Research On Thermal Deformation Behavior And Microstructure Evolution Of Near-β Titanium Alloy TC18 For Aerospace Applications

Near-βtitanium alloys have high strength,high hardenability,good plasticity and fracture toughness,and are a key class of aerospace lightweight structural materials,mainly used to make key load-bearing parts such as landing gear,airframe and rotor systems.The thermoplastic flow behavior and microstructure evolution are the key basis for the preparation and performance modulation of near-βtitanium alloys.This paper takes TC18 alloy as the research object,two aspects of the study were carried out by thermal compression experiment.At the macro level,based on the plastic flow data,a constitutive model was established to accurately describe the thermoplastic flow behavior of alloy based on artificial neural network method optimized by genetic algorithm（GA-BPANN）,and its reliability was verified by hot rolling simulation and experiments.At the micro level,the dynamic recrystallisation and dynamic recovery ofβphase,globularization of lamellarαphase,dynamic recrystallisation of equiaxedαphase and coevolution ofα/βduring thermal deformation were analyzed by SEM,EBSD and TEM characterization techniques.The main research work and results are as follows:（1）The dynamic recrystallisation behavior and kinetics inβphase region of TC18 alloy with singleβstructure were studied.The results show that there are two dynamic recrystallisation nucleation mechanisms inβphase:Discontinuous dynamic recrystallisation（DDRX）characterized by grain boundaries bulging and continuous dynamic recrystallisation（CDRX）characterized by progressive sub-grains rotation.When the deformation temperature was 950°C and the strain rate increased from 0.001 to 1 s^-1,the DRX volume fraction ofβphase decreased from 31.7%to 4.0%.At a strain rate of 0.01 s^-1,the DRX fraction ofβphase decreased first and then increased with increasing deformation temperature,and its DRX kinetic index and flow softening rate both showed unexpected decrease,and the main DRX mechanism had transformed from DDRX to CDRX.Considering the effect of thermodynamic parameters and their coupling on grain size,a kinetic model of DRX grain size forβphase was established.（2）The microstructure evolution of TC18 alloy with lamellar structure during theα+βphase region deformation was investigated.The results show that theα/αsubgrain boundary with LAGBs is formed in the lamellarαphase based on dislocation and twin boundary evolution,and the misorientation is increased by subgrain rotation.Theα/αsubgrain boundaries are transformed into HAGBs,and the lamellarαphase undergoes CDRX.Theβmatrix wedges into the thermal etching grooves at theα/αinner interface,and theα/βphase boundary continues to migrate under the action of diffusion,and finally the lamellarαphase are separated and globularized along the interface.High temperature and low strain rate are favorable to the globularization of lamellarαphase.With increasing of height reduction,the orientation ofαandβphases changes significantly,the preferred orientation intensity of the micro area decreases and the Burgers orientation relationship betweenα/βis gradually destroyed,resulting in the increase of theα/βinterface energy,which facilitates the interfacial diffusion of elements,thus promoting the globularization of the lamellaeαphase.A large number of dislocations accumulated at theα/βinterface and further evolved into subgrain boundaries dividing theβmatrix.Theβsubgrains increase misorientation by progressive rotation and develop into CDRX grains with HAGBs.The lamellarαandβinteract with each other and evolve synergistically.Theβphase CDRX is activated preferentially,and the stress field generated byβmatrix dislocations plugging at the interface promotesαphase slip system initiation and subsequent CDRX;the globularizedαgrains promote DRX nucleation of theβphase at theα/βphase interface.With the increase of strain rate,the DRX degree of lamellarαandβphases decreased first and then increased.With the increase of deformation temperature,the DRX degree ofαphase decreased gradually,and the DRX degree ofβphase increased slightly first and then decreased.The annealing study of the hot compression deformation microstructure shows that the interface separation mechanism plays a leading role at the early stage of annealing,the coarsening mechanism plays a more obvious role at the later stage of annealing.（3）The microstructure evolution of TC18 alloy with equiaxed structure during deformation in theα+βphase region was investigated.The results show that the equiaxedαgrains are first elongated to form the inner interface with LAGBs,then transformed into HAGBs and formed thermal grooves on the surface of the equiaxedαgrains.Theβmatrix wedged into long stripαgrains along the grooves to separate them into fine equiaxedαgrains.The DRX volume fraction of equiaxedαphase increased with the increase of deformation temperature.With the increase of strain rate,the DRX volume fraction of equiaxedαphase decreased first and then increases.In both cases,the refinement effect was enhanced.Theβphase evolution was dominated by DRV,at the same time,a small amount ofβphase also undergoed DRX.The equiaxedαphase is the hard phase,and the non-coherentα/βinterface impedes the dislocation motion,leading to dislocation plugging in the nearbyβmatrix,effectively increasing the dislocation substructure,and thus promotingβphase CDRX.（4）The flow softening behavior of TC18 alloy with lamellar,equiaxed and singleβstructure was investigated,and the deformation constitutive models of above three structure were established by GA-BPANN method.The results show that compared with the equiaxed structure,the lamellar structure has higher deformation temperature rise,larger dynamic phase transformation driving force,which will cause stronger thermal softening effect and dynamic phase transition softening effect.In addition,the microstructure evolution of the lamellaeαphase caused more significant softening by its own bending,globularization and strong interaction with theβmatrix.Therefore,the lamellar structure show higher decline ratio of flow stress.The GA-BPANN modified model has high prediction accuracy and low deviation.After correction of the model using the above method,the AARE of the SCA model for TC18 alloy with lamellar,equiaxed and singleβstructure decreased from 8.01%to 1.02%,4.99%to 0.76%and7.11%to 0.48%,respectively.The cross-validation tests show that the modified model had a strong generalization capability,and can accurately describe and predict the thermoplastic flow behavior of the alloy.The modified model was applied to the hot rolling simulation with high reliability,and the width and length errors of the sheets are less than 5.30% and 6.40% respectively.