429 research outputs found
Characterisation of corrosion damage in T91/F91 steel exposed to static liquid lead-bismuth eutectic at 700–715 °C
Precipitation of the ordered α2 phase in a near-α titanium alloy
Precipitate evolution in a near-α alloy was studied using transmission electron microscopy (TEM) and correlative atom probe tomography (APT) after ageing at 550-700 for times up to 28 days. It is found that precipitation occurs much faster and is more prolific in samples heat treated at higher temperatures. Particles were spherical after ageing at 550 °C, while after ageing at 700 °C they become ellipsoids with the major axis lying close to the [0001] direction. At longer ageing times, the α2 precipitates were found to contain greater amounts of Sn + Si, indicating that Sn and Si are stronger Ti3(Al,Sn,Si) formers than Al
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Point excess solute: A new metric for quantifying solute segregation in atom probe tomography datasets including application to naturally aged solute clusters in Al-Mg-Si-(Cu) alloys
Data availability: The raw and processed data required to reproduce these findings cannot be shared at this time due to legal reasons.Supplementary data are available online at: https://www.sciencedirect.com/science/article/pii/S1044580323007611#s0060 .Copyright © 2023 The Authors. Accurate, repeatable and quantitative analysis of nanoscale solute clustering in atom probe tomography (APT) datasets is a complex challenge which is made more difficult by the positional uncertainty and lack of absolute resolution inherent to the technique. In this work a new method, the point excess solute, is introduced for quantifying solute segregation in datasets with limited spatial resolution. This new method is based on measuring the matrix concentration using a dataset sampling method. We show the new method can accurately reproduce the values expected from synthetic datasets a priori and when the dataset spatial resolution and or phase contrast is too low for accurate quantification this is observable. The method is then applied to naturally aged solute clusters in the Al-Mg-Si-Cu system. Datasets were collected with a range of natural ageing times from 8 min to 76 weeks. The formation of the solute clusters is shown to be unaffected by the Cu content of the alloy.The authors would like to thank Constellium for providing the materials and financially supporting this research alongside the Engineering and Physical Science Research Council (EPSRC) through studentship 1922133. The authors are grateful to EPSRC for funding of the LEAP 5000XR for the UK National Atom Probe Facility through grant EP/M022803/1
Indium clustering in a -plane InGaN quantum wells as evidenced by atom probe tomography
Atom probe tomography (APT) has been used to characterize the distribution of In atoms within non-polar a-plane InGaN quantum wells (QWs) grown on a GaN pseudo-substrate produced using epitaxial lateral overgrowth. Application of the focused ion beam microscope enabled APT needles to be prepared from the low defect density regions of the grown sample. A complementary analysis was also undertaken on QWs having comparable In contents grown on polar c-plane sample pseudo-substrates. Both frequency distribution and modified nearest neighbor analyses indicate a statistically non-randomized In distribution in the a-plane QWs, but a random distribution in the c-plane QWs. This work not only provides insights into the structure of non-polar a-plane QWs but also shows that APT is capable of detecting as-grown nanoscale clustering in InGaN and thus validates the reliability of earlier APT analyses of the In distribution in c-plane InGaN QWs which show no such clustering.The European Research Council has provided financial
support under the European Community’s Seventh Framework
Programme (FP7/2007-2013)/ERC Grant Agreement No.
