Nickel Titanium Base Shape Memory Alloy Phase Transition Behavior Of Synchrotron Radiation And Neutron Diffraction Experiment In Situ

Ni-Ti-based shape memory alloy (SMA) are a unique metallic alloy which can undergo large deformations (8%) while reverting back to their undeformed shape through either the application of heat (shape memory effect) or the removal of the load (superelastic effect). These unique properties have led to the development of applications in the biomedical field, aerospace field, and commercial industry. However, successful implementation of these applications requires systematic studies on their martensitic transformation behaviors which are strongly dependent on the composition and post-thermomechanical treatment.The Ti-Ni-Nb shape memory alloys are distinct from other TiNi shape memory alloys because of their wide phase transformation hysteresis. This wide hysteresis has important engineering utility, as parts made of this alloy can avoid storing and installing at cryogenic temperature. On the other hand, the Ti-Ni-Cu alloys show narrow transformation temperature hysteresis and relatively high martensitic transformation temperature. Thus, the Ti-Ni-Cu thin film is considered as one of the most promising actuator materials in the fields of microelectromechanical system (MEMS).The in situ diffraction techniques when applied during thermomechanical tests are capable of bringing corresponding experimental information without interfering with the transformation process, particularly with respect to detection of changes of phase fractions, texture and stresses in austenite and martensite phases. Due to the high penetration depth of high-energy X-ray and neutron, the in situ synchrotron-based high-energy X-ray (S-XRD) and neutron diffraction (ND) are powerful tools for studying the microstructure evolution of bulk materials. This work deals with the application of in situ diffraction methods to investigate the mechanics of a stress induced martensitic transformation in three kinds of typical Ni-Ti-based alloys, i.e., bulk Ni50.1Ti49.9, Ti50.1Ni40.8Cu9.1thin film and bulk Ni47Ti44Nb9. All of the studies are helpful for understanding the physical mechanisms of preferred selection of martensitic variants under stress field, stress induced phase transition, and superelasticity. The current work provides the direct experimental evidence for the further development of physical models for functional behaviors in SMAs.Combined with the four types of deformation characters in the macro stress-strain curves of double phase Ni50.1Ti49.9alloy, the micromechanical interactions and phase transformation are determined by in-situ neutron diffraction. The volume fraction of initial austenite before deformation is about22%. The contrast transform of (110)B2and (002)B19’ lattice strains reveal that stress-induced austenite to martensite phase transformation appears with the volume fraction of austenite decreasing rapidly and<011>Ⅱ type twinning increasing at the low strain harding stage. At the same time, the initial martensite grains change their orientation to favorable direction and the new{20-1} type martensite twinning are induced with the applied stress increasing. which cannot recover after unloading. At the high strain harding stage. the twinning deform is the main mechanism corresponding to the small changes of full width at half maximum (FWHM). However, the slipping caused by dislocation is the main deform mechanism corresponding to the obviously increasing of FWHM at the strain harding statured stage.Stress-induced martensitic transformation of as-sputtered and post-annealed Ti50.1Ni40.8Cu9.1thin films was investigated using in-situ synchrotron X-ray diffraction (S-XRD) technique. For the as-deposited film, in-situ S-XRD analysis showed an elastic deformation of martensite during initial loading, followed by reorientation of martensite variants via detwinning. This detwinning process induced a strong <020> fiber texture along the loading direction and a strong<002> fiber texture perpendicular to the loading direction. For the650℃annealed film, there is only elastic deformation, followed by a martensitic transformation during deformation.The microstructural characteristics of bulk Ni47Ti44Nb9in response to multiple external parameters (temperature and stress fields) are traced, through high-energy X-ray diffraction technique. When tensioning the bulk Ni47Ti44Nbg at both room temperature (about25℃) and low temperature (-70℃), martensitic variant with crystal plane (001) perpendicular to the loading direction ((hkl)⊥LD) is dominant in the specimen. However, the deformation mechanism at low temperature is more complex than that at room temperature. It is found only <011> Type Ⅱ twinning at the room temperature but more three kind twinning modes at the low temperature. The twinning modes of B19’ martensite was found to play an important role on the deformation process.