Tension Behavior Of Two-phase Titanium Alloy

?+? titanium alloys have been widely used in the military and civil fields due to their low weight and high strength. These alloys maybe subjected to extreme environments and extreme loading conditions, therefore it is necessary to investigate the accurate measurement and reliable characterization for mechanical responses of titanium alloys. In this paper, tensile responses of TC11 over a wide range of temperatures and strain rates are studied on the experimental research, macro-constitutive characterization and micro-numerical simulation. The main work is presented as follows:1. New improvements are introduced to conduct tensile loading and unloading tests at higher strain rates. The variable cross-sectional short metal bar and the high rigidity stopper component are designed. The effects of strain rate and temperature on the uniaxial tension behavior of TC11 alloy are systematically investigated over a wide range of initial temperatures,213-873K, and strain rates,0.001?1150 s-1. Dynamic tension and recovery tests are conducted using a split-Hopkinson tension bar to obtain the adiabatic and isothermal stress-strain responses of the alloy at high strain rates,102-103s-1. Furthermore, temperature jump and strain rate jump tests are performed. Experimental results show that the tension behavior of TC11 alloy exhibits typically elastic-plastic deformation characterization and obvious temperature and strain rate sensitivity. The detailed performances are showed as follows. The initial yield stress increases with increasing strain rate, which presents a considerable strain-rate strengthening effect. There exists the transition of rate sensitivity for initial yield stress under moderate-rate loading conditions. The initial yield stress decreases with increasing temperature. However, the relationship between the initial yield stress and the temperature shows "an abnormal temperature range" where the initial yield stress exhibits temperature insensitivity within the temperature range of 573-723K. Moreover, the isothermal strain hardening behavior changes little at different temperatures and strain rates. The adiabatic temperature rise is the main reason for the reduction of strain hardening rate during the high-rate deformation process. The temperature and strain rate jump tests indicate that the value of temperature rise conversion coefficient is 0.9, approximately. The tensile mechanical behavior has no apparent history effects on strain rate and temperature.2. The micro-structure observation results of the deformed TC11 specimens show that the local plastic deformation for both primary ? grain and ?+? lamellar colony in necking zone is up to 50%, which presents much greater than that in the uniform deformation area. Moreover, the internal micro-structure exhibits a non-uniform local deformation phenomenon due to the complexity of the crystal structure and the stress state. The crack propagates through both primary ? grains and lamellar Widmanstatten grains. The SEM fractographs of the alloy display three kinds of dimple morphologies, namely round dimples, shallow dimples and parabolic dimples corresponding to fibrous zone, radiation area and shearing lip, which presents a typical ductile fracture mechanism of TC11 during the tension process. The value of microscopic fracture strain is much greater than that of unstable strain and increases with increasing temperature and strain rate.3. Considering the tensile mechanical characteristics, a modified Johnson-Cook model is proposed to describe the behavior of TC11 alloy. The new improved model accounts for the effect of adiabatic temperature rise on the strain hardening behavior at high strain rates, and the non-identical effects on the initial yield stress and the isothermal strain hardening. Material parameters are determined using the isothermal and adiabatic stress-strain curves at different temperatures and strain rates, and temperature and strain rate jump test results are used to evaluate the validity of the model. The model correlations are in good agreement with the experimental data, indicating that the modified JC model is capable of describing the rate-temperature dependent deformation behavior of TC11 alloy within the investigated range of strain rates and temperatures.4. Based on the crystal plasticity theory, a microstructure-based numerical constitutive model for duplex ?+? titanium alloy is established. The isostress homogenization approach is used to model the properties of lamellar colony. The numerical model incorporates the random distribution and the orientation of primary ? grains and lamellar Widmanstatten grains. The material parameters in the numerical model are preset and/or confined, so the relationship between the macroscopic stress-strain behavior and the CRSS and initial strain hardening module of dislocation slip systems is found by the finite element simulations. The values of CRSS are determined using the interpolation optimization process and the tension responses are simulated at various temperatures and strain rates. The numerical resultss are in good agreement with the experimental data, indicating that the numerical model is suitable to reveal the effects of temperature and strain on the tension deformation mechanism for TC11.