| With aircraft and spacecraft becoming high-speed, large-scale and complex, high strength titanium alloy is expected much higher mechanical performance for aircraft frame beam structure component, fasteners and high strength spring. Super high strength titanium alloy is one of key materials for new generation of aerospace vehicles. As an important method to improve mechanical properties of metal materials, heat treatment technology is also widely used in the field of titanium alloy. However, it is necessary to master phase transformation and microstructure evolution of titanium alloy for optimizing heat treatment process. The study of solid transformation and microstructure evolution of titanium alloy was mainly carried out under constant temperature. However, obtained conclusion couldn’t predict phase transformation during continuous heating or cooling. As well as crystal defects are important factors that affect phase transformation of alloy. Therefore, it is urgent to study on phase transformation and effect of crystal defects on solid transformation of titanium alloy during continuous heating and cooling for developing heat treatment technology. Therefore, dilatometric method, OM, SEM, TEM, EBSD and XRD were carried out to study on phase transformation and microstructure evolution of Ti-1300 alloy, which has been independently developed by Northwest Institute for Nonferrous Metal Research of China, during continuous heating, solid treatment, aging treatment and continuous cooling. The main work and conclusions are as follows:Overall activation energy of the α+βâ†'β phase transformation of Ti-1300 alloy during continuous heating was calculated, as well as the mechanism of α+βâ†'β phase transformation is a typical nucleation and diffusion control type transformation on the basis of modified Johnson-Mehl-Avrami equation. Furthermore, based on Arrhenius and Beck equation, the β grain growth models were established during isothermal solution treatment at 840℃ to 950℃. It was also calculated that the overall activation energy of the β grain boundary migration was Q=350k J/mol. It was found that the morphology of the primary α phase of the alloy gradually evolves from fine needlelike to thick lathlike when the alloy was solution treated at α+β two phase region, and the coarsening mechanism is mainly governed by the atoms diffusion along the prior grain β boundaries.Influence of β grain size, prior α phase and deformation rate(0.008s-1~0.08 s-1) on room temperature deformation of the solution treated Ti-1300 alloy were investigated systematically. It was found that stress/strain induced martansite did not occur during tensile deformation for the solution treated Ti-1300 alloy. However, the deformation mechanism of the alloy was mainly dislocation slip at room temperature, which can be explained by chemical composition. Moreover, it was found that the preferential orientation of the solution treated Ti-1300 alloy after the deformation occurred. The β grain orientation of Ti-1300 alloy gradually tended to be <110> with the increase of cold deformation reduction.The lever rule was used to deal with change in length of Ti-1300 alloy during the isothermal aging. As a result the temperature constant K and the Avrami exponent n of metastable β phase decomposition kinetics equation were obtained. Furthermore, the decomposition kinetics equation of metastable β phase was established on the basis of above result during isothermal aging for the alloy. When the solution treated Ti-1300 alloy was isothermal aged at 350℃, the decomposition mechanism of metastable β phase was βâ†'ω+βâ†'α+β. When the alloy was isothermal aged at 400℃for 60 min, the decomposition mechanism of metastable β phase wasβâ†'β′+βâ†'α+β. When the alloy was isothermal aged between 500℃~700℃, the decomposition mechanism of metastable β phase was βâ†'α+β. Moreover, the nucleation site of the secondary α phase was mainly in the prior β grain boundaries and the internal nucleation of the prior β grain, and the nucleation rate gradually decayed.It was investigated that cold pre-deformation had influenced aging behavior of the solution treated Ti-1300 alloy. It was found that crystal defects which occurred during cold pre-deformation promoted the nucleation of α precipitates and these α precipitates present clearly variant selection when the solution treated Ti-1300 alloy was aged after cold pre-deformation. By continuous heating experimental research, it was found that cold pre-deformation delayed the βâ†'ω phase transformation of the solution treated Ti-1300 alloy during continuous heating. The average activation energy for ω phase of Ti-1300 alloy under different condition was calculated on the basis of KissingerAkahira-Sunose equation. It was clearly found that the average activation energy for ω phase increased with the increase of cold pre-deformation reduction. It was a reason for delaying the βâ†'ω phase transformation.Microstructure evolution during continuous cooling of Ti-1300 alloy from β single phase zone was investigated with dilatometric and metallographic method. It was found that different orientations of α phase precipitated with a thin-like continuous distribution at β grain boundaries when Ti-1300 alloy was cooled from β phase zone during continuous cooling. With the decrease of the temperature, the volume fraction of α phase increased. Moreover, α precipitates formed at β grain boundaries began gradually to grow in the direction of inside of β grain. When the cooling rate exceeded 0.3℃/s, the residual metastable β phase in the alloy began to appear.When the cooling rate exceeded 3℃/s, the alloy was composed of single metastable β phase. With the increase of cooling rate, the microhardness of Ti-1300 alloy increased then decreased. When the cooling rate was 0.3℃/s, the microhardness of the alloy reached the maximum, and then gradually decreased. Furthermore, the continuous cooling transformation kinetics diagram(CCT diagram) of the alloy was established. |
PDF Full Text Request