Imperial College London

ProfessorBaptisteGault

Faculty of EngineeringDepartment of Materials

Professor of Atomic-Scale Characterization
 
 
 
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Contact

 

b.gault

 
 
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Location

 

Royal School of MinesSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

439 results found

Wang Y, Chen X, Zhao H, Sun W, Zhang Q, Gault B, Hutchinson Cet al., 2024, Effect of cluster chemistry on the strengthening of Al alloys, Acta Materialia, Vol: 269, ISSN: 1359-6454

When a supersaturated Al solid solution is held at room temperature, natural ageing occurs through the formation of clusters/zones providing a strengthening of the material. Recently, it was shown that this process can be greatly amplified by applying a cyclic plastic strain to a supersatured Al alloy. The cyclic strengthening also leads to a microstructure dominated by clusters/zones but the strengths are much higher than those obtained by natural aging, and can even exceed the peak aged strength. This work uses a combination of atom probe tomography and small angle x-ray scattering to provide a quantitative comparison of the cluster states in commercial Al alloys AA2024 and AA7075, after both natural aging and cyclic strengthening. We show that the chemistry of the clusters formed by these processes is different, and the clusters formed by cyclic strengthening tend to have chemistries closer to the composition of the relevant local equilibrium precipitate. The average force required to shear an individual cluster/zone is back-calculated from the strengthening observed experimentally and a strong correlation is found between the shearing force and the solute content of the cluster (i.e. 1-XAl, where XAl is the Al content of the cluster). This correlation applies not only for AA2024 and AA7075, but also AA6111 and AA7050 data taken from the literature, as well as some precipitates formed at elevated temperatures. This work shows that the first order effect controlling cluster strengthening is simply the cluster chemistry and its proportional effect on the force required to shear a cluster.

Journal article

Chen X, Zhou X, De Geuser F, da Silva AK, Zhao H, Woods E, Liu C, Ponge D, Gault B, Raabe Det al., 2024, Atom probe tomography-assisted kinetic assessment of spinodal decomposition in an Al-12.5 at.%Zn alloy, Acta Materialia, Vol: 268, ISSN: 1359-6454

The rates of atomic clustering and precipitation hardening are closely related to the diffusivity of solutes and the concentration of vacancies during the natural aging of aluminum alloys. The measurement of the diffusivity of solutes at room temperature, especially in systems with an equilibrium vacancy concentration, is beneficial to the design of the aging process. However, this measurement has long been challenging because of the extremely low diffusion rates of solutes in aluminum at room temperature and the presence of supersaturated vacancies. In this work, we propose a method to quantify the diffusivity of solutes based on the kinetic evaluation of the spinodal decomposition process. This evaluation involves conducting atom probe tomography experiments, analyzing the radial distribution function, and modeling the phase separation process using the Cahn–Hilliard theory. The aging experiments were conducted on nanoscale samples, where excess vacancies can be eliminated at free surfaces due to a high surface-to-volume ratio. The results yielded a diffusivity of Zn in the Al-12.5 at.% Zn alloy of (1.32±0.46)×10−25 m2/s at 295 K. This work introduces a novel approach to assess the solute diffusivity under conditions of equilibrium vacancy concentration at room temperature and expands the temperature range for measuring the diffusivity in systems with spinodal decomposition, particularly in cases where kinetic data at low temperatures are scarce.

Journal article

López Freixes M, Peguet L, Warner T, Gault Bet al., 2024, Nanoscale perspective on the stress-corrosion cracking behavior of a peak-aged 7XXX-Al alloy, Corrosion Science, Vol: 229, ISSN: 0010-938X

High-strength 7xxx Al-alloys are used in aerospace and are expected to be increasingly employed in the automotive sector for “lightweighting” but they can be sensitive to stress-corrosion cracking (SCC) depending on temper and loading conditions. Here, we performed double cantilever beam (DCB) crack growth tests and study at the nanoscale stress-corrosion cracks on two compositional variants of AA7140 from different regions of a thick-plate. We report on precipitate dissolution, matrix solute enrichment near the crack tips and a modified microstructure ahead of the crack. We relate our findings with the SCC behaviour on both samples and propose an array of active mechanisms.

