| Researches on the mechanical properties and deformation mechanisms ofα+βtitanium alloy under quasi-static loadings at elevated temperatures have been carried out adequately. Researches on the dynamic behaviors are mainly focused on the compressive loading conditions. However, the tensile responses ofα+βtitanium alloy at intermediate and high strain rates are less investigated. This research, supported by the National Natural Science Foundation of China (No.10472110), aims to investigate the tensile properties and deformation mechanisms of TC21, a new designedα+βtitanium alloy in China, under intermediate and high strain rate loadings at elevated temperatures.The dynamic tensile experiments of TC21 with two different microstructures, Bimodal (BM) and Widmanstatten (W), were carried out in a temperatures range of 298K to 1023K and under strain rates of 240s-1 and 1270s-1 using the high temperature tensile impact technique. Also, the quasi-static tensile experiments under strain rates of 0.001s-1 and 0.05s-1 and corresponding temperatures were carried out using MTS809 testing system. The intermediate tensile experiments under strain rates of 2s-1, 18s-1 and 50s-1 at room temperature were carried out using intermediate strain rate tensile technique. The dynamic tensile recovery experiments under 240s-1 at room temperature were carried out using the tensile impact recovery technique. The test results indicate that the values of yield stress and fracture strain of BM TC21 are higher than those of W TC21 under the same loading conditions, while the strain-hardening rate of BM TC21 is lower than that of W TC21. The tensile properties of BM TC21 and W TC21 have similar dependence on temperature and strain rate. There exhibits a discontinuity in the yield stress-temperature curve under quasi-static loadings, above which yield stress drops sharply with increasing temperature. Under high strain-rate loadings, no discontinuity is found in the yield stress-temperature curve, indicating that the discontinuity temperature increases with increasing strain rate. Below discontinuity temperature, the temperature dependence of yield stress of TC21 under quasi-static and dynamic loading is similar with cp-Ti that has the same interstitial solute concentration. At 298-873K, the strain-rate sensitivity of yield stress under high strain rate loadings is higher than that under quasi-static loadings and the strain rate sensitivity changes in the range of intermediate strain rates. Yield stress exhibit a nearly bi-linear relationship with the logarithm of strain rate.At 298-873K, the strain-hardening rates of both microstructures decrease with increasing strain rate. Under quasi-static loadings, the strain-hardening rates increase firstly and then decrease and then increase with increasing temperature, while under dynamic loadings, the strain hardening rates increase firstly and then decrease. The temperature corresponding to the maximum value of strain-hardening rate is 673K for both quasi-static and dynamic loadings.At 298-673K, the unstable strains of dynamic loadings are less than those of quasi-static loadings, while at 873-1023K, the unstable strains of dynamic loadings are greater than those of quasi-static loadings, indicating a high-velocity ductility phenomenon. The unstable strains increase firstly and then decrease with increasing temperature within the whole strain rate range. At 298-873K, the fracture strains under dynamic loadings are larger than those under quasi-static loadings, while at 973-1023K, the fracture strains under dynamic loadings are smaller than those under quasi-static loadings. Under quasi-static loadings, the fracture strains decrease firstly and then increase with increasing temperature, while under dynamic loadings, the fracture strains increase firstly and then decrease with increasing temperature. The values of strain-hardening rate, fracture strain and unstable strain of the isothermal stress-strain curve at 240s-1 and room temperature are higher than those of the adiabatic stress-strain curve under the same conditions. The strain-hardening rate of the isothermal stress-strain curve is strain-rate insensitive.SEM fractographic observations show that the fracture mode of BM TC21 is transgranular fracture under all conditions, while W TC21 show a mixed manner of transgranular fracture and intergranular fracture under quasi-static loadings at room temperature and transgranular fracture under other conditions. No localized melting area due to adiabatic shear is found on the fracture surfaces of all tensile samples. Metallographic observations show that no adiabatic shear band is found in the perpendicular sections near the fracture surfaces. The void nucleation and growth is the main reason for the tensile fracture of TC21.TEM observations show that both microstructures exhibitαcolony, in which there is a near Burgers orientation relationship betweenαphase and neighboringβphase. The size ofαcolony of BM TC21 is smaller than that of W TC21, resulting in higher yield stress and fracture strain of BM TC21. The main deformation mechanism of both microstructures is the screw dislocation slip. Under quasi-static loadings, the main slip system is type slip system below discontinuity temperature and the main slip system are type and type slip systems above discontinuity temperature, indicating that the discontinuity of the yield stress-temperature curve is associated with the change of slip type. Under dynamic loadings, the main slip system is type in the whole temperature range, resulting in no discontinuity in the yield stress-temperature curve. The main slip systems of both microstructures are the same under all conditions, resulting in the same dependence of mechanical properties on temperature and strain rate. The increase of strain-rate sensitivity from quasi-static loadings to dynamic loadings is probably caused by the increase of the dislocation generation rate.A bi-linear constitutive model based on bimodal Weibull distribution is proposed to describe the tensile behavior of TC21 under different strain rates and temperatures. Compared with the J-C model and KHL model, the bi-linear relationship between the yield stress and the logarithm of strain rate can be well described and the transition strain rate corresponding to the yield stress can be predicted. Furthermore, the variation of strain-hardening rate coupling with temperature and strain rat can be well described. A decompose-synthetize method is used to identify the values of the model parameters. The fitted model results are in good agreement with the experimental data obtained within the present investigated temperature and strain rate range.
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