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Atom Probe Tomography (APT) is a powerful analytical technique for 3D characterization of materials at the atomic scale which has been widely used to study irradiation induced features. However, the accuracy and reliability of the atom probe reconstruction and post-processing methods such as cluster analysis is often neglected. In an effort to account for both of these limitations we introduce a two-step method for quantifying the quality of an atom probe reconstruction and the resulting secondary analysis.First, we developed a pointwise measure of uncertainty for APT data based on linear error propagation. This approach provide a systematic way of estimating the uncertainty in the atom positions and the most influential reconstruction parameters. The pointwise uncertainty measure can be used to assess the local quality of APT data and govern alternate reconstruction directions which minimize uncertainty. Furthermore, focusing the error analysis not on resolution but on parameter and coordinate uncertainty enables error to be propagated through complex processes such as the measurement of isotopically enriched thin films.Second, we developed a method which extends monte-carlo consensus clustering from K-based clustering algorithms to density-based clustering algorithms. In doing so a measure of relative stability is introduced to describe the ambiguity of clustering observed in an APT sample and automate the selection of the distance parameter for DBSCAN (Density-based spatial clustering of applications with noise). Our approach uses Monte-Carlo perturbation statistics, and thus could be linked to use the pointwise uncertainty established in the first part of this work, to generate alternate atom probe datasets and then apply DBSCAN to each of these datasets. In doing so the sample size for which to calculate grows in magnitude enabling more thorough post-clustering filtration methods with which to extract clusters from high-noise scenarios. We use statistical methods to analyze the results and determine the optimal DBSCAN parameters that maximize the clustering performance and minimize the uncertainty.The efficacy and utility of pointwise error propagation is demonstrated through a case study on the measurement of an isotopically enriched iron thin film while our novel clustering algorithm, Density-based Monte-Carlo Consensus Clustering (DMC3), is benchmarked against a round robin study on binary Fe-Cu systems with an emphasis on irradiation induced hardening. Our approaches provides a quantitative and objective way of assessing the quality of APT data and improving the reliability of APT data analysis.