Journal article

Dubosq R, Camacho A, Rogowitz A, Zhang S, Gault Bet al., 2024, Influence of high-strain deformation on major element mobility in garnet: Nanoscale evidence from atom probe tomography, Journal of Metamorphic Geology, Vol: 42, Pages: 355-372, ISSN: 0263-4929

Garnet is a common rock-forming mineral that occurs in a variety of rock types and over a wide range of pressure (P)–temperature (T) conditions in the Earth's lithosphere. Because garnet is considered a high-strength mineral stable across an extensive range of conditions (1–25 GPa, <300–2000°C), it is generally accepted that garnets can retain their microstructures and chemical composition during deformation and metamorphism. Therefore, garnet is commonly used as a geothermobarometer and geochronometer to provide P–T and timing constraints on tectonic events. Herein, we study garnet from an eclogite facies mylonite (central Australia) to investigate the mechanisms of element mobility during high-strain deformation under relatively dry, lower crustal conditions. Electron backscatter diffraction (EBSD) and electron channelling contrast imaging (ECCI) reveal evidence of crystal plasticity associated with brittle deformation in the form of heterogeneous misorientation patterns and low-angle grain boundaries developed over length scales of 20–50 μm in the rims of garnet porphyroclasts. Atom probe tomography (APT) analysis of a low-angle grain boundary within a highly strained portion of a clast shows Ca enrichment and Mg depletion along dislocations, whereas APT data along the rim of a mostly undeformed clast reveal a homogeneous distribution of garnet major components in the specimen matrix with the exception of Ca, Fe and Mg enrichment within a healed microfracture. The above-mentioned results suggest that under relatively dry conditions, crystal plasticity enhances bulk element mobility via pipe diffusion, highlighting the importance of deformation-induced microstructures on element mobility, with important implications for the robust and reliable use of garnet as a petrological tool.

Journal article

Favelukis B, Chakrabartty S, Kumar V, Kim S, ElZoka A, Krämer M, Raabe D, Gault B, Eliaz N, Natan A, Sokol M, Rosen BAet al., 2024, Improved durability of Ti3C2Tz at potentials above the reversible hydrogen electrode by tantalum substitution, Advanced Functional Materials, Vol: 34, ISSN: 1616-301X

MXenes have gained significant attention, particularly Ti3C2Tz, as materials with favorable properties for energy storage and conversion applications. The overwhelming majority of electrochemical durability studies are based on durability in the hydrogen evolution window, well below the reversible hydrogen electrode where degradation via electrochemical oxidation is less relevant. Consequently, few strategies have been put forward to protect Ti3C2Tz at higher potentials and widen their applicability to electrochemical systems. Here, the electrochemical degradation of pristine Ti3C2Tz and tantalum (Ta)-substituted (Ti0.95Ta0.05)3C2Tz is reported. X-ray photoelectron spectroscopy and electron microscopy revealed that pristine and Ta-doped MXene went through entirely different degradation mechanisms, and that these mechanisms are driven by electrochemical, rather than chemical effects. Density functional theory is used to explain the role of Ta doping with respect to the binding of oxygen and the formation of metal oxide phases. The influence of the degradation mechanism is observed by accelerated stress tests and anode reversal tests on a polymer electrolyte membrane fuel cell. Therefore, the substitution of titanium (Ti) with other oxyphilic metals in Ti3C2Tz may be an effective route to improve the durability of the otherwise fragile MXene phase.

Journal article

Kim SH, Bhatt S, Schreiber DK, Neugebauer J, Freysoldt C, Gault B, Katnagallu Set al., 2024, Understanding atom probe’s analytical performance for iron oxides using correlation histograms and ab initio calculations, New Journal of Physics, Vol: 26, ISSN: 1367-2630

Field evaporation from ionic or covalently bonded materials often leads to the emission of molecular ions. The metastability of these molecular ions, particularly under the influence of the intense electrostatic field (1010Vm−1), makes them prone to dissociation with or without an exchange of energy amongst them. These processes can affect the analytical performance of atom probe tomography (APT). For instance, neutral molecules formed through dissociation may not be detected at all or with a time of flight no longer related to their mass, causing their loss from the analysis. Here, we evaluated the changes in the measured composition of FeO, Fe2O3 and Fe3O4 across a wide range of analysis conditions. Possible dissociation reactions are predicted by density-functional theory calculations considering the spin states of the molecules. The energetically favoured reactions are traced on to the multi-hit ion correlation histograms, to confirm their existence within experiments, using an automated Python-based routine. The detected reactions are carefully analyzed to reflect upon the influence of these neutrals from dissociation reactions on the performance of APT for analysing iron oxides.

