Temperature And Strain Rate Dependency Of Tension And Compression Behavior For ? Titanium Alloy TA7

TA7 titanium alloy(Ti-5Al-2.5Sn alloy)is one of the most widely used a-titanium alloys.Due to its excellent physical and chemical properties,TA7 titanium alloy has been used in many engineering fields.TA7 titanium alloy used as a structural material may endure high-rate loadings at low and elevated temperatures in the process of manufacturing and applications.Therefore,understanding the mechanical properties of TA7 titanium alloy at different temperatures and strain rates is of great significance for its engineering application.The mechanical behavior of some metals,including titanium alloys,exhibits tension-compression asymmetry.The experimental investigation and analysis on the tension-compression asymmetry and its temperature and rate dependence for metallic materials are essential for their processings and applications.In this thesis,the tension and compression responses of TA7 titanium alloy were systematically studied over wide ranges of temperatures and strain rates.The morphology of the annealed TA7 titanium alloy rod used in this research was observed.Observation shows that the undeformed sample has only equiaxed grains with a structure and the annealed rod has a weak<1210>//RD fiber texture.Uniaxial tension tests were carried out over wide ranges of temperatures(-120?600?)and strain rates(0.001?1050s-1).Experimental results indicate that the tension behavior of TA7 titanium alloy exhibits significant temperature and strain-rate dependence.The tensile yield stress of TA7 titanium alloy decreases with the increase of temperature,and the tensile strain hardening increases slightly with increasing temperature.The tensile yield stress increases with the increase of strain rate,and the tensile strain hardening decreases when the strain rate is increased.EBSD observation was performed on the deformed tensile samples.Results show that there exist a small amount of {1012}tension twin and {1122} compression twin during uniaxial tension.Moreover,it was noted that the prismatic a slip system dominates the tensile plastic deformation of TA7 titanium alloy.The isothermal tensile stress-strain responses of TA7 titanium alloy at high strain rates were obtained by means of dynamic tension recovery testing.Results show that the tensile plastic deformation of TA7 titanium alloy at high strain rates exhibits significant adiabatic temperature-rise softening phenomenon.The isothermal strain hardening decreases with increasing strain rate.Based on the characteristics of macroscopic plastic deformations,the physically-based VA constitutive model was modified and used to characterize the tension responses of TA7 titanium alloy.Comparison between the model results and experimental data shows that the modified VA constitutive model is capable of describing the temperature and rate dependent tension behavior of TA7 titanium alloy.Uniaxial compression tests at various temperatures(-70?300?)and strain rates(0.001?3000s-1)were carried out.Results show that the uniaxial compression behavior of TA7 titanium alloy is also significantly sensitive to temperature and strain rate.The compression yield stress of TA7 titanium alloy decreases with increasing temperature,while the compression strain hardening increases slightly with increasing temperature.The compression yield stress increases with the increase of strain rate,while the compression strain hardening decreases with increasing strain rate.EBSD observations on the deformed compression samples show that {1012} tension twin and {1121}tension twin were formed during the plastic deformation.Twins are more likely to occur at low temperatures and high strain rates.The compression stress-strain responses of TA7 titanium alloy were described by the modified VA constitutive model.Model results agree well with the experimental data over wide ranges of temperatures and strain rates.The uniaxial tension and uniaxial compression responses of TA7 titanium alloy were compared.Results show that the mechanical behavior of TA7 titanium alloy exhibits significant tension-compression asymmetry.The values of both yield stress and strain hardening are lower at tension than those at compression.The tension-compression asymmetry for TA7 titanium is more pronounced at high strain rates,while temperature has no apparent effect on such asymmetry.The microscopic plastic deformation characteristics between tension and compression loadings were compared.The mechanism of tension-compression asymmetry for TA7 titanium alloy was investigated.Microscopic observations indicate that the twin density at tension is much lower than that at compression,and the twin type is different.The asymmetry formation of twins at tension and compression is one of the reason for the macroscopic tension-compression asymmetry of TA7 titanium alloy.Moreover,results reported in the literatures indicated that the pyramidal<c+a>slip system in a titanium exhibits tension-compression asymmetry,which is another reason for the tension-compression asymmetry of macroscopic stress-strain responses for TA7 titanium alloy.Based on the initial texture of the annealed TA7 titanium alloy rod,a polycrystalline plastic finite element model was established.The effects of geometric and physical parameters used in the model on the numerical results were investigated.Especially,the effects of tension-compression asymmetry for the related pyramidal<c+a>slip parameters on the macroscopic stress-strain responses were explored,and the reliability of the model was examined.Numerical results show that the model has the ability to simulate the macroscopic mechanical behavior and texture evolution of TA7 titanium alloy subjected to tension and compression loadings.Furthermore,the microscopic mechanism of plastic deformation was investigated.Numerical results indicate that the prismatic slips dominates the plastic deformation of TA7 titanium alloy.The proportion of prismatic slips decreases as plastic deformation develops,while proportion of basal slips and pyramidal slips increases.The orientation of the grains changes significantly during plastic deformation.The glide of the slip system in TA7 titanium alloy causes the tension-compression asymmetry of texture evolution,which ultimately contributes to the tension-compression asymmetry of macroscopic stress-strain responses of TA7 titanium alloy.