The overarching goal of this research is to advance understanding of, model and predict embrittlement primarily due to hardening by precipitates in Reactor Pressure Vessel (RPV) steels as it pertains to 80-year life extension of fission reactors. The RPV is responsible for primary radioactive containment and pressurizing water in the nuclear reactor. The most immediate materials safety problem facing the RPV steels is neutron irradiation embrittlement, which is the reduction of fracture toughness. Embrittlement is caused by irradiation hardening (Δσy), mainly as a result of precipitation, and manifested as upward ductile-to-brittle transition temperature shifts (ΔT). The excess concentration of defects created under irradiation leads to radiation enhanced diffusion (RED), which greatly accelerates precipitation normally limited by extremely slow kinetics at ≈ 300ᴼC, or at typical reactor operating temperatures. Concomitantly, the radiation induced segregation (RIS) of solute elements leads to enrichment at microstructural features which likely plays a role in the heterogeneous nucleation of precipitates. At lower fluences (ϕt), or displacements per atom (dpa), the hardening features are nm-scale copper rich precipitates (CRPs) and solute defects vacancy complexes at trace impurity levels of > ≈ 0.06 wt% Cu. Effective Cu concentrations are less than ≈ 0.25 wt%. However, at higher ϕt much larger quantities of alloying elements precipitate to form Mn, Ni, Si nm-scale intermetallic precipitates known as MNSPs. Large volume fractions (fv) of MNSPs cause severe hardening and embrittlement. The variables controlling the formation and character of severely embrittling Mn-Ni-Si precipitates (MNSP) under neutron irradiation are neither fully understood nor explicitly treated in regulatory models. There are five primary questions addressed by this work. 1) What is the quantitative effect of temperature (Ti), fluence/dose (ϕt), flux/dose rate (ϕ), particle type, product form and alloy composition on precipitate size, number density, volume fraction, composition and magnetic character of CRPs and MNSPs? 2) Are very high Ni alloys, with high strength and toughness, irradiation tolerant at low Mn; and how does precipitation and hardening occur in steels with a much wider range of Mn, Ni and Si that conventional normalized and tempered bainitic steels?3) How can charged particle irradiations (CPI) compliment neutron data to gain insight into precipitation mechanisms?4) Are MNSPs enhanced or induced by irradiation (equilibrium versus non-equilibrium) and can this ongoing controversy by establishing their detailed character, long-time thermal stability, and formation mechanisms?5) How can these insights help build an improved high fluence, low flux predictive embrittlement model for steels used in in 80-year extended power reactor service?Atom probe tomography (APT), small angle neutron scattering (SANS) and transmission electron microscopy (TEM) show the presence of significant volume fractions of MNSPs in all RPV steels at the high 80-year fluence. Precipitation is reflected in Δσy, as characterized by tensile, shear and microhardness tests. Ni generally plays the strongest role in the formation of MNSPs. In the absence of sufficient Cu and CRPs, and at low to intermediate Ni, the MNSPs homogeneous nucleation rates are negligible in a defect free matrix. Heterogenous precipitation occurs at microstructural features, such as segregated network dislocation and irradiation induced interstitial loop sites. Units of % for compositions refers to atomic percent (%) unless otherwise noted. For typical RPV steel compositions (0.75% to 1.6% Ni with ≥ 0.8% Mn and 0.4 – 1.2% Si) MNSPs compositions are generally similar to G (Mn6Ni16Si7) and Γ2 (Mn2Ni3Si) phases. The formation of these phases is predicted by Calphad and has been confirmed by synchrotron X-Ray and TEM Diffraction measurements in a number of cases. However, in alloys with very low or very high Ni, coupled with variations in Mn and Si, other phase compositions are selected. For example, Ni-silicide type compositions are found in alloys with very low ≤ 0.24 Mn and high ≈ > 3% Ni. Notably at normal levels of > 0.8% Mn, very large MNSP fv form in 3.5% Ni steels at high fluence. The MNSP high Ni fv decreases approximately linearly with the decreasing alloy Mn content. Thus precipitation hardening is much lower in high ≈ 3.5% Ni and ≤ 0.24% Mn steels due to what is described as Mn starvation. For the so-called ATR-2 irradiation condition, which is the focus of this study, the high Ni fv ≈ 2.44 and 0.69% at 1.04 and 0.24% Mn, respectively. The resulting Δσy, which is well correlated with the √fv (as predicted by dispersed barrier hardening models), are 472 and 260 MPa.At high ϕ, a very high N and fv of MNSPs and CRP-MNSPs are observed in low Cu and Cu bearing alloys, respectively. In the latter case, Cu core Mn-Ni-Si shell CRP structures formed at lower dpa evolve into CRP-MNSP appendage co-precipitate features at high dpa. MNSP compositions formed in in rapid and convenient self-ion charged particle irradiation (CPI) are very similar to those found in neutron irradiations (NI). High dpa CPI produce fewer and larger precipitates than in NI. Further, higher dpa are needed to form the same precipitate fv for CPI versus NI conditions. The delayed precipitation is consistent with enhanced recombination of vacancies and SIA defects at the higher CPI dpa rates, which reduces the efficiency of RED. The MNSP grow slowly, but eventually reach large fv at very high dpa. Notably, fv correlates well with the G and Γ2 phase solute product, (Ni16Mn6Si7)(1/29) and (Ni3Mn2Si1)(1/6) and, at high dpa, is close to the equilibrium values, slightly modified by the Gibbs-Thomson effect. However, in steels with very low Mn and high Ni, Ni2-3Si silicide phase type precipitate compositions are observed; and when Ni is low, the precipitate compositions are close to the MnSi phase field. A comparison of dispersed barrier model predictions with measured hardening data suggests that the Ni-Si dominated precipitates are weaker dislocation obstacles than the G phase type MNSPs. Ultimately, fv from the CPI can be used to estimate y (and T) at lower service relevant dpa. While not quantitatively precise, this allows scoping studies of the embrittlement sensitivity of new RPV alloys. Post irradiation annealing (PIA) was used to clarify the irradiation induced versus enhanced controversy regarding dominant nanoscale Mn-Ni-Si precipitate (MNSP) formation mechanisms in pressure vessel steels. Radiation induced, non-equilibrium, MNSPs would dissolve under high temperature PIA, while radiation enhanced precipitates would be stable above a critical radius (rc). A Cu-free, high Ni steel was irradiated with 2.8MeV Fe2+ ions at two temperatures to generate MNSPs with average radii (r ̅) above and below an estimated rc for PIA at 425°Cup to 52 weeks. The complementary APT and Energy Dispersive X-ray Spectroscopy studies show MNSPs with r < rc dissolved, while those with r > rc slightly coarsened, consistent with thermodynamic predictions. Note, the MNSPs would be even more thermodynamically stable at much lower neutron irradiation RPV service temperatures around 290°C.Finally, the ultimate goal of this research is to create and analyze the high ϕt, intermediate ϕ ATR-2 database on both Δσy and microstructural changes in a large number of irradiated alloys. In the final section, ATR-2 results are integrated with a variety of other (mostly UCSB and surveillance) databases to develop a new high ϕt-low ϕ predictive embrittlement chemistry factor. Special emphasis is placed on the Δσy contributions of MNSPs, which are observed in a wide range of RPV steels at high ϕt. This directly informs the Odette Research Group’s development of an advanced embrittlement model which accurately predicts ΔT for low ϕ high ϕt conditions up to 80 full power years of operation.

