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| 2. |
- Athanasopoulos, Stefanos, et al.
(författare)
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A novel multiscale neutron diffraction based experimental approach for granular media
- 2019
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Ingår i: Géotechnique Letters. - : Thomas Telford Ltd.. - 2045-2543. ; 9:4, s. 284-289
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Tidskriftsartikel (refereegranskat)abstract
- The complexity of granular (geo-)materials is associated with the mobility and interaction of their constituent particles. Under the effect of loading, these materials exhibit a highly inhomogeneous behaviour, which might vary significantly as the loading develops (i.e. certain grain assemblies take up the load whilst other, neighbouring assemblies fall into states of lower load). To that end, understanding the (micro-)mechanisms related to the distribution and evolution of forces/stresses through granular media requires appropriate, spatially resolved measurements. Neutron strain scanning (NSS) is an experimental technique based on diffraction measurements that has been successfully used in recent years to infer force/stress distribution in granular materials under load, by measuringthe crystallographic strains of grains. In this work, first results from a new experimental approach involving simultaneous NSS and digital image correlation (DIC) of quartz sand under load in a specially designed plane-strain apparatus are presented. The combined use of these techniques allows the investigation of deformation mechanisms in granular media, such as sand, at different scales. Therefore, a completely novel multiscale analysis of granular (geo-)materials can be made – that is, associating the traditional macroscale measurements with the mesoscale characterisation of the strain field (through DIC) and the inferred microscale stress distribution (through NSS).
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| 3. |
- Athanasopoulos, Stefanos D., et al.
(författare)
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Mapping Grain Strains in Sand Under Load Using Neutron Diffraction Scanning
- 2018
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Ingår i: Micro to MACRO Mathematical Modelling in Soil Mechanics. - Cham : Springer International Publishing. - 2297-0215 .- 2297-024X. - 9783319994734 - 9783319994741 ; , s. 23-33
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Bokkapitel (refereegranskat)abstract
- Towards the improvement of the understanding of force/stress distribution in granular media under load, a new experimental approach is suggested. Neutron diffraction, a non-conventional experimental technique, has been successfully used to map the evolution of intragranular strains in sand specimens loaded in a novel plane-strain apparatus. Representative preliminary results from recent experiments are presented, focusing on the correlation between the macro- and micro-scale response of the material, to highlight the potential of the experimental approach.
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| 5. |
- Das, Yadunandan B., et al.
(författare)
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The influence of temperature on deformation-induced martensitic transformation in 301 stainless steel
- 2018
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Ingår i: Materials Science and Technology. - : Taylor & Francis Group. - 0267-0836 .- 1743-2847. ; 34:17, s. 2114-2125
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Tidskriftsartikel (refereegranskat)abstract
- Deformation-induced martensitic transformations are increasingly being used to create desirable mechanical properties in steels. Here, the kinetics of the deformation-induced martensitic transformation is investigated at 300, 263, 223, 173 and 100 K using in situ neutron diffraction during tensile loading. The results from these experiments show a distinct change in the transformation behaviour between 300 K and the tests conducted at 263 K and below, causing a difference in martensite structure. The difference in transformation kinetics is correlated to the suppression of slip at low temperatures, as evidenced using diffraction peak intensity analysis for different grain families and corroborated using transmission electron microscopy. A direct correlation between the deformation-induced martensite fraction and the work-hardening rate is shown.
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| 7. |
- Jaladurgam, Nitesh Raj, 1993, et al.