Journal article

Chakraborty P, Mouton I, Gault B, Tehranchi A, Neugebauer J, Hickel Tet al., 2024, Effect of Sn on stacking fault energies in zirconium and its hydrides, Physical Review Materials, Vol: 8

Hydrogen embrittlement in Zr-alloy fuel cladding is a primary safety concern for water-based nuclear reactors. Here we investigated the stabilization of planar defects within the forming hydrides by Sn, the primary alloying element of Zircaloy-4 used in the cladding. In order to explain the formation of hydrides and planar defects observed in our experiments, we performed atomic-scale ab initio calculations focusing on the solute interactions with generalized stacking faults in hcp α-Zr and fcc zirconium hydrides. Our calculations showed that an increase in Sn concentration leads to a stabilization of stacking faults in both the α-Zr and hydride phases. However, the solution enthalpy of Sn is lower in the α-Zr as compared to the other hydride phases, indicative of two competing processes of Sn depletion/enrichment at the Zr hydride/matrix interface. This is corroborated by experimental findings, where Sn is less soluble in hydrides and is mostly found trapped at interfaces and planar defects, indicative of stacking faults inside the hydride phases. Our systematic investigation enables us to understand the presence and distribution of solutes in the hydride phases, which provides a deeper insight into the microstructural evolution of such alloy's properties during its service lifetime.

Journal article

Wang Y, Zhao H, Chen X, Gault B, Brechet Y, Hutchinson Cet al., 2024, The effect of shearable clusters and precipitates on dynamic recovery of Al alloys, Acta Materialia, Vol: 265, ISSN: 1359-6454

The formability of Al alloys is strongly influenced by their strain hardening capacity. Whilst the effect of precipitates on yield strength has been thoroughly studied, their effects on the strain hardening behaviour have been comparatively less studied. This is especially true for the case of shearable particles, such as those formed during natural ageing, or underaging. This work presents a detailed study of the effect of shearable clusters/precipitates on the room temperature dynamic recovery of 7xxx Al alloys. The dynamic recovery behaviour is characterised by the slope of the stage III hardening (β) curve in a Kocks–Mecking plot, and the cluster/precipitate state has been characterised using small angle x-ray scattering and atom probe tomography. The rate of dynamic recovery is shown to depend non-monotonically on the yield strength of the alloy. For alloys in the solution treated and quenched state, or with an extremely fine distribution of clusters, dynamic recovery becomes more difficult with increasing alloy yield strength. However, as the cluster/particle spacing increases, such as during artificial ageing, dynamic recovery becomes easier. A phenomenological model is presented showing that the critical microstructural features controlling this non-monotonic dependence of dynamic recovery on yield strength is the ratio of the cluster/precipitate spacing and the critical annihilation distance between dynamically recovering dislocations. The model is general and describes well the experimental data. It can be used as a predictive tool to guide microstructure design for combinations of yield strength and strain hardening behaviour.

Journal article

Schwarz TM, Woods E, Singh MP, Chen X, Jung C, Aota LS, Jang K, Krämer M, Kim S-H, McCarroll I, Gault Bet al., 2024, In Situ Metallic Coating of Atom Probe Specimen for Enhanced Yield, Performance, and Increased Field-of-View., Microsc Microanal

Atom probe tomography requires needle-shaped specimens with a diameter typically below 100 nm, making them both very fragile and reactive, and defects (notches at grain boundaries or precipitates) are known to affect the yield and data quality. The use of a conformal coating directly on the sharpened specimen has been proposed to increase yield and reduce background. However, to date, these coatings have been applied ex situ and mostly are not uniform. Here, we report on the controlled focused-ion beam in situ deposition of a thin metal film on specimens immediately after specimen preparation. Different metallic targets e.g. Cr were attached to a micromanipulator via a conventional lift-out method and sputtered using Ga or Xe ions. We showcase the many advantages of coating specimens from metallic to nonmetallic materials. We have identified an increase in data quality and yield, an improvement of the mass resolution, as well as an increase in the effective field-of-view. This wider field-of-view enables visualization of the entire original specimen, allowing to detect the complete surface oxide layer around the specimen. The ease of implementation of the approach makes it very attractive for generalizing its use across a very wide range of atom probe analyses.

