The physical properties of polycrystalline materials depend on their microstructure, which is the nano- to centimeter scale arrangement of phases and defects in their interior. Such microstructure depends on the shape, crystallographic phase and orientation, and interfacing of the grains constituting the material. This article presents a new non-destructive 3D technique to study centimeter-sized bulk samples with a spatial resolution of hundred micrometers: time-of-flight three-dimensional neutron diffraction (ToF 3DND). Compared to existing analogous X-ray diffraction techniques, ToF 3DND enables studies of samples that can be both larger in size and made of heavier elements. Moreover, ToF 3DND facilitates the use of complicated sample environments. The basic ToF 3DND setup, utilizing an imaging detector with high spatial and temporal resolution, can easily be implemented at a time-of-flight neutron beamline. The technique was developed and tested with data collected at the Materials and Life Science Experimental Facility of the Japan Proton Accelerator Complex (J-PARC) for an iron sample. We successfully reconstructed the shape of 108 grains and developed an indexing procedure. The reconstruction algorithms have been validated by reconstructing two stacked Co-Ni-Ga single crystals, and by comparison with a grain map obtained by post-mortem electron backscatter diffraction (EBSD).
Conventional shape memory alloys cannot be employed for applications in the elevated temperature regime due to rapid functional degradation. Co-Ni-Ga has shown the potential to be used up to temperatures of about 400°C due to a fully reversible superelastic stress-strain response. However, available results only highlight the superelastic response for single cycle tests. So far, no data addressing cyclic loading and functional fatigue are available. In order to close this gap, the current study reports on the cyclic degradation behavior and tensioncompression asymmetry in [001]-oriented Co 49 Ni 21 Ga 30 single crystals at elevated temperatures. The cyclic stressstrain response of the material under displacement controlled superelastic loading conditions was found to be dictated by the number of active martensite variants and different resulting stabilization effects. Co-Ni-Ga shows a large superelastic temperature window of about 400°C under tension and compression, but a linear Clausius-Clapeyron relationship could only be observed up to a temperature of 200°C. In the present experiments, the samples were subjected to 1000 cycles at different temperatures. Degradation mechanisms were characterized by neutron diffraction and transmission electron microscopy. The results in this study confirm the potential of these alloys for damping applications at elevated temperatures.
Ti-Ta based alloys are an interesting class of high-temperature shape memory materials. When fabricated as thin films, they can be used as high-temperature micro-actuators with operation temperatures exceeding 100°C. In this study, microstructure, shape memory effect and thermal cycling stability of room-temperature sputter deposited Ti 67 Ta 33 thin films are investigated. A disordered a 00 martensite (orthorhombic) phase is formed in the as-deposited Ti 67 Ta 33 films. The films show a columnar morphology with the columns being oriented perpendicular to the substrate surface. They are approximately 200 nm in width. XRD texture analysis reveals a martensite fiber texture with {120} and {102} fiber axes. The XRD results are confirmed by TEM analysis, which also shows columnar grains with long axes perpendicular to the {120} and {102} planes of a 00 martensite. The shape memory effect is analyzed in the temperature range of -10 to 240°C using the cantilever deflection method, with special emphasis placed on cyclic stability. Ti 67 Ta 33 thin films undergo a forward martensitic transformation at M s % 165°C, with a stress relaxation of approximately 33 MPa during the transformation. The actuation response of the film actuators degrades significantly during thermal cycling. TEM analysis shows that this degradation is related to the formation of nanoscale v precipitates (5-13 nm) which form above the austenite finish temperature. These precipitates suppress the martensitic transformation, as they act as obstacles for the growth of martensite variants.
A prototype quasiparasitic thermal neutron beam monitor based on isotropic neutron scattering from a thin natural vanadium foil and standard 3 He proportional counters is conceptualized, designed, simulated, calibrated, and commissioned. The European Spallation Source designed to deliver the highest integrated neutron flux originating from a pulsed source is currently under construction in Lund, Sweden. The effort to investigate a vanadium-based neutron beam monitor was triggered by a list of requirements for beam monitors permanently placed in the ESS neutron beams in order to provide reliable monitoring at complex beam lines: low attenuation, linear response over a wide range of neutron fluxes, near to constant efficiency for neutron wavelengths in a range of 0.6-10 Å, calibration stability and the possibility to place the system in vacuum are all desirable characteristics. The scattering-based prototype, employing a natural vanadium foil and standard 3 He proportional counters, was investigated at the V17 and V20 neutron beam lines of the Helmholtz-Zentrum in Berlin, Germany, in several different geometrical configurations of the 3 He proportional counters around the foil. Response linearity is successfully demonstrated for foil thicknesses ranging from 0.04 mm to 3.15 mm. Attenuation lower than 1% for thermal neutrons is demonstrated for the 0.04 mm and 0.125 mm foils. The geometries used for the experiment were simulated allowing for absolute flux calibration and establishing the possible range of efficiencies for various designs of the prototype. The operational flux limits for the beam monitor prototype were established as a dependency of the background radiation and prototype geometry. The herein demonstrated prototype monitors can be employed for neutron intensities ranging from 10 3-10 10 n=s.
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