Complex precipitation behavior of Cu-rich particles (CRPs) was investigated and simulated in continuously cooled and quench-aged super duplex stainless steel. Atom probe tomography (APT) and scanning electron microscopy showed that slow cooling resulted in nonuniform multimodal CRP precipitation and spinodal decomposition, while in the fast cooled and quench-aged conditions, more uniform precipitation of CRPs with no visible spinodal decomposition was found. Depletion of Cu, Ni, and Mn was observed in the ferrite next to the CRPs during growth, but not during dissolution. Some evidence of Ostwald ripening was seen after slow cooling, but in the quench-aged condition, particle coalescence was observed. Large CRPs disappeared next to a ferrite–austenite phase boundary after slow cooling when Cu was depleted due to the diffusion to austenite as also predicted by moving boundary Dictra simulation. Comparing Cu depleted areas next to CRPs analyzed by APT and moving boundary Dictra simulation of CRP–ferrite showed that the effective Cu diffusion coefficient during the early-stage precipitation was about 300 times higher than the Cu diffusion coefficient in ferrite at 475 °C. Using the effective diffusion coefficient and a size-dependent interfacial energy equation, CRP size distribution was successfully predicted by the Langer–Schwartz model implemented in Thermo-Calc Prisma. Applying a short aging time and continuous cooling increased the hardness and decreased the toughness values compared to the solution annealed condition. A nonuniform distribution of Cu in ferrite, the duplex structure, and partitioning of alloying elements among different phases are factors making CRP precipitation in duplex stainless steels complex.

This study deals with early stages of sigma phase growth in a high end austenitic stainless steel - Alloy 28 (EN 1.4563/UNS N08028). Its precipitation kinetics was followed by a series of heat treatments at 800 degrees C for holding times up to 30 000 s. The samples were studied with high resolution scanning electron microscopy and atom probe tomography. Detailed image analysis of the micrographs made it possible to quantify the growth rate of the precipitates. It was shown that diffusion limited growth along grain boundaries was about 15 times faster than growth perpendicular to a grain face. By combining the image data with quantitative chemical analysis of the phase boundaries, it was possible to estimate diffusion coefficients in the lattice and in the grain boundaries; grain boundary diffusion coefficients were about 250 times those of the lattice.

Funding: The Oxford Atom Probe facility is funded by EPSRC (EP/M022803/1) and the Glasgow plasma focused ion beam system was funded by EPSRC grant EP/P001483/1. Thermoelectric materials were developed under joint EPSRC grants EP/N017218/1 and EP/N01717X/1. Cu-doping and crystallographic site occupations within the half-Heusler (HH) TiNiSn, a promising thermoelectric material, have been examined by atom probe tomography. In particular, this investigation aims to better understand the influence of atom probe analysis conditions on the measured chemical composition. Under a voltage-pulsing mode, atomic planes are clearly resolved and suggest an arrangement of elements in-line with the expected HH (F-43m space group) crystal structure. The Cu dopant is also distributed uniformly throughout the bulk material. For operation under laser-pulsed modes, the returned composition is highly dependent on the selected laser energy, with high energies resulting in the measurement of excessively high absolute Ti counts at the expense of Sn and in particular Ni. High laser energies also appear to be correlated with the detection of a high fraction of partial hits, indicating nonideal evaporation behavior. The possible mechanisms for these trends are discussed, along with suggestions for optimal analysis conditions for these and similar thermoelectric materials. Postprint

Post irradiation annealing (PIA) clarified the induced versus enhanced controversy regarding nanoscale Mn-Ni-Si precipitate (MNSP) formation in pressure vessel steels. Radiation induced MNSPs would dissolve under high temperature PIA, while radiation enhanced precipitates would be stable above a critical radius (rc). A Cu-free, high Ni steel was irradiated with 2.8MeV Fe2+ ions at two temperatures to generate MNSPs with average radii ( r ¯ ) above and below an estimated rc for PIA at 425 °C up to 52 weeks. Atom probe tomography and energy dispersive x-ray spectroscopy showed MNSPs with r rc slightly coarsened, consistent with thermodynamic predictions.

This paper describes a reconstruction method for atom probe tomography based on a bottom-up approach accounting for (i) the final tip morphology (which is frequently induced by inhomogeneous evaporation probabilities across the tip surface due to laser absorption, heat diffusion effects, and inhomogeneous material properties), (ii) the limited (and changing) field of view, and (iii) the detector efficiency. The reconstruction starts from the final tip morphology and reverses the evaporation sequence through the pseudo-deposition of defined small reconstruction volumes, which are then stacked together to create the full three-dimensional (3D) tip. The subdivision in small reconstruction volumes allows the scheme to account for the changing tip shape and field of view as evaporation proceeds. Atoms within the same small reconstruction volume are reconstructed at once by placing atoms back onto their possible lattice sites through a trajectory-matching process involving simulated and experimental hit maps. As the ejected ion trajectories are simulated using detailed electrostatic modeling inside the chamber, no simplifications have been imposed on the shape of the trajectories, projection laws, or tip surface. We demonstrate the superior performance of our approach over the conventional reconstruction method (Bas) for an asymmetrical tip shape. ispartof: MICROSCOPY AND MICROANALYSIS vol:28 issue:4 pages:1102-1115 ispartof: location:England status: published

