Study On The Dynamic Deformation Behavior Of Pure Ti And Ti-xAl(X=2,4,6) Alloys

Since the1970s, with the growing demand for weight-efficient weapon system, more researchers began to focus on the dynamic performance of titanium alloys. Many studies have been performed on Ti-6A1-4V and other a+β type titanium alloys as well as some β type titanium, and attentions have been paid to the performance characterization, influencing factors analysis and microscopic observation of adiabatic shear band etc. The present study then focuses on the dynamic deformation behavior of a type titanium alloy, including twinning behavior, dynamic shear instability and the related influencing factors.Firstly, the dynamic mechanical response of Gr.2commercial purity titanium were obtained using split Hopkinson pressure bar by means of compression, and the deformed microstructure were observed by optical microscopic. By contrasting the strain hardening modulus and twinning fraction of quasi-static (10-3S-1) and dynamic (6000s’1) compression deformation process, it was found that a sharp increase in twinning fraction was accompanied by a substantial increase in strain hardening modulus. This indicates that increasing the strain rate will boost the density of twinning and enhance the strain hardening effect through the law of Hall-Petch.In order to analyze the twinning behavior, EBSD technique was applied to confirm that quasi-statically deformed structure with εp=0.36consisted of two types of twinning namely{1012} and{1122}, while dynamically deformed structure with εp=0.22consisted of three types of twinning namely{1012},{1122} and{1121}. Twinning type was dependant on the spatial relationship between crystal orientation and compressive direction. Schmid factor of twinning and spherical coordinates in HCP unit cell were induced to calculate the Schmid distribution. By combining the calculated results with twinning type and Euler angles of grain matrix, the Schmid factor was proved to be able to predict the activation of twinning under both quasi-static and dynamic states. With the help of the calculated SF map, the corresponding critical resolved shear stress range was determined to be321-361MPa,367-393MPa and525-538MPa respectively. Moreover, strain rate was found to have a minor effect on the value of CRSS of twinning. In addition, by means of single-crystal titanium compression it has been proved that among the six twinning variants of a specific type the one with the maximum Schmid factor will be activated.In order to investigate the phenomenon of plabtic instability in Gr.2titanium a dynamic shear loading system was proposed. According to the pattern of pre-gridded scratch lines on the deformed specimen surface, a critical shear strain of1.1was found to activate plastic localization, while the shear stress was475MPa at a strain rate of18000s-The critical condition can be analyzed by considering the competition between strain hardening and thermal softening. This was realized mathematically by coupling the Johnson-Cook constitute model and temperature rise equation, based on which adiabatic shear stress and strain curve in a strain rate range from10s"1to104S-1was constructed. According to peak points of calculated curves, the critical condition can be determined as0.9-1.3for critical shear strain and395-480MPa for maximum shear stress. The calculated result fits well with the experimental value of1.1and475MPa. The relationship between the critical condition and metallurgical factors were also discussed. In order to lower ASB propensity the most important material parameters are those indicating strain hardening capacity, thermal softening propensity and thermal conductivity.Then, the influence of Al on the dynamic performance and deformation behavior of Ti-xAl(x=2,4,6) alloys was explored through the above mentioned methods on dynamic performance test, the deformation microstructure observation and data processing. While Al content increased from2wt.%to6wt.%, the heat conductivity at room temperature was lowered by approximately58%, the quasi-static yield strength was increased by144%, the critical resolved shear stress of{1012} type twinning was increased by100%, and strain hardening modulus at εp-0.07was lowered by50%. These changes directed the dynamic shear instability critical condition of Ti-2A1along the forecasted path to that of Ti-6A1.Finally, polycrystalline pure titanium with different grain size and apparent texture feature was used to discuss the effect of equiaxed grain size and loading direction on the dynamic performance of a titanium. The result shows that increasing the grain size does not significantly change the dynamic shear resistance, however significantly reduces the critical shear strain. When the shear loading direction tends to perpendicular to the c-axis, a titanium will manifest a superior combination of dynamic strength and ductility.