An In-situ X-ray Diffraction And Imaging Research Of NiTi SMAs Under Quasi-static Tensile Loading

NiTi shape memory alloys(SMA)are remarkable materials which possess the capability of recover its undefromed shape,when subjected to mechanical loading followed by either apply heat(shape memory effect)or unloading(superelastic effect).Owe to their unique properties,high engineer strength and ductility,applications of NiTi SMAs has huge potential in many industries such as automobile,biomedical field and aerospace field etc.Systematic study of the relationships between microstructure,mechanical properties and martensitic transformation behaviors is needed,because applications closely related to not only mechanical properties but also microstructure and martensitic transformation.The in-situ X-ray diffraction and imaging techniques applied in the thesis,are capable of collecting information about crystal lattice parameters,phase fraction of materials,prefer orientation etc.,when undergoing mechanical loading process.The applies of in-situ X-ray diffraction in phase transformation research is irreplaceable.Synchrotron high-energy X-ray radiation in-situ diffraction and imaging techniques are used.X-ray diffraction patterns and X-ray phase contrast images(XPCI)are obtained during Nitinol subjected to quasi-static loading under room temperature.Bulk Nitinol plate are cutted along rolling direction and transverse direction respectively,stress-strain curves show mechanical anisotropy related to texture of samples.Simultaneous X-ray imaging outcomes have no sign of marco martensitic transformation band propagation(Lüders band),digital image correlation(DIC)method found small strain focus regions may related to grain size effect,twins and dislocations.High energy X-ray white-beam diffraction patterns show overlapped diffraction peaks get lower,wider and shift toward lower Bragg degrees,those phenomena related martensitic transformation considering CuK? radiation diffraction results.To illustrate texture influence on martensitic transformation behaviors,two different texture and grain size samples are used.First,X-ray pole figure measurement and Electron Backscattering Diffraction(EBSD)techniques are implemented to obtain texture information and grain size distribution information.Then experiments are carried out on independently developed mini material test system(MMTS),quasi-static tensile loading under room temperature.Profile fitting and simplified Rietevld refinement are applied to X-ray diffraction patterns,outcomes show peak evolution related to phase transformation are quite distinguish for two samples.Stress-induced martensite peaks have less prefer orientation effect for grain size small sample.Shape recovery ability is influenced by microstructure and loading strain.Optical imaging recorded Lüders bands evolution during loading and unloading.Research methods,equipment applied in the thesis and related discussions provide ideas for develop laboratory X-ray in-situ loading technique.The current work give some experimental evidences for further research on relationships between prefer orientations,marco loading response and martensitic transformation of NiTi polycrystalline.