Journal article

Singh MP, Woods EV, Kim SH, Jung C, Aota LS, Gault Bet al., 2024, Facilitating the Systematic Nanoscale Study of Battery Materials by Atom Probe Tomography through in-situ Metal Coating, Batteries and Supercaps, Vol: 7

Through its capability for 3D mapping of Li at the nanoscale, atom probe tomography (APT) is poised to play a key role in understanding the microstructural degradation of lithium-ion batteries (LIB) during successive charge- and discharge cycles. However, APT application to materials for LIB is plagued by the field induced delithiation (deintercalation) of Li-ions during the analysis itself that prevents the precise assessment of the Li distribution. Here, we showcase how a thin Cr-coating, in-situ formed on APT specimens of NMC811 in the focused-ion beam (FIB), preserves the sample's integrity and circumvent this deleterious delithiation. Cr-coated specimens demonstrated remarkable improvements in data quality and virtually eliminated premature specimen failures, allowing for more precise measurements via. improved statistics. Through improved data analysis, we reveal substantial cation fluctuations in commercial grade NMC811, including complete grains of LiMnO. The current methodology stands out for its simplicity and cost-effectiveness and is a viable approach to prepare battery cathodes and anodes for systematic APT studies.

Journal article

Krämer M, Favelukis B, El-Zoka AA, Sokol M, Rosen BA, Eliaz N, Kim S-H, Gault Bet al., 2024, Near-atomic-scale perspective on the oxidation of Ti3C2Tx MXenes: insights from atom probe tomography, Advanced Materials, Vol: 36, ISSN: 0935-9648

MXenes are a family of 2D transition metal carbides and nitrides with remarkable properties, bearing great potential for energy storage and catalysis applications. However, their oxidation behavior is not yet fully understood, and there are still open questions regarding the spatial distribution and precise quantification of surface terminations, intercalated ions, and possible uncontrolled impurities incorporated during synthesis and processing. Here, atom probe tomography (APT) analysis of as-synthesized Ti3 C2 Tx MXenes reveals the presence of alkali (Li, Na) and halogen (Cl, F) elements as well as unetched Al. Following oxidation of the colloidal solution of MXenes, it is observed that the alkalis are enriched in TiO2 nanowires. Although these elements are tolerated through the incorporation by wet chemical synthesis, they are often overlooked when the activity of these materials is considered, particularly during catalytic testing. This work demonstrates how the capability of APT to image these elements in 3D at the near-atomic scale can help to better understand the activity and degradation of MXenes, in order to guide their synthesis for superior functional properties.

Journal article

Zhao H, Yin Y, Wu Y, Zhang S, Mingers AM, Ponge D, Gault B, Rohwerder M, Raabe Det al., 2024, How solute atoms control aqueous corrosion of Al-alloys., Nat Commun, Vol: 15

Aluminum alloys play an important role in circular metallurgy due to their good recyclability and 95% energy gain when made from scrap. Their low density and high strength translate linearly to lower greenhouse gas emissions in transportation, and their excellent corrosion resistance enhances product longevity. The durability of Al alloys stems from the dense barrier oxide film strongly bonded to the surface, preventing further degradation. However, despite decades of research, the individual elemental reactions and their influence on the nanoscale characteristics of the oxide film during corrosion in multicomponent Al alloys remain unresolved questions. Here, we build up a direct correlation between the near-atomistic picture of the corrosion oxide film and the solute reactivity in the aqueous corrosion of a high-strength Al-Zn-Mg-Cu alloy. We reveal the formation of nanocrystalline Al oxide and highlight the solute partitioning between the oxide and the matrix and segregation to the internal interface. The sharp decrease in partitioning content of Mg in the peak-aged alloy emphasizes the impact of heat treatment on the oxide stability and corrosion kinetics. Through H isotopic labelling with deuterium, we provide direct evidence that the oxide acts as a trap for this element, pointing at the essential role of the Al oxide might act as a kinetic barrier in preventing H embrittlement. Our findings advance the mechanistic understanding of further improving the stability of Al oxide, guiding the design of corrosion-resistant alloys for potential applications.

Journal article

Saksena A, Sun B, Dong X, Khanchandani H, Ponge D, Gault Bet al., 2024, Optimizing site-specific specimen preparation for atom probe tomography by using hydrogen for visualizing radiation-induced damage, International Journal of Hydrogen Energy, Vol: 50, Pages: 165-174, ISSN: 0360-3199