(författare)
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Microstructure-dependent deformation behaviour of a low γ′ volume fraction Ni-base superalloy studied by in-situ neutron diffraction
- 2020
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454. ; 183, s. 182-195
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Tidskriftsartikel (refereegranskat)abstract
- Ni-base superalloys are critical materials for numerous demanding applications in the energy and aerospace sectors. Their complex chemistry and microstructure require detailed understanding of the operating deformation mechanisms and interaction between the matrix and the hardening phase during plastic deformation. Here we use in-situ neutron diffraction to show that the dependence of the deformation mechanisms and load redistribution on $\gamma^\prime$ particle size in a Ni-base superalloy with a $\gamma^\prime$ volume fraction of around $20 \%$ can exhibit distinct differences compared to their high volume fraction counterparts. In particular, the load redistribution in the coarse microstructure occurs immediately upon yielding in the present case, whereas high $\gamma^\prime$ volume fractions have been observed to initially lead to shear mediated co-deformation before work hardening allows looping to dominate and cause load partitioning at higher stresses. The fine microstructure, on the other hand, behaved similar to high volume fraction alloys, exhibiting co-deformation of the phases due to particle shearing. A recently developed elasto-plastic self-consistent (EPSC) crystal plasticity model, specifically developed for the case of coherent multi-phase materials, could reproduce experimental data with good accuracy. Furthermore, the finite strain formulation of the EPSC model allowed deformation induced texture predictions. The correct trends were predicted by the simulations, but the rate of lattice rotation was slower than experimentally observed. The insights point towards necessary model developments and improvements in order to accurately predict e.g. texture evolution during processing and effect of texture and microstructure on component properties.
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| 8. |
- Makowska, Malgorzata G., et al.
(författare)
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Effect of stress on NiO reduction in solid oxide fuel cells: a new application of energy-resolved neutron imaging
- 2015
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Ingår i: Journal of Applied Crystallography. - 1600-5767. ; 48, s. 401-408
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Tidskriftsartikel (refereegranskat)abstract
- Recently, two new phenomena linking stress field and reduction rates in anode-supported solid oxide fuel cells (SOFCs) have been demonstrated, so-called accelerated creep during reduction and reduction rate enhancement and nucleation due to stress (Frandsen et al., 2014). These complex phenomena are difficult to study and it is demonstrated here that energy-resolved neutron imaging is a feasible technique for combined mechanics-chemical composition studies of SOFC components, including commercially produced ones. Cermet anode supports, which prior to the measurements were reduced under varying conditions such as different temperatures, various times and different values of applied stress, have been measured. Thus, samples with different contents (and gradients) of Ni and NiO phases were investigated. The first Bragg edge transmission neutron measurements applied for the studies of the reduction progress in these samples were performed at two neutron beamline facilities (ISIS in the UK, Helmholtz Zentrum Berlin in Germany). The obtained results demonstrate the possibility to image and distinguish NiO and Ni phases within SOFC anode supports by energy-resolved neutron imaging and the potential of the neutron imaging method for in situ studies of reduction processes.
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| 9. |
- Pacheco, Victor
(författare)
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Processing–Structure–Properties Relationship in Metal Additive Manufacturing
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The last three decades have seen the transition of additive manufacturing, from applications exclusively in rapid-prototyping to an emerging production method in the manufacturing industry that is rapidly gaining more relevance. Within additive manufacturing methods, selective laser melting (SLM) is one of the most widely used and mature technologies and is the focus of this thesis. In particular, this work aims at characterizing novel microstructures and/or alloys produced with SLM and to understand how the process parameters influence the microstructure and properties.Hitherto, the most prevalent material selection approach for SLM has been the use and optimization of well-known alloys, such as steels, Ni- and Ti-based alloys, among others. Favorable microstructures are usually achieved with a combination of appropriate parameters and post-processing techniques. Another approach, especially interesting from a research perspective, is the exploration of materials and microstructures suited for the inherent characteristics of SLM. In alignment with the latter strategy, three types of materials are successfully produced and analyzed in this work: the amorphous Zr-based AMZ4 alloy, 316L stainless steel with strong preferential orientation (i.e., similar orientation of the crystalline structure of the grains) and the intermetallic MnAl(C) with strong preferential orientation. The latter contains a ferromagnetic phase with potential applications as a permanent magnet.SLM was found to be an effective method to produce the amorphous phase in the Zr59.3Cu28.8Al10.4Nb1.5 system (AMZ4). The laser power and oxygen impurities were found to have a central role in the formation of crystalline particles in the amorphous matrix. These crystalline particles and the oxygen impurities reduced the thermal stability of the alloy in comparison to specimens fabricated by suction casting. For the more conventional 316L stainless steel, it was demonstrated that the scan strategy can be used to influence the type of texture, with a notable effect on the mechanical properties. In the case of MnAl(C), it was established that the high temperature polymorph – ε-phase, can be retained during the printing process. This phase can be subsequently transformed to the ferromagnetic τ-phase with annealing procedures. It was observed that a strong preferred orientation of the ε-phase can be achieved, although it did not translate into a strong texture in the τ-phase (after the heat treatments). The research methodology used in this thesis and the findings regarding the processing–structure–properties relationship in SLM provide an important reference for future studies of novel materials and microstructures produced by additive manufacturing.