A position-energy-sensitive detector has been developed for APT instruments in order to deal with some mass peak overlap issues encountered in APT experiments. Through this new type of detector, quantitative and qualitative improvements could be considered for critical materials introducing mass peak overlaps, such as nitrogen and silicon in TiSiN systems, or titanium and carbon in cemented carbide materials. This new detector is based on a thin carbon foil positioned on the front panel of a conventional MCP-DLD detector. According to several studies, it has been demonstrated that the impact of ions on thin carbon foils has the effect of generating a number of transmitted and reflected secondary electrons that mainly depends on both the kinetic energy and the mass of incident particles. Despite the fact that this phenomenon is well known and has been widely discussed for decades, no studies have been performed to date for using it as a mean to discriminate particles energy. Therefore, this study introduces the first experiments on a potential new generation of APT detectors that would be able to resolve mass peak overlaps through the energy-sensitivity of thin carbon foils., Comment: 36 pages, 13 figures, 2 tables, conference paper

Fuel cells recombine water from H-2 and O-2 thereby can power, for example, cars or houses with no direct carbon emission. In anion-exchange membrane fuel cells (AEMFCs), to reach high power densities, operating at high pH is an alternative to using large volumes of noble metals catalysts at the cathode, where the oxygen-reduction reaction occurs. However, the sluggish kinetics of the hydrogen-oxidation reaction (HOR) hinders upscaling despite promising catalysts. Here, the authors observe an unexpected ingress of B into Pd nanocatalysts synthesized by wet-chemistry, gaining control over this B-doping, and report on its influence on the HOR activity in alkaline conditions. They rationalize their findings using ab initio calculations of both H- and OH-adsorption on B-doped Pd. Using this "impurity engineering" approach, they thus design Pt-free catalysts as required in electrochemical energy conversion devices, for example, next generations of AEMFCs, that satisfy the economic and environmental constraints, that is, reasonable operating costs and long-term stability, to enable the "hydrogen economy."

International audience; The effects of boron segregation and austenite grain size on the bainitic isothermal transformation of a high-strength steel were studied independently. Dilatometry, microstructural observations and previous atom probe tomography analyses showed that bainitic transformation rate decreases with increasing boron excess at austenite grain boundaries. We also found that boron segregation causes an unexpected grain size effect: large austenite grains transform faster than small ones. A kinetic model assuming slow nucleation at austenite triple junctions and rapid growth of bainitic nuclei successfully described our experimental data. These results confirm that bainite nucleation is inhibited by segregated boron at grain boundaries. On this basis, austenitic grain size could be optimized to make the most of boron addition in advanced high-strength steels.

International audience; The mechanisms and the sequence of crystallization of a Fe64B24Y4Nb6Al0.4 metallic glass were investigated experimentally. To this end, the microstructure of the metallic glass after interrupted isothermal treatments at a temperature between glass transition and primary crystallization was studied by complementary techniques, namely atom probe tomography, transmission electron microscopy and X-ray diffractometry. During the early stages of crystallization of the glass, some nanometric (Fe,Nb)B crystals nucleate first by rejecting Y at their interface, which hinders their growth. The Fe62B14Y3 dendritic phase starts growing in a second step. Longer annealings lead to the development of chemical gradients in the amorphous matrix surrounding Fe62B14Y3 crystals, which favors the nucleation of additional nanometric (Fe,Nb)2.4B crystals.

In an effort to understand the effect of high dose neutron irradiation on fast reactor cladding candidate materials, oxide-dispersion strengthened (ODS) alloy MA957 was irradiated to doses exceeding 100 displacements per atom (dpa) at various irradiation temperatures. The finely distributed Y-Ti-O particles, which provide MA957 its attractive properties, were examined by atom probe tomography (APT). Significant increases in oxide cluster number density and reductions in oxide cluster size were observed in specimens irradiated at 412 °C and below. A substantial hardness increase, measured by nanoindentation, was also observed at these low irradiation temperatures. It was found that the increase in oxide cluster number density, reduction in oxide cluster size, and associated increase in hardness is due to the inhibition of reformation processes of the Y-Ti-O particles following ballistic dissolution by incident radiation. Redistribution of oxide particle material along the grain boundaries is also observed at the low irradiation temperatures. The intermetallic phase α’ was observed in the low temperature samples. This observation of this phase provides additional experimental evidence for the location of the phase boundary for this low temperature precipitate. The conclusion of this work is that MA957 is microstructurally stable under neutron irradiation at and above 495 °C.