Atom probe tomography (APT) is extensively used to measure the local chemistry of materials. Site-specific preparation via a focused ion beam (FIB) is routinely implemented to fabricate needle-shaped specimens with an end radius in the range of 50 nm. This preparation route is sometimes supplemented by transmission Kikuchi diffraction (TKD) to facilitate the positioning of a region of interest sufficiently close to the apex. Irradiating the specimen with energetic electrons and ions can lead to the generation of vacancies and even amorphization of the specimen. These extrinsically created vacancies become crucial for probing the hydrogen or deuterium distribution since they act as a strong trap. Here, we investigated the feasibility of site-specific preparation of a two-phase medium-Mn steel containing austenite (fcc) and ferrite (bcc). Following gaseous charging of APT specimens in deuterium (D2), clusters enriched by up to 35 at.% D, are imaged after Pt deposition, conventional Ga-FIB preparation, and TKD conducted separately. These D-rich clusters are assumed to arise from the agglomeration of vacancies acting as strong traps. By systematically eliminating these preparation-induced damages, we finally introduce a workflow allowing for studying intrinsic traps for H/D inherent to the material.

Journal article

Adabifiroozjaei E, Maccari F, Schäfer L, Jiang T, Recalde-Benitez O, Chirkova A, Shayanfar N, Dirba I, Kani NA, Shuleshova O, Winkler R, Zintler A, Rao Z, Pfeuffer L, Kovács A, Dunin-Borkowski RE, Skokov K, Gault B, Gruner M, Gutfleisch O, Molina-Luna Let al., 2024, Martensitic phase transformation in short-range ordered Fe<inf>50</inf>Rh<inf>50</inf> system induced by thermal stress and mechanical deformation, Acta Materialia, Vol: 264, ISSN: 1359-6454

Metallic/intermetallic materials with BCC structures hold an intrinsic instability due to phonon softening along [110] direction, causing BCC to lower-symmetry phases transformation when the BCC structures are thermally or mechanically stressed. Fe50Rh50 binary system is one of the exceptional BCC structures (ordered-B2) that has not been yet showing such transformation upon application of thermal stress, although mechanical deformation results in B2 to disordered FCC (γ) and L10 phases transformation. Here, a comprehensive transmission electron microscopy (TEM) study is conducted on thermally-stressed samples of Fe50Rh50 induced by quenching in water and liquid nitrogen from 1150 °C and 1250 °C. We demonstrated that samples quenched from 1150 °C into water and liquid nitrogen show the presence of 1/4{110} and 1/2{110} satellite reflections, the latter of which is expected from phonon dispersion curves obtained by density functional theory calculation. Therefore, it is proposed that Fe50Rh50 maintains the B2 structure that is in premartensite state. Once Fe50Rh50 is quenched from 1250 °C into liquid nitrogen, formation of two short-range ordered tetragonal phases with various c/a ratios (∼1.15 and 1.4) is observed in line with phases formed from mechanically deformed (30 %) sample. According to our observations, an accurate atomistic shear model ({110}〈11¯0〉) is presented that describes the martensitic transformation of B2 to these tetragonal phases.

Journal article

Yan K, Xu Y, Niu J, Wu Y, Li Y, Gault B, Zhao S, Wang X, Li Y, Wang J, Skokov KP, Gutfleisch O, Wu H, Jiang D, He Y, Jiang Cet al., 2024, Unraveling the origin of local chemical ordering in Fe-based solid-solutions, Acta Materialia, Vol: 264, ISSN: 1359-6454

Local chemical ordering (LCO) can exert pronounced effects on both structural and functional properties, tailoring LCO domains at (sub-)nanoscale could offer an alternative material-design concept for yet unexplored performance. However, the origin of LCO remains an open question, making accurate manipulation of LCO extremely challenging. Here we selected the Fe-Ga magnetostrictive materials and demonstrated that LCO tetragonal structures play a significant role in optimizing the magnetostrictive properties. The “full-lifecycle”, including formation, evolution and dissolution of LCO, is concretely studied from the atomic-scale up by combined experimental and theoretical studies. The dynamic precipitation and dissolution processes of LCO L60 domains during isothermal aging are directly observed based on in-situ high-resolution transmission electron microscopy images, and the corresponding mechanisms are revealed by first-principles calculation. Based on the results, we evidence that LCO domain is a frozen-intermediate-state of a kinetically-slow solid-state phase transformation leading to the formation of the long-range-ordered equilibrium phase with a face-center-cubic structure. We confirm the reversibility of LCO during cycling treatments. Our findings shed light on the origin of LCO in a range of material systems, and we discuss directions for developing materials with superior performance by manipulating LCO domains.