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| 10. |
- Yu, Cheng-Han, 1992-
(författare)
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Anisotropic mechanical behaviors and microstructural evolution of thin-walled additively manufactured metals
- 2020
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Additive manufacturing (AM), also known as 3D printing, is a concept and method of a manufacturing process that builds a three-dimensional object layer-by-layer. Opposite to the conventional subtractive manufacturing, it conquers various limitations on component design freedom and raises interest in various fields, including aerospace, automotive and medical applications. This thesis studies the mechanical behavior of thin-walled component manufactured by a common AM technique, laser powder bed fusion (LPBF). The studied material is Hastelloy X, which is a Ni-based superalloy, and it is in connection to a component repair application in gas turbines. The influence of microstructure on the deformation mechanisms at elevated temperatures is systematically investigated. This study aims for a fundamental and universal study that can apply to different material grades with FCC crystallographic structure.It is common to find elongated grain and subgrain structure caused by the directional laser energy input in the LPBF process, which is related to the different printing parameters and brands of equipment. This thesis will start with the study of scan rotation effect on stainless steel 316L in an EOS M290 equipment. The statistic texture analysis by using neutron diffraction reveals a clear transition when different level of scan rotation is applied. Scan rotation of 67° is a standard printing parameter with intention to lower anisotropy, yet, the elongated grain and cell structure is still found in the as-built microstructure. Therefore, the anisotropic mechanical behavior study is carried out on the sample printed with scan rotation of 67° in this thesis.Thin-walled effects in LPBF are investigated by studying a group of plate-like HX specimens, with different nominal thicknesses from 4mm down to 1mm, and a reference group of rod-like sample with a diameter of 18mm. A texture similar to Goss texture is found in rod-like sample, and it becomes <011>//BD fiber texture in the 4mm specimen, then it turns to be <001> fiber texture along the transverse direction (TD) in the 1mm specimen. Tensile tests with the strain rate of 10−3 s−1 have been applied to the plate-like specimens from room temperature up to 700 ℃. A degradation of strength is shown when the sample becomes thinner, which is assumed to be due to the overestimated load bearing cross-section since the as-built surface is rough. A cross-section calibration method is proposed by reducing the surface roughness, and a selection of proper roughness parameters is demonstrated with the consideration of the calculated Taylor’s factor and the residual stress. The large thermal gradient during the LPBF process induces high dislocation density and strengthens the material, hence, the LPBF HX exhibits better yield strength than conventionally manufactured, wrought HX, but the work hardening capacity and ductility are sacrificed at the same time.Two types of loading condition reveal the anisotropic mechanical behavior, where the vertical and horizontal tests refer to the loading direction being on the BD and TD respectively. The vertical tests exhibit lower strength but better ductility that is related to the larger lattice rotation observed from the samples with different deformation level. Meanwhile, the elongated grain structure and grain boundary embrittlement are responsible for the low horizontal ductility. A ductile to brittle transition is traced at 700 ℃, so a further study with two different slow strain rates, 10−5 s−1 and 10−6 s−1, are carried out at 700 ℃. Creep damage is shown in the slow strain rates testing. Deformation twinning is found only in the vertical tests where it forms mostly in the twin favorable <111> oriented grain along the LD. The large lattice rotation and the deformation twinning make the vertical ductility remain high level under the slow strain rates. The slow strain rate tensile testing lightens the understanding of creep behavior in LPBF Ni-based superalloys.In summary, this thesis uncovers the tensile behavior of LPBF HX with different variations, including geometry-dependence, temperature-dependence, crystallographic texture-dependence and strain rate-dependence. The generated knowledge will be beneficial to the future study of different mechanical behavior such as fatigue and creep, and it will also enable a more robust design for LPBF applications.
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