Study Of The Evolution Of Deformation Microstructure In Ti-25Nb-25Ta Biomedical Alloy By Electron Microscopy

In this study, the evolution and intersection phenomenon of {332}<113> twinning deformation structures in metastable beta type Ti-25Nb-25 Ta alloy during plastic deformation at room temperature was investigated by the combination of various characterization techniques for both structure and property, including optical microscopy(OM), electron backscattered diffraction(EBSD), transmission electron microscopy(TEM). At the same time, careful analysis was made on the deformation structures of the alloy at the final stage of plastic deformation. The aim is to reveal the proliferation characteristics and the evolution tendency of {332}<113> twinning during deformation, and to determine deformation characteristics of the final stage of plastic deformation, so as to find some new ways or factors to control microstructures intentionally, to improve its mechanical properties effectively, to strengthen the relationship between microstructures and the mechanical properties of the alloy, and to provide valuable information for developing new type biomedical ?-titanium alloys with high-performance as well.Firstly, tensile deformation of Ti-25Nb-25 Ta alloy at room temperature was carried out in order to study the evolution of {332}<111> twinning in plastic deformation. It is clearly revealed that, with the increasing of deformation, the thickness of twin laths keep increasing, along with their morphology changing gradually from straight thin laths to those with various width during the tensile deformation of Ti-25Nb-25 Ta alloy. At the final stage of the deformation, some twins were broken with their twin boundaries being seriously distorted. At the same time, crystallographic misorientation between {332}<113> twinning and the matrix changed gradually and fluctuation range increased gradually, which change from 50.5° at the initial stage to 46°~56° at the final stage. The difference of crystallographic misorientation between two sides of {332}<113> twinning and the matrix changed from 0° at the initial stage to 6° at the final stage as well.Secondly, it was found that when the tensile deformation rate reached 20%, a variety of {332}<113> twinning variants were produced within grains, and a network of twin laths structure was formed by different twinning variants. The interaction phenomenon of two types of {332}<113> twinning variants was investigated by EBSD. The results show that the interaction effect of different {332}<113> twinning variants resulted in large local lattice distortion occurred in the intersected zone and the most serious distorted zones were in the vicinity of intersection boundaries. As a result of the interaction effect, the crystal orientation of intersected zone was changed to some extent with respect to both the matrix and the twins involved in the intersection as well.Finally, the samples were observed carefully with TEM in order to reveal the structure characteristics of Ti-25Nb-25 Ta alloy at the final stage of tensile deformation. The results showed that there were a variety of configurations of twin-omega concomitant structure, including zigzag configuration of twin-omega phase-twin and twin-multiple omega phase-multiple twin. On the basis of dislocation decomposition theory, the formation reason of concomitant structure was discussed. At the same time, the stress-induced omega phase and secondary {112}<111> twins were formed within {332}<113> twins which produced in the early deformation, which segments and refines grains further.