I12-JEEP: Joint Engineering, Environmental and Processing
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Diamond Proposal Number(s):
[26376]
Open Access
Abstract: The structural performance of polycrystalline alloys is strongly controlled by the characteristics of individual grains and their interactions, motivating this study to understand the dynamic micromechanical response within the microstructure. Here, a high ductility single-phase ferritic steel during uniaxial deformation is explored using three-dimensional X-ray diffraction. Grains well aligned for dislocation slip are shown to possess a wide intergranular stress range, controlled by per-grain dependent hardening activity. Contrariwise, grains orientated poorly for slip have a narrow stress range. A grain neighbourhood effect is observed of statistical significance: the Schmid factor of serial adjoining grains influences the stress state of a grain of interest, whereas parallel neighbours are less influential. This phenomenon is strongest at low plastic strains, with the effect diminishing as grains rotate during plasticity to eliminate any orientation dependent load shedding. The ability of the ferrite to eliminate such neighbourhood interactions is considered key to the high ductility possessed by these materials.
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Mar 2024
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James A. D.
Ball
,
Claire
Davis
,
Carl
Slater
,
Himanshu
Vashishtha
,
Mohammed
Said
,
Louis
Hébrard
,
Florian
Steinhilber
,
Jonathan P.
Wright
,
Thomas
Connolley
,
Stefan
Michalik
,
David M.
Collins
Open Access
Abstract: A novel complex-phase steel alloy is conceived with a deliberately unstable austenite, , phase that enables the deformation-induced martensitic transformations (DIMT) to be explored at low levels of plastic strain. The DIMT was thus explored, in-situ and non-destructively, using both far-field Three-Dimensional X-ray Diffraction (3DXRD) and Electron Back-Scatter Diffraction (EBSD). Substantial martensite formation was observed under 10 % applied strain with EBSD, and many
ɛ
grain formation events were captured with 3DXRD, indicative of the indirect transformation of martensite via the reaction
ɛ
. Using
ɛ
grain formation as a direct measurement of grain stability, the influence of several microstructural properties, such as grain size, orientation and neighbourhood configuration, on stability have been identified. Larger grains were found to be less stable than smaller grains. Any grains oriented with {100} parallel to the loading direction preferentially transformed with lower stresses. Parent
ɛ
-forming grains possessed a neighbourhood with increased ferritic/martensitic volume fraction. This finding shows, unambiguously, that the nearby presence of and promotes
ɛ
formation in neighbouring grains. The minimum strain work criterion model for
ɛ
variant prediction was also evaluated, which worked well for most grains. However,
ɛ
-forming grains with a lower stress were less well predicted by the model, indicating crystal-level behaviour must be considered for accurate
ɛ
formation. The findings from this work are considered key for the future design of alloys where the deformation response can be controlled by tailoring microstructure and local or macroscopic crystal orientations.
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Dec 2023
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[18972]
Open Access
Abstract: Multiple oxides were observed during the transient oxidation stage of the polycrystalline Ni-based superalloy, RR1000, before a protective Cr2O3 scale formed. Thermogravimetric analysis, synchrotron grazing incidence X-ray diffraction, and electron microscopy were performed on samples subjected to isothermal exposures at 800 °C for up to 100 hours. Transient effects governed the early stages up to 40 hours. NiO, spinels (NiCr2O4, (Ni,Co)(Cr,Co)2O4), Cr2O3/(Cr0.88Ti0.12)2O3, NiTiO3, and CrTaO4 formed during the initial stage with pseudo-linear kinetics. At the onset of parabolic kinetics, extensive Cr2O3 and TiO2 growth dominated scale formation with the former emerging as the major passivating oxide.
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Nov 2023
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Open Access
Abstract: The ability to characterise the three-dimensional microstructure of multiphase materials is essential for understanding the interaction between phases and associated materials properties. Here, laboratory-based diffraction-contrast tomography (lab-based DCT), a recently-established materials characterization technique that can determine grain phases, morphologies, positions and orientations in a voxel-based reconstruction method, was used to map part of a dual-phase steel alloy sample. To assess the resulting microstructures that were produced by the lab-based DCT technique, an electron backscatter diffraction (EBSD) map was collected within the same sample volume. To identify the two-dimensional (2D) slice of the three-dimensional (3D) lab-based DCT reconstruction that best corresponded to the 2D EBSD map, a novel registration technique based solely on grain-averaged orientations was developed – this registration technique requires very little a priori knowledge of dataset alignment and can be extended to other techniques that only recover grain-averaged orientation data such as far-field 3D X-ray diffraction microscopy. Once the corresponding 2D slice was identified in the lab-based DCT dataset, comparisons of phase balance, grain size, shape and texture were performed between lab-based DCT and EBSD techniques. More complicated aspects of the microstructural morphology such as grain boundary shape and grains less than a critical size were poorly reproduced by the lab-based DCT reconstruction, primarily due to the difference in resolutions of the technique compared with EBSD. However, lab-based DCT is shown to accurately determine the centre-of-mass position, orientation, and size of the large grains for each phase present, austenite and martensitic ferrite. The results reveals a complex ferrite grain network of similar crystal orientations that are absent from the EBSD dataset. Such detail demonstrates that lab-based DCT, as a technique, shows great promise in the field of multi-phase material characterization.