Formation of a high density of Mn-Ni-Si nanoscale precipitates in irradiated reactor pressure vessel steels could lead to severe, unexpected embrittlement, which may limit the lifetimes of our nation’s light water reactors. While the existence of these precipitates was hypothesized over 20 years ago, they are currently not included in embrittlement prediction models used by the Nuclear Regulatory Commission. This work aims to investigate the mechanisms and variables that control Mn-Ni-Si precipitate (MNSP) formation as well as correlate their formation with hardening and embrittlement.A series of RPV model steels with systematic variations in Cu and Ni contents, two variables that have been shown to have a dominant effect on hardening, were irradiated in a series of test reactor and power reactor surveillance irradiations. Atom probe tomography (APT) measurements show that large volume fractions (fv) of MNSPs form in all the steels irradiated at high fluence, even those containing no added Cu, which were previously believed to have low sensitivity to embrittlement. It is demonstrated that while Cu enhances the rate of MNSP formation, it does not appear to significantly alter their saturation fv or composition. The high fluence MNSPs have compositions consistent with known intermetallic phases in the Mn-Ni-Si system and have fv very near those predicted by equilibrium thermodynamic models. In addition, X-ray diffraction experiments by collaborators shows that these precipitates also have the expected crystal structure of the predicted Mn-Ni-Si phases. Post irradiation annealing experiments are used to measure the hardness recovery at various temperatures as well as to determine if the large fv of MNSPs that form under high fluence neutron irradiation are thermodynamically stable phases or non-equilibrium solute clusters, enhanced or induced by irradiation, respectively. Notably, while post irradiation annealing of a Cu-free, high Ni steel at 425°C results in dissolution of most precipitates, a few larger MNSPs appear to remain stable and may begin to coarsen after long times. A cluster dynamics model rationalizes the dissolution and reduction in precipitate number density, since most are less than the critical radius at the annealing temperature and decomposed matrix composition. The stability of larger precipitates suggests that they are an equilibrium phase, consistent with thermodynamic models. Charged particle irradiations using Fe3+ ions are also used to investigate the precipitates which form under irradiation. Two steels irradiated to a dose of 0.2 dpa using both neutrons and ions show precipitates with very similar compositions. The ion irradiation shows a smaller fv, likely due to the much higher dose rate, which has been previously shown to delay precipitation to higher fluences. While the precipitates in the ion irradiated condition are slightly deficient in Mn and enriched in Ni and Si compared to neutron irradiated condition, the overall similarities between the two conditions suggest that ion irradiations can be a very useful tool to study the susceptibility of a given steel to irradiation embrittlement.Finally, the large fv of MNSPs that are shown to form in all steels, including those low in Cu, at high fluence, even those without added Cu, result in large amounts of hardening and embrittlement. A preliminary embrittlement prediction model, which incorporates MNSPs at high fluence, is presented, along with results from a recent test reactor irradiation to fluences representative of extended lifetimes. This model shows very good agreement with the data.

Due to their low density, magnesium alloys are very appealing for light-weight constructions. However, the use of the most common magnesium alloy, AZ91 (Mg 9 wt.% Al, 1 wt.% Zn), is limited to temperatures below 150 °C due to creep failure. Several alloys with an improved creep resistance have been developed in the past, for example the alloy MRI 230D or Ca-alloyed AZ91 variants. However, there is an ongoing discussion in the literature regarding the mechanisms of the improved creep resistance. One factor claimed to be responsible for the improved creep resistance is the intermetallic phases which form during casting. Another possible explanation is an increased creep resistance due to the formation of precipitates. To gain more insight into the improved creep resistance of MRI 230D, nanoindentation measurements have been performed on the different phases of as-cast, creep-deformed and heat-treated samples of MRI 230D and Ca-alloyed AZ91 variants. These nanoindentation measurements clearly show that the intermetallic phase (IP) of the alloy MRI 230D does not lose strength during creep deformation in contrast to the Ca-alloyed AZ91 variants. High-temperature nanoindentation measurements performed at 200 °C clearly show that the intermetallic phases of the MRI 230D alloy maintain their strength. This is in clear contrast to the Ca-alloyed AZ91 variants, where the IP is significantly softer at 200 °C than at room temperature. Atom probe measurements have been used to gain insight into the differences in terms of chemical composition between the IPs of MRI 230D and the Ca-alloyed AZ91 variants in order to understand the dissimilar behaviour in terms of strength loss with increasing temperature.

A method based on object kinetic Monte Carlo that can account for segregation and precipitation in metallic alloys in the presence of both vacancies and self-interstitials is presented. Here the model has been applied specifically to FeCr alloys, but could be used for other alloys with proper parametrization. The model is based on the division of the simulation box into cells where only the local concentration of the alloy is considered and not the explicit location of each alloy atom, in a mean field type of approach. This concentration, together with those of neighboring cells, is used to bias defect migration taking into account the proper thermodynamics of the alloy at hand and the stiffness coefficient to include neighboring effects. The novelty of this implementation is, among other things, the explicit description of mixed pairs of point defects with the alloy element: vacancy-Cr (VCr) and self-interstitial-Cr (ICr) whenever necessary. This explicit description allows the temperature dependence of defect evolution to be correctly reproduced. In this paper we present the model in detail for the particular case of processes that take place in the presence of vacancies, to show the robustness and applicability of this method. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 under grant agreement No 633053. The views and Opinions expressed herein do not necessarily reflect those of the European Commission. This work has received funding from the Euratom research and training programme 2014–2018 under grant agreement No. 755039 (M4F project) and contributes to the EERA (European Energy Research Alliance) Joint Programme on Nuclear Materials.

International audience; P-doped 6H-SiC substrates were implanted with Fe ions. During implantation the samples were maintained at 550 °C with energies ranging from 30 to 160 keV in order to produce a diluted magnetic semiconductor (DMS) with a 2% Fe homogeneous concentration to a depth of about 100 nm thickness. The effects of rapid thermal annealing on the microstructure were examined by atom probe tomography (APT). The study shows evidence of the formation of Fe rich nanoclusters after annealing which contain core magnetic phases that contribute to the magnetic properties.