Journal article

Woods EV, Singh MP, Kim S-H, Schwarz TM, Douglas JO, El-Zoka AA, Giulani F, Gault Bet al., 2023, A versatile and reproducible cryo-sample preparation methodology for atom probe studies, Microscopy and Microanalysis, Vol: 29, Pages: 1992-2003, ISSN: 1083-0375

Repeatable and reliable site-specific preparation of specimens for atom probe tomography (APT) at cryogenic temperatures has proven challenging. A generalized workflow is required for cryogenic specimen preparation including lift-out via focused ion beam and in situ deposition of capping layers, to strengthen specimens that will be exposed to high electric field and stresses during field evaporation in APT and protect them from environment during transfer into the atom probe. Here, we build on existing protocols and showcase preparation and analysis of a variety of metals, oxides, and supported frozen liquids and battery materials. We demonstrate reliable in situ deposition of a metallic capping layer that significantly improves the atom probe data quality for challenging material systems, particularly battery cathode materials which are subjected to delithiation during the atom probe analysis itself. Our workflow design is versatile and transferable widely to other instruments.

Journal article

Woods EV, Kim S-H, El-Zoka AA, Stephenson LT, Gault Bet al., 2023, Scalable substrate development for aqueous sample preparation for atom probe tomography, Journal of Microscopy, ISSN: 0022-2720

Reliable and consistent preparation of atom probe tomography (APT) specimens from aqueous and hydrated biological specimens remains a significant challenge. One particularly difficult process step is the use of a focused ion beam (FIB) instrument for preparing the required needle-shaped specimen, typically involving a 'lift-out' procedure of a small sample of material. Here, two alternative substrate designs are introduced that enable using FIB only for sharpening, along with example APT datasets. The first design is a laser-cut FIB-style half-grid close to those used for transmission electron microscopy (TEM) that can be used in a grid holder compatible with APT pucks. The second design is a larger, standalone self-supporting substrate called a 'crown', with several specimen positions, which self-aligns in APT pucks, prepared by electrical discharge machining (EDM). Both designs are made nanoporous, to provide strength to the liquid-substrate interface, using chemical and vacuum dealloying. Alpha brass, a simple, widely available, lower-cost alternative to previously proposed substrates, was selected for this work. The resulting designs and APT data are presented and suggestions are provided to help drive wider community adoption.

Journal article

Jenkins BM, Haley J, Chen L, Gault B, Burr PA, Callow A, Moody MP, Grovenor CRMet al., 2023, Experimental and modelling evidence for hydrogen trapping at a β-Nb second phase particle and Nb-rich nanoclusters in neutron-irradiated low Sn ZIRLO, Journal of Nuclear Materials, Vol: 587, ISSN: 0022-3115

Zirconium-based alloys used for fuel cladding in nuclear fission reactors are susceptible to hydrogen embrittlement during operation, but we currently lack the necessary mechanistic understanding of how hydrogen behaves in the materials during service to properly address this issue. Imaging the distribution of hydrogen within material microstructures is key to creating or validating models that predict the behaviour and influence of hydrogen on material properties, but is experimentally difficult. Studying hydrogen in zirconium-alloys is further complicated by the fact that the most common routes for preparing specimens for Transmission Electron Microscopy and Atom Probe Tomography (APT) analysis, electropolishing and focused ion beam (FIB) milling, are known to induce hydride formation. This introduces uncertainty as to whether the hydrogen distribution in the analysed specimen is actually representative of the entire sample a priori. Recent work has shown that this effect can be mitigated by performing the final specimen thinning stages at cryogenic temperatures. In this paper we use cryo-FIB to prepare APT specimens of neutron-irradiated low Sn ZIRLO, showing that hydrogen is trapped within a β-Nb SPP and at Nb-rich nanoclusters formed by exposure to neutron irradiation. We then use density functional theory calculations to explain these experimental observations. These results highlight the importance of including niobium-rich features in models used to predict hydrogen pick-up in zirconium alloys during service and delayed hydride cracking during storage.

Journal article

Josten N, Franzka S, Rao Z, Smoliarova T, Kovács A, Scheibel F, Staab F, Acet M, Çaklr A, Durst K, Gault B, Dunin-Borkowski RE, Gutfleisch O, Farle Met al., 2023, Location and morphology of ferromagnetic precipitates in Ni-Mn-Sn, Physical Review Materials, Vol: 7

Ni50Mn45Sn05 heated above 600 K decomposes into ferromagnetic Ni2MnSn precipitates in an antiferromagnetic NiMn matrix. If an external magnetic field is applied during annealing, magnetic hysteresis curves with high coercive fields of up to 5 T can be achieved. The origin of this hysteresis has been attributed to the coupling of the antiferromagnetic matrix with the ferromagnetic precipitates, whose location and morphology were not known. To close this knowledge gap, four samples with varying annealing treatments were investigated using switching magnetization magnetic force microscopy. One sample was additionally analyzed with transmission electron microscopy and atom probe tomography. The decomposition type is identified to be a cellular precipitation starting at grain boundaries and growing into the grains. This leads to a multilayer thin film like lamellar structure with a lamella thickness in the nm range. Our results provide a basis for understanding the magnetic interactions, which lead to the magnetic hysteresis with ultra high coercivity.

