Research On Plastic Deformation Characteristics And Globularization Behavior Of TA15 Titanium Alloy With Lamellar Microstructure In Alpha And Beta Phase Field

Titanium alloys are the mainly structural material in aerospace and aeronautical field for their high specific strength, excellent corrosion and heat resistance. Because of the high structural stability at elevated temperature, large plastic deformation in alpha and beta phase field is necessary to obtain the globular microstructure. Compared to beta phase field, there are several problems for the deformation in alpha and beta phase field, such as, lower deformation temperature, higher deformation resistance, poor processing plasticity and various deformation defects. In addition, forming parts with uniform microstructure and excellent performance are difficult to be obtained due to inhomogeneous deformation resulting from poor thermal conductivity of titanium alloys. Therefore, it is essential to investigate the plastic deformation characteristics and globularization behavior of titanium alloys with lamellar microstructure to realize the control of microstructure and performance during the deformation in alpha and beta phase field.By means of isothermal constant strain rate compression tests in the temperature range of 750℃~950℃and strain rate range of 0.001s-1~10s-1 at alpha and beta phase field, the influences of hot deformation parameters and the thickness of lamellar alpha phase on flow stresses of TA15 titanium alloy with two different thickness of lamellar alpha phase were studied. The corresponding activation energies of these two TA15 titanium alloys were respectively calculated and a constitutive equation applicable to these two TA15 titanium alloys was constructed. The computational accuracy of this equation is desirable and the average errors of flow stress between experimental and calculated value are less than 4.83%.The hot deformation regions of TA15 titanium alloy with two different thickness of lamellar alpha phase were predicted to optimize the hot deformation parameters in alpha and beta phase field by using processing map technology based on Dynamic Material Model. The results show that the preferable hot deformation regions are mainly at low strain rate domains (0.001s-1~0.01s-1) and the instable regions of plastic flow are mainly focused on the domains of low temperature and high strain rate (750℃~ 850℃,0.1s-1~10s-1). The reliability of the prediction results was verified by the observation of microstructure. It is shown that the predicting results of processing maps for TA15 titanium alloy with lamellar microstructure are accurate and creditable. The deformation microstructure of the parameters in preferable regions of processing maps exhibits a large proportion of globular alpha phase. The corresponding microstructure in suitable regions is globular structure to a scale. Various plastic flow instabilities occur in the microstructure of the instable regions, such as flow localization, micro-cavity, micro-crack and 45°macroscopic shear crack. The generation and forming mechanisms of these instability defects were studied to clarify the essential relationships among them, which provide significant theoretical reference for controlling the microstructure and performance of titanium alloys with lamellar microstructure during the deformation processing in alpha and beta phase field.The influences of hot deformation parameters and the thickness of lamellar alpha phase on globularization behavior of TA15 titanium alloy with lamellar alpha phase in the temperature range of 750℃~950℃and strain rate range of 0.001s-1~10s-1 were researched by quantitative metallographic observation, scanning and transmission electron microscopes etc. The results indicate that the percent of globular alpha phase increases with the decreasing of strain rate and the thickness of lamellar alpha phase and the increasing of deformation temperature and true strain. The critical true strains of globularization for TA15 titanium alloy with lamellar microstructure are varied in 0.20 to 0.50. There are some lamellar alpha phase still remained even at the deformation parameter of 900℃, 0.001s-1 and true strain of 1.20. The dynamic model of globularization for TA15 titanium alloy with lamellar microstructure was established. In this model the experimental and calculated value fit well, accordingly it can represent the dynamic process of globularization for TA15 titanium alloy.The globularization mechanism of TA15 titanium alloy with lamellar microstructure was studied on the base of the dynamic model of globularization. The globularization procedure of lamellar alpha phase is caused by the combined action of diffusion and deformation mechanisms. Both dynamic recrystalization and deformation twin occur simultaneously, compete and supply each other in globularization procedure. Diffusion process runs through the globularization procedure of TA15 titanium alloy with lamellar microstructure in the whole processing. The forked structures occurred at the end and side faces of alpha phase and the penetration of beta phase into the interface of alpha and alpha phase virtually both were caused by the diffusion of element alloys such as Al, Mo and V etc. The thickness of lamellar alpha phase decreases by separating from forked structure, which is beneficial to globularization procedure. The penetration of beta phase into the interface of alpha and alpha phase caused by dynamic recrystalization and deformation twin plays an important role in the globularization procedure.