International audience; We report on the microscopic behavior of residual hydrogen on nanometric field emitters. By using homogeneous or heterostructured semiconductor specimens analyzed in a laser-assisted atom probe, it is possible to study how the relative abundances of the ionic species H+, H2+, and H3+ depend on the microscopic electric field, estimated through post-ionization statistics. In the case of Ga-containing semiconductors, the relative abundances of H+, H2+, and H3+ follow a common trend, independent of the nonmetallic component of the matrix. The dependence of the total H flux on the electric field exhibits a more complex behavior, which depends also on the spatial direction of the variation of the field (in-depth or on-surface). The analysis of multiple detection events provides further insights into surface chemistry. Noticeably, H+–H3+ ion pairs are both correlated in number and separated by very small distances on the detector space, suggesting a possible reaction 2H2 → H3++H++2e– taking place on the field emitter surface.

We investigated metal-organic vapor phase epitaxy grown (InGa)(AsSb)/GaAs/GaP Stranski–Krastanov quantum dots (QDs) with potential applications in QD-Flash memories by cross-sectional scanning tunneling microscopy (X-STM) and atom probe tomography (APT). The combination of X-STM and APT is a very powerful approach to study semiconductor heterostructures with atomic resolution, which provides detailed structural and compositional information on the system. The rather small QDs are found to be of truncated pyramid shape with a very small top facet and occur in our sample with a very high density of ∼4 × 1011 cm−2. APT experiments revealed that the QDs are GaAs rich with smaller amounts of In and Sb. Finite element (FE) simulations are performed using structural data from X-STM to calculate the lattice constant and the outward relaxation of the cleaved surface. The composition of the QDs is estimated by combining the results from X-STM and the FE simulations, yielding ∼InxGa1 − xAs1 − ySby, where x = 0.25–0.30 and y = 0.10–0.15. Noticeably, the reported composition is in good agreement with the experimental results obtained by APT, previous optical, electrical, and theoretical analysis carried out on this material system. This confirms that the InGaSb and GaAs layers involved in the QD formation have strongly intermixed. A detailed analysis of the QD capping layer shows the segregation of Sb and In from the QD layer, where both APT and X-STM show that the Sb mainly resides outside the QDs proving that Sb has mainly acted as a surfactant during the dot formation. Our structural and compositional analysis provides a valuable insight into this novel QD system and a path for further growth optimization to improve the storage time of the QD-Flash memory devices.

International audience; In ferrite/austenite dissimilar metal welds, carbon can diffuse from the ferritic low-alloy steel to the austenitic weld metal during heat treatment and thermal aging due to the large carbon and chromium content differences between the two alloys. This carbon influx leads to the precipitation of carbides in the high-alloy weld metal, near the fusion boundary, which can affect the properties of the joint. This precipitation was investigated using multiple characterization techniques: electron probe microanalysis, small angle X-ray scattering and atom probe tomography. The studied joint consisted of low-alloy steel and stainless steel base metals that were arc welded using a nickel-based filler metal. Given the heterogeneous microstructure of such joints, a microstructure navigation strategy was implemented to ensure all characterizations were carried out in the same regions of interest and the associated data could later be correlated. The precipitates were found to be mostly M23C6. The precipitation was quantified using two different methods that gave comparable results, whose differences highlight the capabilities and limitations of each technique. Precipitate fractions are higher following thermal aging but remain far from equilibrium in all cases, indicating that precipitation kinetics should be accounted for to model the microstructural evolution of such dissimilar metal welds more accurately.

International audience; Diffusion barriers play an important role in numerous phase formation processes. A well known exam- ple in microelectronics is the reactive diffusion growth of silicide thin films which are applied as contact materials. In this work, the effect of a Ti interlayer on the kinetics of the formation of CoSi by reactive diffusion is investigated. Therefore, Co(100nm)/Ti(5nm) deposited on Si(111) is annealed at various tem- peratures. Transmission electron microscopy and atom probe tomography allow to study the evolution of microstructure and local compositions after each annealing. It is observed that the CoSi growth starts at the Ti/Si interface and it is controlled by Co diffusion through the Ti interlayer. The Ti interlayer keeps its microstructure during the growth of CoSi. At higher annealing temperatures, Si diffusion through the Ti interlayer to the Co layer is evidenced. First, it segregates at grain boundaries of the polycrystalline Co layer on top of the Ti interlayer before it reacts to cobalt silicides. Also Ti diffusion to the CoSi/Si in- terface occurs that may affect diffusion processes and phase formation reaction at this interface. Beyond the experimental observations, a model is developed to quantify the diffusivity of Co in the Ti interlayer on the base of the investigated CoSi growth. Furthermore, a model is developed in order to estimate the diffusivity of Si in the Ti interlayer on the base of the formed solid solution and segregation in the Co layer. Both models can be generally applied for similar material configurations to estimate diffusivities in interlayers or to predict phase growth.