Journal article

Freixes ML, Zhou X, Aymerich-Armengol R, Vega-Paredes M, Peguet L, Warner T, Gault Bet al., 2023, Crack arrest markings in stress corrosion cracking of 7xxx aluminium alloys: Insights into active hydrogen embrittlement mechanisms, SCRIPTA MATERIALIA, Vol: 237, ISSN: 1359-6462

Journal article

Rivas NAR, Manjon AG, Vega-Paredes M, Kim S-H, Gault B, Jun H, Jung C, Berova V, Hengge K, Jurzinsky T, Scheu Cet al., 2023, Chemistry and microstructure of C-supported Ru catalyst nanoparticles: A correlative study, ULTRAMICROSCOPY, Vol: 254, ISSN: 0304-3991

Journal article

Kim SH, Jun H, Jang K, Choi PP, Gault B, Jung Cet al., 2023, Exploring the Surface Segregation of Rh Dopants in PtNi Nanoparticles through Atom Probe Tomography Analysis, Journal of Physical Chemistry C, Vol: 127, Pages: 22721-22725, ISSN: 1932-7447

Proton-exchange membrane fuel cells hold promise as energy conversion devices for hydrogen-based power generation and storage. However, the slow kinetics of oxygen reduction at the cathode imposes the need for highly active catalysts, typically Pt or Pt based, with a large available area. The scarcity of Pt increases the deployment and operational cost, driving the development of novel highly active material systems. As an alternative, a Rh-doped PtNi nanoparticle has been suggested as a promising oxygen reduction catalyst, but the three-dimensional distributions of constituent elements in the nanoparticles have remained unclear, making it difficult to guide property optimization. Here, a combination of advanced microscopy and microanalysis techniques is used to study the Rh distribution in the PtNi nanoparticles, and Rh surface segregation is revealed, even with an overall Rh content below 2 at. %. Our findings suggest that doping and surface chemistry must be carefully investigated to establish a clear link with catalytic activity that can truly be established.

Journal article

Li Y, Wei Y, Wang Z, Liu X, Colnaghi T, Han L, Rao Z, Zhou X, Huber L, Dsouza R, Gong Y, Neugebauer J, Marek A, Rampp M, Bauer S, Li H, Baker I, Stephenson LT, Gault Bet al., 2023, Quantitative three-dimensional imaging of chemical short-range order via machine learning enhanced atom probe tomography., Nat Commun, Vol: 14

Chemical short-range order (CSRO) refers to atoms of specific elements self-organising within a disordered crystalline matrix to form particular atomic neighbourhoods. CSRO is typically characterized indirectly, using volume-averaged or through projection microscopy techniques that fail to capture the three-dimensional atomistic architectures. Here, we present a machine-learning enhanced approach to break the inherent resolution limits of atom probe tomography enabling three-dimensional imaging of multiple CSROs. We showcase our approach by addressing a long-standing question encountered in body-centred-cubic Fe-Al alloys that see anomalous property changes upon heat treatment. We use it to evidence non-statistical B2-CSRO instead of the generally-expected D03-CSRO. We introduce quantitative correlations among annealing temperature, CSRO, and nano-hardness and electrical resistivity. Our approach is further validated on modified D03-CSRO detected in Fe-Ga. The proposed strategy can be generally employed to investigate short/medium/long-range ordering phenomena in different materials and help design future high-performance materials.

