| The globularization of lamellar structure to obtain equiaxed structure is an important method to obtain good toughness and fatigue properties in lightweight components of titanium alloy.However,the globularization process is very complex and difficult to predict and control due to the various initial states of the lamellar structure.Therefore,revealing the globularization law and mechanism of the lamellar structure under different initial states is the key to control the parameter of equiaxed structure,obtain fine equiaxed structure,and improve the toughness and fatigue properties of titanium alloy.In this paper,two kinds of initial structure of TC11 titanium alloy were obtained by double heat treatment,namely,bi-lamellar structure(coarse lamellar ?+fine lamellar ?)and tri-modal structure(equiaxed?+coarse lamellar ?+fine lamellar ?).The globularization process of lamellar ? and the effect of equiaxed ? on lamellar globularization were studied by thermal simulation compression and heat treatment.The main findings are as follows:During thermal deformation,fine lamellar ? in these two initial structures will globularize first,and coarse lamellar ? and fine lamellar ? will globularize by shearing at lower deformation temperatures(800? and 850?)and boundary splitting through groove at higher deformation temperatures(900 ?).The two kinds of structures contain coarse lamellar ? and fine lamellar ? with cross arrangement,cross arrangement of lamellar ? will hinder the rotation of each other during the deformation process,which is conducive to the local dislocation stacking and the generation of subboundaries,thereby promote the globularization of lamellar ?.The preferential globularized lamellar ? during thermal deformation due to the loss of Burgers orientation relationship,which is conducive to the accumulate dislocations near these grain boundaries and promote the globularization of surrounding lamellar ?.The lamellar ? and the equiaxed ? of the tri-modal structure also hinder each other during the hot deformation,which makes the lamellar ? around equiaxed ? easy to globularize,while the equiaxed ? will form dislocations and subgrains at the interface.In the process of thermal deformation and heat treatment,the heterogeneous globularization will preserve in bi-lamellar structure.In the region of low globularization fraction,the loss of Burgers orientation relationship in coarse lamellar? and fine lamellar ? is small,which leads to the low dislocation density in deformed microstructure and the slow globularization rate of lamellar ? during heat treatment.In the region of high globularization fraction,the Burgers orientation relationship of some lamellar ? is destroyed during thermal deformation,which is beneficial to accumulate dislocations in the microstructure and promote globularization and coarsening of the surrounding lamellar ? during heat treatment.However,lamellar ?with less loss of Burgers orientation relationship is easy to retain and coarsen during heat treatment due to lower interfacial energy.The proportion of low-angle grain boundaries increased by static recovery of equiaxed ? and lamellar ? in tri-modal structure during heat treatment.The transition from low-angle grain boundary to high-angle grain boundary through continuous static recrystallization,but it is difficult to occur completely,so it has little effect on the distribution of grain boundary misorientation in tri-modal structure during heat treatment.The grain boundary evolution caused by static recovery and continuous static recrystallization is beneficial to the occurrence of boundary splitting in equiaxed? and lamellar ?,which will reduce the proportion of high-angle grain boundary and increase the degree of globularization.The low-angle grain boundary will also produced by grain merging of lamellar ?,which will lead to increase the proportion of low-angle grain boundary and decrease the aspect ratio of combined lamellar ?,which may also in favor of increase the degree of globularization. |
PDF Full Text Request