Numerical Simulation Study On Formation And Instability Of TiAl Lamellar Microstructure

TiAl intermetallic based alloys exhibit a great potential as high temperature structural materials due to their light-weight and excellent high temperature performance.And TiAl alloys have been successfully applied in some of the most advanced aircraft such as Boeing 787 to replace Ni-based superalloys for manufacturing low pressure blades of turbineengine,with the benefit of weight reduction of as much as?50%,realizing the lightweight of aero engine.TiAl alloys have a variety of typical microstructures,among which the lamellar structure has attracted particular attention due to its good comprehensive properties,and TiAl lamellae play a crucial role in different kinds of TiAl microstructures.However,under complex and severe service conditions,stability of TiAl lamellar structure is facing challenges,resulting in performance degradation.Therefore,investigation on lamellar structure formation and instability of TiAl alloy is of significance for TiAl microstructure controlling and service life improvement.However,the process of the microstructure evolution under complex conditions is hard to be observed by experimental method.Therefore,the formation of TiAl lamellar microstructure and its influcing factors,and the mechanisms of instability of TiAl lamellar microstructure(especially mechanical instability)have not yet been fully studied.Numerical simulation method can reproduce complex dynamic processes,and reveal mechanisms deeply,which makes it an important method for studying microstructure evolution of materials.In view of the above problems,formation and instability of TiAl lamellar microstructure have been studied by phase field and classical molecular dynamics methods in this work.The main research contents and conclusions are as follows:CALPHAD database and WBM(Warren-Boettinger-Mc Fadden)model were used to express chemical energy and KHS method was use to describe elastic energy,then a multi-phase field model of TiAl lamellar microstructure evolution was built and it was solved by finite element method.The formation of TiAl lamellar structure was simulated by the above established model.Results indicate that,gradient energy coefficient and elastic energy contributed together in the phase transformation process of TiAl alloy.There are three different interfaces between?phase variants,named as PT(pseudo-twin),RB(rotational boundary)and TT(true-twin)interface respectively.Under the influence of elastic energy,?phase variants with TT interface are tend to nucleate in a correlated way.Moreover,the interface energy of TT interface is minimum compared with the other two lamellar interfaces.TT interface is much easier to form and the fraction of TT interface is larger under the influence of both elastic energy and small interface energy.Influence of the intial TiAl lamellar structure and the applied stress on?₂ phase parallel dissolution(?2??phase transformation)during TiAl lamellar structure thermal instability were studied by using phase field method.Results indicate that,during the thermal dissolution of the?2 lamella,the adjacent?lamella will act as a heterogeneous core and then grow up.Besides,compared with thermal dissolution of the?₂ lamella with no applied stress,the normal stress has almost no influence on the thickness and fractions of three lamellar interface variants in the last microstructure,while it only changes the driving force of phase transformation.When the shear stress is applied,some of the?phase are premoted and some are repressed.Specifically,?phase variant is premoted when its stress-free shear strain is of same direction with the shear stress,while?phase variant is depressed when its stress-free strain is of opposite direction with the shear stress.At last,the lamellar structure became thick and TT interface fraction decreased dramatically.By using classical molecular dynamics method,stacking fault energies of different plane defects of?-TiAl are calculated dislocation nucleation and evolution during mechanical deformation are investigated,different plane defects are classified and their influence on TiAl lamellar microstructure are discussed.Stacking fault energies of different plane defects of?-TiAl are calculated,results indicate that,for slip systems of?-TiAl alloy,stacking fault energies of ordinary slip system is smaller than that of super slip system.For twinning system,(111)[11(?)]and(?)[(?)2]twinning systems are of smaller stacking fault energy than other twinning systems.Dislocation evolution was studied by numerical simulation under the condition of nanoindentation test.According to the relation between plane defect and lamellar interface,plane defects are divided into two groups:longitudinal slip/twinning system when plane defects are parallel to the lamellar interface and transverse slip/twinning system when plane defects intersect the lamellar interface.And interaction between longitudinal twinning and lamellar interface leads to mechanical instability of TiAl lamellar microstructure.The mechanism of mechanical instability of TiAl lamellar microstructure is studied deeply by using classical molecular dynamics simulation.Interactions between longitudinal twinning and lamellar interfaces in a quasi-three-dimensional TiAl lamellar structure were simulated.Results indicate that,after interaction with longitudinal twinning,PT lamellar interface and RB interface transform to each other,while the TT lamellar interface migrates and disappears at last.In other words,compared with TT lamellar interface,PT and RB lamellar interfaces are of higher mechanical stability.What's more,Microstructure evolution of a three-dimensional polycrystalline fully lamellar TiAl alloy under mechanical deformation is also investigated by numerical simulation.The result is similar with former quasi-three-dimensional model,and the mechanical stability of lamellar microstructure with finer lamellar thickness is found to be worse.