279361 (MACONS). This work was also funded in part by
the EPSRC (Grant Nos. EP/H047816/1, EP/H0495331 and
EP/J003603/1).This is the author accepted manuscript. The final version is available via AIP at http://scitation.aip.org/content/aip/journal/apl/106/7/10.1063/1.4909514
The effect of boron on oxide scale formation in a new polycrystalline superalloy
Boron addition to a new polycrystalline nickel-based superalloy promotes the formation of a hitherto unreported aluminoborate phase, in the scale produced by oxidation in air at 900 °C for ~ 10,000 h. Atom probe tomography provides unambiguous confirmation of this. The ramifications of its formation are discussed
Neutron cryo-crystallography captures the protonation state of ferryl heme in a peroxidase
Heme enzymes activate oxygen through formation of transient iron-oxo (ferryl) intermediates of the heme iron. A long-standing question has been the nature of the iron-oxygen bond and, in particular, the protonation state. We present neutron structures of the ferric derivative of cytochrome c peroxidase and its ferryl intermediate; these allow direct visualization of protonation states. We demonstrate that the ferryl heme is an Fe(IV)=O species and is not protonated. Comparison of the structures shows that the distal histidine becomes protonated on formation of the ferryl intermediate, which has implications for the understanding of O–O bond cleavage in heme enzymes. The structures highlight the advantages of neutron cryo-crystallography in probing reaction mechanisms and visualizing protonation states in enzyme intermediates
Analyzing Ideological Communities in Congressional Voting Networks
We here study the behavior of political party members aiming at identifying
how ideological communities are created and evolve over time in diverse
(fragmented and non-fragmented) party systems. Using public voting data of both
Brazil and the US, we propose a methodology to identify and characterize
ideological communities, their member polarization, and how such communities
evolve over time, covering a 15-year period. Our results reveal very distinct
patterns across the two case studies, in terms of both structural and dynamic
properties
A nexus between 3D atomistic data hybrids derived from atom probe microscopy and computational materials science: a new analysis of solute clustering in Al-alloys
Solute clusters affect the physical properties of alloys. Knowledge of the atomic structure of solute clusters is a prerequisite for material optimisation. In this study, solute clusters in a rapid-hardening Al-Cu-Mg alloy were characterised by a combination of atom probe tomography and density functional theory, making use of a hybrid data type that combines lattice rectification and data completion to directly input experimental data into atomistic simulations. The clusters input to the atomistic simulations are thus observed experimentally, reducing the number of possible configurations. Our results show that spheroidal, compact clusters are more energetically favourable and more abundant
Direct observation of local K variation and its correlation to electronic inhomogeneity in (Ba1-xKx)Fe2As2 Pnictide
Local fluctuations in the distribution of dopant atoms are a suspected cause
of nanoscale electronic disorder or phase separation observed within the
pnictide superconductors. Atom probe tomography results present the first
direct observations of dopant nano-clustering in a K-doped 122-phase pnictides.
First-principles calculations suggest the coexistence of static magnetism and
superconductivity on a lattice parameter length scale over a large range of
doping concentrations. Collectively, our results provide evidence for a mixed
scenario of phase coexistence and phase separation originating from variation
of dopant atom experiments distroibutions.Comment: 4 pages, 4 figures and 1 table, accepted by Physical Review Letter
201
Insight into the impact of atomic- and nano-scale indium distributions on the optical properties of InGaN/GaN quantum well structures grown on m -plane freestanding GaN substrates
We investigate the atomic scale structure of m-plane InGaN quantum wells grown on bulk m-plane GaN templates and reveal that as the indium content increases there is an increased tendency for non-random clustering of indium atoms to occur. Based on the atom probe tomography data used to reveal this clustering, we develop a k.p model that takes these features into account, and links the observed nanostructure to the optical properties of the quantum wells. The calculations show that electrons and holes tend to co-localise at indium clusters. The transition energies between the electron and hole states are strongly affected by the shape and size of the clusters. Hence, clustering contributes to the very large line widths observed in the experimental low temperature photoluminescence spectra. Also, the emission from m-plane InGaN quantum wells is strongly linearly polarised. Clustering does not alter the theoretically predicted polarisation properties, even when the shape of the cluster is strongly asymmetric. Overall, however, we show that the presence of clustering does impact the optical properties, illustrating the importance of careful characterisation of the nanoscale structure of m-plane InGaN quantum wells and that atom probe tomography is a useful and important tool to address this problem
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