The T91 grade and similar 9Cr tempered-martensitic steels (also known as ferritic-martensitic) are leading candidate structural alloys for fast fission nuclear and fusion power reactors. At low temperatures (300–400 °C) neutron irradiation hardens and embrittles these steels, therefore it is important to investigate the origin of this mode of life limiting property degradation. T91 steel specimens were separately neutron irradiated to 2.14 dpa at 327 °C and 8.82 dpa at 377 °C in the Idaho National Laboratory Advanced Test Reactor. Atom probe tomography was used to investigate the segregation driven formation of Mn–Ni–Si-rich (MNSPs) and Cu-rich (CRP) co-precipitates. The precipitates increase in size and, slightly, in volume fraction at the higher irradiation temperature and dose, while their corresponding compositions were very similar, falling near the Si(Mn,Ni) phase field in the Mn–Ni–Si projection of the Fe-based quaternary phase diagram. While the structure of the precipitates has not been characterised, this composition range is distinctly different than that of the typically cited G-phase. The precipitates are composed of CRP with MNSP appendages. Such features are often observed in neutron irradiated reactor pressure vessel (RPV) steels. However, the Si, Ni, Mn, P and Cu solutes concentrations are lower in the T91 than in typical RPV steels. Thus, in T91 precipitation primarily takes place in solute segregated regions of line and loop dislocations. These results are consistent with the model for radiation induced segregation driven precipitation of MNSPs proposed by Ke et al. Cr-rich alpha prime (α’) phase formation was not observed.

High temperature corrosion remains a challenge for many high temperature applications. The corrosion resistance may be improved by the addition of minor amounts of reactive alloying elements. However, in the presence of corrosive species, such as alkali and chlorine, the corrosion rates are often highly accelerated. This study implements STEM/EDX and APT in order to shed light on a possible destructive minor element effect by investigating how the presence of minor amounts of corrosive species in the iron oxide formed on a low-alloyed steel may impair the corrosion resistance by increasing the oxide growth rate and possibly also reduce the oxide integrity. Graphical Abstract High temperature corrosion remains a challenge for many high temperature applications. The corrosion resistance may be improved by the addition of minor amounts of reactive alloying elements. However, in the presence of corrosive species, such as alkali and chlorine, the corrosion rates are often highly accelerated. This study implements STEM/EDX and APT in order to shed light on a parallel minor element effect by investigating how the presence of minor amounts of corrosive species in the iron oxide formed on a low-alloyed steel may impair the corrosion resistance by increasing the oxide growth rate and possibly also reduce the oxide integrity.

International audience; During ageing, the ferritic phase of Cast Austenitic Stainless Steels (CASS) undergoes decomposition by spinodal decomposition and G-phase precipitation, which leads to increasing hardness and decreasing impact toughness. Mo-bearing steels are more prone to ageing than Mo-free steels. To study the effects of Ni, Mo, and Mn, duplex model alloys were thermally aged at 350 and 400°C. The microstructure evolution during ageing was characterized by atom probe tomography. Only Ni additions seem to affect the spinodal decomposition kinetics. Ni accelerates the spinodal decomposition kinetics regardless of the presence of Mo or Mn. Additions of Mo and Mn promote the precipitation of G-phase.

The effect of surface orientations on the formation of iridium oxide species during the oxygen evolution reaction (OER) remains yet unknown. Herein, we use a needle‐shaped iridium atom probe specimen as a nanosized working electrode to ascertain the role of the surface orientations in the formation of oxide species during OER. At the beginning of electrolysis, the top 2–3 nm of (024), (026), (113), and (115) planes are covered by IrO−OH, which activates all surfaces towards OER. A thick subsurface oxide layer consisting of sub‐stoichiometric Ir−O species is formed on the open (024) planes as OER proceeds. Such metastable Ir−O species are thought to provide an additional contribution to the OER activity. Overall, this study sheds light on the importance of the morphological effects of iridium electrocatalysts for OER. It also provides an innovative approach that can directly reveal surface species on electrocatalysts at atomic scale., An approach to directly examine the oxidation state and surface composition of nanosized Ir electrodes after oxygen evolution reaction (OER) was developed. Atom probe tomography reveals compositional changes in the topmost surface and subsurface on various Ir planes during OER. This approach provides atomic‐scale insights into surface changes during electrochemical reactions.

International audience; The evolution of radiation damage in steels is a major issue for the safe operation of nuclear power plants. Mn and Ni contribution to the formation and evolution of microstructural features remains a controversial issue. The present study aims at investigating their effects on microstructure by irradiating undersaturated BCC Fe-3.3at.%Ni and Fe-2.8at.%Mn model alloys. Two different ion irradiation conditions have been applied to study the effect of irradiation dose rate in both of these Fe-based alloys at a temperature of 673 K. In all cases, irradiation promotes the formation of dislocation loops. In the FeNi alloy, radiation-induced segregation is observed on loops, leading to precipitation of a FCC phase in the low dose rate irradiation condition only. In the case of FeMn, even if no precipitation was revealed, highly enriched decorated dislocation loops are observed, with a higher Mn segregation at radiation defects than Ni.

