Microstructure Evolution Research Of TC21 Titanium Alloy Thermal Deformation And Heat Treatment

TC21 alloy is a kind of(α+β) two-phase titanium alloy with high strength and high toughness which has been independent research and development by China. It can be obtained a good comprehensive performance through processing of thermal deformation and heat treatment. Microstructure evolution of TC21 titanium alloy in the process of hot working strongly influences the mechanical properties. Therefore, research on microstructure evolution of TC21 titanium alloy thermal deformation and heat treatment process has important significance not only in theory but also in engineering application.Firstly, by using Gleeble3500 thermal simulation experiment machine, the isothermal and constant test of TC21 titanium alloy was conducted, and its plastic rheological constitutive equation were investigated with the deformation temperature from 900 to 1020 ℃and the strain rate from 0.001 to 10 s-1 by analyzing the stress-strain curve and microstructure. Obtained the hot deformation activation energy Q is 258.600 KJ/mol. According to the recrystallization nucleation and growth theory as well as dynamic characteristics, the β phase area hot deformation process of dynamic recrystallization critical strain model, volume fraction model, grain size and dislocation density evolution model were set up.Secondly, the isothermal transformation behavior of TC21 titanium alloy was studied. Time-temperature-transformation(TTT) curve of TC21 was investigated by simulation calculation using JMat Pro software and experiments. The experimental results show that the start time for isothermal transformation of βâ†'α phase can not be detected by dilatometric technique due to the lower volume effect for the phase transition, however, metallographic method can be used to accurately measure the TTT curve and then it is showed that in the temperature range of 800 to 900 α ℃phase precipitation occurs rapidly. The results of JMat Pro software simulation and experiment are generally agreement.Thirdly, the microstructure evolution of TC21 titanium alloy during double annealing were researched. The experimental results show that the needle of α phase was precipitated after secondary annealing. There has been little particulate dispersed distribution of the grain boundary α phase due to the original β phase grain boundaries varying degrees of broken.Microstructure retains the typical dynamic recrystallization characteristics as well as dynamic response in addition to generating basketweave microstructure.Finally, TC21 alloy microstructure evolution simulation on the hot deformation process is combined with dynamic recrystallization mathematical model and dislocation density model as well as cellular automata(CA) method by using DEFORM-3D finite element software. The simulation results show that the strain rate under temperature of 975 ℃ for 0.01 s-1 sample in the process of hot deformation microstructure occurred the phenomenon of dynamic recrystallization and large deformation zone grain size smaller than that of other regions. Under the temperature of 975 ℃when the strain rate increasing, the dynamic recrystallization alloy grain size decrease and the average grain size is refined. The simulation results with experimental results better.