Static Globularization And Its Evolution Mechanisms Of The Lamellar Microstructure For TC17 Titanium Alloy

The manufacturing technologies of dual-property blisk of titanium alloys,which can lose weight effectively and optimize aerodynamic configuration,become the key technology to improve comprehensive performance of aero engine.In China,the whole forging technology is used to manufacture the dual-property blisk.The different positions of the blisk will obtain different microstructure morphologies by controlling the isothermal deformation and heat treatment parameters in ?+? phase field,the disk still keep the lamellar microstructure,while the lamellar microstructure in blades will be transformed into equiaxed microstructure due to globularization behavior.The lamellar microstructure has excellent toughness and fatigue crack growth resistance.On the contrary,a microstructure comprising equiaxed-alpha in transformed beta matrix will process a better balance of ductility and strength and crack nucleation resistance.The obtained performance advantages in different positions of the blisk will fully exploit the potential performance of titanium alloys.The globularization of the lamellar microstructure is a key to control microstructure evolution of the blisk,and the systematic study of globularization behavior will be significance to manufacture dual-property blisk.The objective of this work is to investigate the globularization behavior and mechanisms of TC17 alloy with the initial lamellar microstrmcture using the methods of experimental research,theoretical analysis and numerical simulation.The main research contents and conclusions are as follows:Microstrmcture evolution behavior and mechanism of the lamellar structure of TC17 alloy have been investigated with the assistance of isothermal forging experiments.The results indicate that dynamic globularization is influenced by strain and deformation temperature,and the globularization process is controlled by boundary splitting mechanism.Boundary splitting process is completed via the two modes:shearing surface and substructure.These two modes are enabled by dislocation glide,and the values of Schmid factor indicate that pyramidal slip is the easiest,prismatic slip takes second place,and basal slip is most difficult.During deformation,the other main feature of microstrmcture evolution is the rotation of alpha lamellae,and the orientation distribution of alpha lamellae changed from the two symmetrical peaks to the single one with the increasing strain.Heat treatment experiments are designed to investigate static globularization behavior of the lamellar structure of TC17 alloy.Mechanisms of static globularization are found and the globularization times are successly predicted.Static globularization is effected by strain,heat treatment temperature and time,and the globularization fraction increases with strain,temperature and time.Static globularization process can be divided into two stages.The first includes microstructural changes during the initial stages of heat treatment,and the second occurs during prolonged heat treatment.The first stage includes segmentation of alpha lamellae by boundary splitting,and the second stage is controlled by termination migration and Ostwald ripening mechanisms.The driving force of boundary splitting in the initial stage of heat treatment is provided by the release of distortion energy,whereas the driving forces of the termination migration and Ostwald ripening derive from energy gradient generated by curvature difference in different locations of microstructure or different microstructures.In addition,the termination migration and Ostwald ripening are also important mechanisms to cause microstructure coarsening during heat treatment.The EBSD technology is used to study the influence mechanism between alpha phase and beta phase during microstructure evolution.For beta phase,the continuous dynamic recrystallization is the main style of microstructure evolution.Alpha phase distributes on the boundary of recrystallized beta particle and hinders the growth of beta particle.Alpha phase plays an important role in recrystallization process.However,microstructure evolution of alpha phase is also influenced by beta phase.The penetration of beta phase can lead to the formation of misorientation inside alpha phase.The low angle boundary will be firstly formed,and then transformed into the high angle boundary.Finally,the new boundary is formed and the initial lamellar structure is separated.Microstructure evolution of the lamellar alpha for TC17 alloy is analyzed quantitatively under different conditions basing on the stereology and quantitative analysis technology.Static globularization kinetics is established by inducting the parameter t0.5(time for the globularization fraction reaches 50%)in terms of Johnson-Mehl-Avarmi-Kolmogorov(JMAK)models.The processes of static globularization are completed by boundary splitting and termination migration.Mullins model and Fick model can be used to predict completion times of boundary splitting and termination migration,respectively.Furtherly,the prediction model of static globularization behavior of TC17 alloy is established basing on the ANN technology.The amount of strain prior to heat treatment,heat treatment temperature and time were taken as inputs,and static globularization fraction as output.The ANN model is trained by a large number of test data,and then the non-training data are used to verify the accuracy and precision of the established model.The trained neural network had a good performance for static globularization behavior of Ti-17 alloy.A comparison of the predicted value by the neural network and calculated results by the regression method was carried out.The result indicates that the ANN model is more accurate and efficient than the regression method in terms of the prediction of static globularization kinetics of TC17 alloy.Besides,static coarsening behavior of TC17 alloy can be interpreted in terms of Lifshitz-Slyozov-Wagner(LSW)theory,and the kinetics equation can be established by the modified LSW model.The coarsening of the lamellar microstructure is influence by bulk diffusion and interface reaction,whereas the equiaxed microstructure is a classical bulk diffusion behavior.The effect of globularization behavior of the lamellar alpha on tensile property and impact toughness are investigated quantitatively combining with actual machining process of dual-property blisk.The globularization behavior can refine the grain,decrease slip length and lead to a smoother alpha/beta interface.Such changes will improve tensile properties.Both strength and plasticity exhibited the increasing tendency with the globularization fraction.The quantitative investigations found that there are linear relationships between tensile properties and the globularization fraction at both room temperature and 400?.However,the impact toughness of TC17 alloy exhibits an opposite trend in contrast to tensile property with the globularization fraction.The globularization behavior has a negative influence on the impact toughness of TC17 alloy.A linear relationship between the impact toughness and globularization fraction can also be observed though the quantitative analysis.