original emphasis of mechanical properties was on static strength, fatigueproperties, and creep, durability and thermal stability at high temperature of materialswithout any damages, now its focuses have been added to two other atems of the staticstrength and fatigue properties of materials at the condition of defects or damages, which wascharacterization of high fracture toughness and low fatigue crack growth rate. As the keymaterials for aircraft structures, the titanium alloys designed based on the damage tolerancephilosophy are TC4-DT and TC21in China, and the common processing of this type alloy aremainly in β phase field such as β annealing and β processing which can get fully lamellarmicrostructures with high fracture toughness and low fatigue crack growth rate.In this paper, all the research work was based on the new type titanium alloy of TC4-DT.High temperature flow behaviors were investigated by conducting isothermal hot compressiontests, and the consititutive equation for precise simulation and processing maps weredeveloped based on the flow stress-strain data. Microstructure evolution under the processingsof deformation in β phase field with different parameters followed by different heattreatments in+β phase field, deformation in+β phase field followed by heat treatments in βand+β phase field were systematically studied. The quantitative relationships betweenmicrostructure paramenters and room temperature tensile properties were established at theprocessing conditions above, and the strengthening mechanisms and room temperature tensiledeformation mechanisms as well as fracture mechanisms were deeply analyzed. The influenceof microstructure types on fracture toughness were revealed through the fracturetoughness obtained by different processes, and the effect of widmansttaten microstructureparamenter on fracture toughness were qualitative researched. The main research work was asfollows:1. The high temperature deformation of TC4-DT alloy was very sensitive to strain rateand temperature, and the flow stress increased rapidly with decrease in temperature andincrease in strain rate. There existed a significant characteristics of dynamic softening in theflow curves which presented a characterization of dynamic recrystallization at lowertemperature range and dynamic recovery at higher temperature range. A modifedsine-hyperbolic Arrhenius constitutive equation considering the compensation of strain could accurately simulate the mechanical response TC4-DT alloy during high temperaturedeformation with a smaller error. For the TC4-DT alloy with equiaxed microstructure, themicrostructure evolution mechanism is not the same in different temperature regions andstrain rates and in α+β phase field it mainly related with the phase transformation induced bytemperature rise,while in β phase field it mainly corresponded to dynamic recrystallizationdetermined by strain rate.2. The processing map developed based on danymic materials model delineated thestable and instability regions of TC4-DT alloy during the deformation at high temperature.Combining with the observation of microstructures. the safe processing regions characterizedby dynamic recovery, dynamic recrystallization and superplasticity and instability regionscharacterized by adiabatic shear bands, localized plastic fow and micro-cracks wereconfirmed. The deformation mechanism map was constructed with the correlation ofprocessing map and deformation mechanisms, from which the preferable hot processingparameters were obtained.3. After isothermal forging with different processing parameters in single phase region,the microstructures of TC4-DT alloy could be divided into two categories, type Ⅰ wasmixed lamellar structure after large deformation, and type Ⅱ was lamellar structure throughonce or more times of small deformation which was close to widmanstatten structure. Byquantitative measurement of microstructure parameters in each type, quantitative relationshipswere established between microstructure parameters and room temperature mechanicalproperties. Based on TEM observations of microstructures after tensile test at roomtemperature, it showed that slip in initial coarse plate α played a dominate role with twiningacting on a supplementary effect in the deformation of typeⅠ, and the observations offracture surfaces demonstrated that the fracture mechanism was mainly ductile fracture ofmicroporous connection type. In type Ⅱ microstructures, it deformed mainly via thecoordination among α colonies and there has not been observation of twinning appeared, andthe fracture mechanism was mainly brittle fracture of intergranular and transgranularcleavage.4. Equiaxed and Widmanstatten structures of TC4-DT alloy have been obtained afterheat treatments in α+β two phase and β single phase field. Microstructure evolution underdifferent heat treatment parameters in each phase field was studied. Quantitative relationshipsbetween mechanical properties and microstructure parameters in equiaxed and widmanstattenstructures have been set up through the quantitative measurement of the microstructureparameters. The room temperature tensile strength of Widmanstatten structure was at the same level with equiaxed structure, but the plasticity was in a lower level. The deformation ofequiaxed structure at room temperature depended mainly on slip in equiaxed α phase and βphase in transformed beta structure, and α platelets in transformed beta structure only bearedpartially deformation which gave priority to twinning. During deformation of equiaxed α,dislocation rearrangement happened and sub-grain boundary have been formed. WhenWidmanstatten structure deformed, α colony as a whole participated in the deformationmainly by slip, but the α lath in α colony deformed mainly on dislocation and twinning. Thedeformation of Widmanstatten structure at room temperature could also result in the change ofsub-structure within alpha lath to form subgrain. Fracture surface observations showed thatfracture mechanism of equiaxed structure was mainly ductile fracture, while Widmanstattenstructure mainly presented characteristics of cleavage fracture or a mixture of cleavage andintergranular brittle fracture.5. Fracture toughness under different processings was studied and its affecting factorswere analyzed. From the point of microstructure type, fracture toughness of equiaxedstructure was the lowest, and that of Widmanstatten structure was the highest with that ofmixed lamellar structure in the medium level. In Widmanstatten structure, prior β grain sizehad no obvious regular influence on fracture toughness. Fracture toughness increased with theincreasing width of grain boundary α, thickness of inner α platelet and size of α colony underthe test conditions.The effect of yield strength on fracture toughness was mainly related to itseffect on the size of plastic zone at the crack tip, and fracture toughness increased with risingyield strength within the same kind of microstructure. The fracture surfaces of equiaxedstructure under fracture toughness test was full of homogeneous dimples in size and thevertical depth showed little change. The fracture sufaces of mixed lamellar structure wasmainly stretched fibrous dimples and ridges between them, and secondary cracks alsoappeared. The fracture sufaces of Widmanstatten structure had a large amount of roughcleavage plane, cleavage steps, large size of elongated dimples and deeper secondary cracks,which indicated that the fracture mechanism was transgranular brittle fracture, and the verticaldepth fluctuated leading to the increase of fracture roughness. The increase of fractureroughness and presence of secondary cracks showed the zigzag of crack propagation whichtook more energy and so resulted in higher fracture toughness.6. The optimalizing processing from which superior comprehensive properties could beacquired was the processing with the deformation in β phase field: βt+20℃、60%、1x10-3s-1、AC（WQ）+800℃/1h、AC. The β annealing adopted nowadays in practice needed furtherresearching.