Journal article

Buttard M, Freixes ML, Josserond C, Donnadieu P, Chéhab B, Blandin JJ, Gault B, De Geuser F, Martin Get al., 2023, Ageing response and strengthening mechanisms in a new Al-Mn-Ni-Cu-Zr alloy designed for laser powder bed fusion, Acta Materialia, Vol: 259, ISSN: 1359-6454

Aluminum alloys designed for laser powder bed fusion (L-PBF) often show a bimodal grain structure and a strong out-of-equilibrium character with heterogeneities developing at scales ranging from the melt pool, i.e. several hundred microns, down to sub-nanometer. When subjected to post-fabrication heat treatments, microstructural evolutions arise at all scales. Herein are established the relationships between microstructure and mechanical properties at room temperature of a novel Al-4Mn-3Ni-2Cu-1Zr alloy, designed for L-PBF and subjected to direct ageing. On the basis of a multiscale microstructural study using scanning-electron microscopy (SEM), automated orientation mapping in the transmission-electron microscope (TEM-ACOM), atom probe tomography (APT), and synchrotron small-angle X-ray scattering (SAXS), we discuss and weigh the role of multiple strengthening mechanisms to the high strength of the material. In the stress-relieved conditions (300 °C/4 h), the yield strength is about 320 MPa and solid solution strengthening accounts for nearly two third of the yield strength (∼200 MPa) thanks to a very high content of Mn retained in solid solution (> 1.5 at.%) and, to a lesser extent, grain boundary strengthening. After ageing at 400 °C/1 h, the yield strength reaches 410 MPa. The additional contribution is brought by precipitation strengthening by L12-ordered Al3Zr, and to a lesser extent, Mn-rich precipitates. The composite effect due to the large fraction of relatively fine (< 1 μm) intermetallic particles (∼20%) is highlighted and cannot be neglected. This work provides guidelines to further optimize the mechanical properties and thermal stability of Al-alloys designed for L-PBF.

Journal article

Tehranchi A, Chakraborty P, López Freixes M, McEniry EJ, Gault B, Hickel T, Neugebauer Jet al., 2023, Tailoring negative pressure by crystal defects: Microcrack induced hydride formation in Al alloys, Physical Review Materials, Vol: 7

Climate change motivates the search for non-carbon-emitting energy generation and storage solutions. Metal hydrides show promising characteristics for this purpose. They can be further stabilized by tailoring the negative pressure of microstructural and structural defects. Using systematic ab initio and atomistic simulations, we demonstrate that an enhancement in the formation of hydrides at the negatively pressurized tip region of the microcrack is feasible by increasing the mechanical tensile load on the specimen. The theoretical predictions have been used to reassess and interpret atom probe tomography experiments for a high-strength 7XXX-aluminium alloy that show a substantial enhancement of hydrogen concentration at structural defects near a stress-corrosion crack tip. These results contain important implications for enhancing the capability of metals as H-storage materials.

Journal article

Saxena A, Polin N, Kusampudi N, Katnagallu S, Molina-Luna L, Gutfleisch O, Berkels B, Gault B, Neugebauer J, Freysoldt Cet al., 2023, A Machine Learning Framework for Quantifying Chemical Segregation and Microstructural Features in Atom Probe Tomography Data., Microsc Microanal, Vol: 29, Pages: 1658-1670

Atom probe tomography (APT) is ideally suited to characterize and understand the interplay of segregation and microstructure in modern multi-component materials. Yet, the quantitative analysis typically relies on human expertise to define regions of interest. We introduce a computationally efficient, multi-stage machine learning strategy to identify compositionally distinct domains in a semi-automated way, and subsequently quantify their geometric and compositional characteristics. In our algorithmic pipeline, we first coarse-grain the APT data into voxels, collect the composition statistics, and decompose it via clustering in composition space. The composition classification then enables the real-space segmentation via a density-based clustering algorithm, thus revealing the microstructure at voxel resolution. Our approach is demonstrated for a Sm-(Co,Fe)-Zr-Cu alloy. The alloy exhibits two precipitate phases with a plate-like, but intertwined morphology. The primary segmentation is further refined to disentangle these geometrically complex precipitates into individual plate-like parts by an unsupervised approach based on principle component analysis, or a U-Net-based semantic segmentation trained on the former. Following the composition and geometric analysis, detailed composition distribution and segregation effects relative to the predominant plate-like geometry can be readily mapped from the point cloud, without resorting to the voxel compositions.

Journal article

Dubosq R, Schneider DA, Camacho A, Gault Bet al., 2023, Strain hardening induced by crystal plasticity: A new mechanism for brittle failure in garnets, EARTH AND PLANETARY SCIENCE LETTERS, Vol: 617, ISSN: 0012-821X

Journal article

Rao Z, Li Y, Zhang H, Colnaghi T, Marek A, Rampp M, Gault Bet al., 2023, Direct recognition of crystal structures via three-dimensional convolutional neural networks with high accuracy and tolerance to random displacements and missing atoms, SCRIPTA MATERIALIA, Vol: 234, ISSN: 1359-6462

Journal article

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