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Abstract: Three-dimensional X-ray diffraction (3DXRD) is shown to be feasible at the I12 Joint Engineering, Environmental and Processing (JEEP) beamline of Diamond Light Source. As a demonstration, a microstructually simple low-carbon ferritic steel was studied in a highly textured and annealed state. A processing pipeline suited to this beamline was created, using software already established in the 3DXRD user community, enabling grain centre-of-mass positions, orientations and strain tensor elements to be determined. Orientations, with texture measurements independently validated from electron backscatter diffraction (EBSD) data, possessed a ∼0.1° uncertainty, comparable with other 3DXRD instruments. The spatial resolution was limited by the far-field detector pixel size; the average of the grain centre of mass position errors was determined as ±∼80 µm. An average per-grain error of ∼1 × 10−3 for the elastic strains was also measured; this could be reduced in future experiments by improving sample preparation, geometry calibration, data collection and analysis techniques. Application of 3DXRD onto I12 shows great potential, where its implementation is highly desirable due to the flexible, open architecture of the beamline. User-owned or designed sample environments can be used, thus 3DXRD could be applied to previously unexplored scientific areas.

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Abstract: To explore an effective route of customizing the superelasticity (SE) of NiTi shape memory alloys via modifying the grain structure, binary Ni55Ti45 (wt) alloys were fabricated in as-cast, hot swaged, and hot-rolled conditions, presenting contrasting grain sizes and grain boundary types. In situ synchrotron X-ray Laue microdiffraction and in situ synchrotron X-ray powder diffraction techniques were employed to unravel the underlying grain structure mechanisms that cause the diversity of SE performance among the three materials. The evolution of lattice rotation, strain field, and phase transformation has been revealed at the micro- and mesoscale, and the effect of grain structure on SE performance has been quantified. It was found that (i) the Ni4Ti3 and NiTi2 precipitates are similar among the three materials in terms of morphology, size, and orientation distribution; (ii) phase transformation happens preferentially near high-angle grain boundary (HAGB) yet randomly in low-angle grain boundary (LAGB) structures; (iii) the smaller the grain size, the higher the phase transformation nucleation kinetics, and the lower the propagation kinetics; (iv) stress concentration happens near HAGBs, while no obvious stress concentration can be observed in the LAGB grain structure during loading; (v) the statistical distribution of strain in the three materials becomes asymmetric during loading; (vi) three grain lattice rotation modes are identified and termed for the first time, namely, multi-extension rotation, rigid rotation, and nondispersive rotation; and (vii) the texture evolution of B2 austenite and B19′ martensite is not strongly dependent on the grain structure.

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Abstract: Tracking texture evolution during in situ loading is critical to understand and simulate the dynamic behaviour of microstructure in polycrystalline materials, yet conventional texture quantification methods are sometimes restricted due to various factors, such as acquisition time, sample environment and complex setup. To address this, a novel approach to extract texture information from single shot Time-Of-Flight neutron diffraction pattern has been developed. Another texture analysis approach based on single shot synchrotron X-ray diffraction has also been demonstrated. The effectiveness of two methods is assessed for polycrystalline Nickel-based superalloy polycrystalline samples possessing different textures. Both methods feature a moderate acquisition time of ~10 min and 30 s respectively, as well as a simplified setup which allows adding complex sample environments and the use of additional equipment. Comparison with the referential EBSD texture suggests that both approaches achieve a satisfactory match, though some details of the complex contour profiles in inverse pole figures may be missing. Besides that, a novel metric has been proposed to quantify the matching quality of pole figures. By employing the EPSC modelling approach, it is shown that the texture deviation due to the technique chosen for its evaluation exerts a subtle influence on th macro- and mesoscale simulation results, highlighting the significance of this approach for underpinning robust computational modelling.

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Abstract: The effect of Strain Path Changes (SPCs) on the mechanical properties and crystal-level features of deformation for a single phase, ferritic steel has been investigated. SPCs were applied via a two-step deformation process, which included pre-straining via cold rolling, followed by uniaxial tension. The pre-strain magnitude and direction, as well as the tensile direction, varied between the specimens. The role of texture and micromechanics were examined in-situ, via Synchrotron X-ray Diffraction (SXRD), and ex-situ, via Electron Backscatter Diffraction (EBSD). Abrupt strain paths (i.e. strain paths where the pre-strain and the subsequent loading directions differ; here they are orthogonal) result in a significant ductility reduction, becoming more prevalent for high pre-strain magnitudes. The macroscopic response, as well as the texture configuration were greatly dependent on the pre-strain direction but were insensitive to the direction of uniaxial tension. Increasing pre-strain magnitudes resulted in a stagnation of lattice strain hardening rates in all macroscopic directions and in a significant increase in the Geometrically Necessary Dislocation (GND) densities. This was vastly increased for specimens rolled perpendicular to the as-received prior rolling direction. No correlation was found between the GND density and the grain orientation, eliminating this as a controlling ductility factor for BCC ferrite. Instead, the initial texture and the texture developed in a subsequent pre-strain influences the density of dislocations accumulated in all grains, and ultimately determines ductility.

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Abstract: During directed energy deposition (DED) additive manufacturing, powder agglomeration and sintering can occur outside of the melt pool when using titanium alloy powders. Using in situ synchrotron radiography we investigate the mechanisms by which sintering of Ti6242 powder occurs around the pool, performing a parametric study to determine the influence of laser power and stage traverse speed on sinter build-up. The results reveal that detrimental sinter can be reduced using a high laser power or increased stage traverse speed, although the latter also reduces deposition layer thickness. The mechanism of sinter formation during DED was determined to be in-flight heating of the powder particles in the laser beam. Calculations of particle heating under the processing conditions explored in this study confirm that powder particles can reasonably exceed 700 °C, the threshold for Ti surface oxide dissolution, and thus the powder is prone to sintering if not incorporated into the melt pool. The build-up of sinter powder layer on deposit surfaces led to lack of fusion pores. To mitigate sinter formation and its detrimental effects on DED component quality, it is essential that the powder delivery spot area is smaller than the melt pool, ensuring most powder lands in the melt pool.