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Aug 2023
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I12-JEEP: Joint Engineering, Environmental and Processing
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Diamond Proposal Number(s):
[24084]
Abstract: Cyclic high temperature deformation, which is a precursor to creep-fatigue damage is one of the main life limiting factors in thermal power plants. Microstructurally informed models such as crystal plasticity have shown great promise in predicting cyclic plasticity and creep deformation; however, further validation of predicted meso-scale deformation is required to ensure accurate damage calculations. Here, a novel 3D X-ray diffraction (3DXRD) experiment was performed to resolve and investigate the response of individual grains within a polycrystalline material under loading at elevated temperature. Specimens were made from 316H stainless steel, which is an alloy commonly used for critical structural components in thermal power generation plants. The 3DXRD experiments were conducted at the UK national synchrotron facility, Diamond Light Source. The measurements provided positions, strain tensors, and orientations of individual grains within a gauge volume. The data generated from 3DXRD was used both as an input and for the validation of a crystal plasticity finite element model (CPFE). The results provided demonstrate the importance of microstructural information in materials modelling.
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Jul 2023
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[28594]
Open Access
Abstract: Improved oxidation kinetics for a polycrystalline Ni-based superalloy used in turbine disc applications has been shown to be possible by controlling the heating rate of the first thermal exposure to 5 °C min−1. The beneficial effect arises from the formation of a protective layer of NiCr2O4, instead of the more usually formed doped Cr2O3. This study shows that it was possible to form the NiCr2O4 at temperatures up to 725 °C, within the operational conditions for this alloy, and that at higher temperatures Cr2O3 formed. The improvements in alloy performance extended to the internal oxidation processes where reduced depths of degradation were observed. It is demonstrated here that Al2O3 formation is less thermodynamically stable when the highly protective NiCr2O4 oxide is present at the alloy surface compared to the doped Cr2O3. Synchrotron XRD was performed on samples removed during the heating stage and provided evidence of the oxidation sequence occurring, enabling refinement in the thermodynamic calculations and suggesting an additional route to the formation of the NiCr2O4.
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Jun 2023
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I12-JEEP: Joint Engineering, Environmental and Processing
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Yuanbo T.
Tang
,
Chinnapat
Panwisawas
,
Benjamin M.
Jenkins
,
Junliang
Liu
,
Zhao
Shen
,
Enrico
Salvati
,
Yilun
Gong
,
Joseph N.
Ghoussoub
,
Stefan
Michalik
,
Bryan
Roebuck
,
Paul A. J.
Bagot
,
Sergio
Lozano-Perez
,
Chris R. M.
Grovenor
,
Michael P.
Moody
,
Alexander M.
Korsunsky
,
David M.
Collins
,
Roger C.
Reed
Diamond Proposal Number(s):
[23674]
Open Access
Abstract: A supersaturated phase microstructure is produced in Ni-based superalloys using laser powder bed fusion (L-PBF) – the cooling rate arising from the process is shown to suppress the solid-state precipitation of the phase. The response of the material to a heat treatment therefore requires new understanding at the fundamental level, since the first population of precipitate forms upon heating, in contrast to cooling from homogenisation above the solvus. Here, we have interrogated two new nickel-based superalloys designed for the L-PBF technology, both in situ and ex situ, at multiple length scales using advanced characterisation methods. First, we conducted in situ synchrotron X-ray diffraction during various heat treatments to trace the evolution of the volume fraction with temperature. The first structural changes were detected at an unexpectedly low temperature of 445 °C. Second, the temperature for nucleation and its sensitivity to heating rate was studied using an electrical resistivity method. Then, the composition upon heating, isothermal holding and cooling is analysed using atom probe tomography (APT), the result is rationalised by further scanning-transmission electron microscopy and nanoscale secondary ion mass spectroscopy. Finally, static recrystallisation during isothermal exposure was investigated, which occurs within minutes. This work sheds light on a new strategy of tailoring microstructure for additively manufactured superalloys by manipulation of the precipitate distribution upon heating.
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Jan 2023
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I12-JEEP: Joint Engineering, Environmental and Processing
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Diamond Proposal Number(s):
[26376]
Open Access
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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Jul 2022
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I12-JEEP: Joint Engineering, Environmental and Processing
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Diamond Proposal Number(s):
[17222]
Abstract: Component failure due to cold dwell fatigue of titanium and its alloys is a long-standing problem which has significant safety and economic implications to the aviation industry. This can be addressed by understanding the governing mechanisms of time dependent plasticity behaviour of Ti at low temperatures. Here, stress relaxation tests were performed at four different temperatures on three major alloy systems: commercially pure titanium (two alloys with different oxygen content), Ti-6Al-4V (two microstructures with differing phase fractions) and Ti-6Al-2Sn-4Zr-Mo (two alloys with different Mo content =2 or 6, and portion of phase). Key parameters controlling the time dependent plasticity were determined as a function of temperature. Both activation volume and energy were found to increase with temperature in all six alloys. It was found that the dwell fatigue effect is more significant by oxygen alloying but is suppressed by the addition of Mo. The presence of the phase did not strongly affect the dwell fatigue, however, it was suppressed at high temperature due to the low strain rate and strain rate sensitivity.
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Apr 2022
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I12-JEEP: Joint Engineering, Environmental and Processing
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Diamond Proposal Number(s):
[21103]
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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Apr 2022
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