Microstructure And Crystallographic Characterization Of Commercially Pure Titanium T40 Sheet During Cold Deformation

Titanium and its alloys have attracted considerable attention due to their good mechanical, chemical and biological properties. They have very important implications in aerospace, transportation, military, medical, and chemical fields since the early 1950s. To further understand their mechanisms and to significantly improve their performances, researchers have carried out extensive studies by investigating the microstructure evolution.How to improve the ductility and formability of titanium and its alloys has been an important issue for along time. The main obstacle to improve the performance of titanium and its alloys during cold deformation is that the understanding to deformation mechanisms is incomplete. There is no consistent explanation for the nucleation, growth and detwinning of twins. Mechanisms of twin-slip dislocation interaction are in lack of systematic and comprehensive understanding. At present, simulation work enlight the direction of this issue, but there is still little visible experiental evidence. Therefore, the experimental study of plastic deformation mechanisms of titanium alloys at room temperature become a hot point in material science filed.To solve the questions mentioned above, this work choose commercially pure titanium plate T40 as the research object, using scanning electron microscopy-backscattered electron diffraction (SEM-EBSD), the grain shape, grain size, crystallographic orientation and appearance order of twins during cold rolling process were characterized and analysed. By establishing a simplified model, the Schmid factor, twinning strain, and plastic deformation energy during twinning for all the potential twinning variants were calculated. The rule of twinning variant selection was summarized and the influence on twinning variants growth was discussed. By this theory, the correlation between the variant number and matrix grain size, matrix grain shape was explained. Finally, we clarified the deformation twinning variant selection mechanism and predicted the deformation twinning system as well as deformation twinning variant.In this work, using a quasi-in-situ electron microscopy-backscattered electron diffraction (SEM-EBSD), the microstructure evolution and crystallographic orientation evolution during a tensile test were characterized, and the morphology, order of appearance, and location of appearance were analysed. Accommodations between dislocation slip and deformation twin were reported. From the material continuity point of view, a physical related shear displacement gradient tensor model was established. By this model, the accommodation between dislocation slip and deformation twin were studied, and dislocation slip-twin stimulation behaviors were explained. Finally, we elaborated the combined effects of local accommodation and macro-loading state on deformation twinning nucleation, growth, and disappearance.Due to the elastic anisotropy of commercially pure titanium, the dislocation strain field in commercially pure titanium grain is complicated, which makes the dislocation identification under transmission electron microscopy very difficult. It is not accurate to identify a dislocation by the dislocation invisible condition. To satisfy the practical needs in identifying dislocation during deformation, a calculation method was proposed based on the crystalllogpaphy theory. This method can significantly simplify the identification process using Transmission electron microscope (TEM). The feasibility and accuracy of this method were verifed by sheared commercially pure titanium sheets.This work further deepens the understanding of deformation behaviors and deformation mechanisms of titanium alloys. It provides experimental parameters and experimental evidence for simulation work. And it laid the theoretical foundation for titanium alloy design optimization, performance prediction and failure control.