Fe and Cr phase separation in ferrite, causing 475°C-embrittlement, was studied after very short aging times in super duplex stainless steel (SDSS) and hyper duplex stainless steel (HDSS) plates and welds. Atom probe tomography showed that hot-rolled SDSS, experiencing significant metal working, had faster kinetics of phase separations compared to the SDSS and HDSS welds after 5 min aging at 475 °C. The surface of the 33-mm SDSS plate had faster Fe and Cr phase separation and larger toughness drop. A higher density of dislocations next to the austenite phase boundary in ferrite, detected by electron channeling contrast, can promote the phase separation at the surface of the plate with lower austenite spacing. The toughness dropped in HDSS welds after aging, but SDSS welds maintained their toughness. An inverse simulation method considering an initial sinusoidal nanometric Cr and Fe fluctuation showed that Ni increases the interdiffusion of Cr in the system, resulting a higher degree of phase separation in SDSS welds than the HDSS weld. Within the composition range of the studied SDSS and HDSS materials, the processing influences the Fe and Cr phase separation more than the variation in composition during short aging or typical fabrication times. The author would like to acknowledge KK-Stiftelsen for funding of “ALWAYS project”

Advanced materials 33(39), 2102356 (2021). doi:10.1002/adma.202102356, Published by Wiley-VCH, Weinheim

In this study, the formation of a periodic microstructural pattern designed by laser powder bed fusion (LPBF), commonly called selective laser melting (SLM), of an AlSi10Mg alloy is revealed at high resolution using scanning transmission electron microscopy and atom probe tomography. Special attention is paid to the description of non-equilibrium structures and compositional fields resulting from the ultrafast cooling of the LPBF process. Observations reveal the existence of a glass state in eutectic areas, wherein short-range ordering of the diamond-Si structure is observed. The apparent very fast solidification of eutectic regions is found to involve a local strain in adjacent Al cells, which extends up to 100 nm on average. In the supersaturated aluminium solid solution retained by the LPBF process, two populations of clusters are identified, for which the potential role of the selection of hardening phases is discussed. It is proposed that the microstructure of former melt pools outside heat-affected areas is described by the repetition of a periodic microstructural pattern consisting of eutectic regions /strain-hardened Al-crystals /strain-free Al-crystals with a high density of solute-rich clusters.

Oxide dispersion strengthened (ODS) steels are leading candidates for structural materials in nuclear fission and fusion power plants. Understanding the nature of nano-oxide particles in ODS steels is vital for a better control of the microstructure and mechanical properties to further their applications. In this study, electron microscopy and atom probe tomography (APT) have been used to investigate the nanocluster features in ODS Eurofer steel. With the addition of V and Ta in ODS Eurofer, the nanoclusters exhibit a higher number density with a decreased average diameter, indicating that V and Ta are beneficial for the formation of small clusters. Irrespective of the composition of the base material, the smaller particles have a variable stoichiometry while the larger particles are likely to have Y2O3 stoichiometry. The nanoclusters were found to have a core/shell structure, where Y, O and Ta are enriched in the core and Cr and V are predominant in the shell. The formation of the complex structure is possibly the result of a competing effect between Ta, Y, V and Cr binding with O. It is deduced that Ta tends to combine with O in the core (Y2O3) of the clusters due to a higher affinity, and pushes V and Cr to the surrounding shell during the formation of nanoclusters.

The introduction of stable isotopes in the fabrication of semiconductor nanowires provides an additional degree of freedom to manipulate their basic properties, design an entirely new class of devices, and highlight subtle but important nanoscale and quantum phenomena. With this perspective, we report on phonon engineering in metal-catalyzed silicon nanowires with tailor-made isotopic compositions grown using isotopically enriched silane precursors 28SiH4, 29SiH4, and 30SiH4 with purity better than 99.9%. More specifically, isotopically mixed nanowires 28Six30Si1–x with a composition close to the highest mass disorder (x ∼ 0.5) were investigated. The effect of mass disorder on the phonon behavior was elucidated and compared to that in isotopically pure 29Si nanowires having a similar reduced mass. We found that the disorder-induced enhancement in phonon scattering in isotopically mixed nanowires is unexpectedly much more significant than in bulk crystals of close isotopic compositions. This effect is explained by a nonuniform distribution of 28Si and 30Si isotopes in the grown isotopically mixed nanowires with local compositions ranging from x = ∼0.25 to 0.70. Moreover, we also observed that upon heating, phonons in 28Six30Si1–x nanowires behave remarkably differently from those in 29Si nanowires suggesting a reduced thermal conductivity induced by mass disorder. Using Raman nanothermometry, we found that the thermal conductivity of isotopically mixed 28Six30Si1–x nanowires is ∼30% lower than that of isotopically pure 29Si nanowires in agreement with theoretical predictions., O.M. acknowledges funding from NSERC-Canada (Discovery Grants) and Canada Research Chair, Fondation de l’École Polytechnique de Montréal. The work at Keio was supported in part by the Grant-in-Aid for Scientific Research by MEXT, in part by NanoQuine, in part by FIRST, and in part by JSPS Core-to-Core Program. J.A. acknowledges the funding from the Generalitat de Catalunya 2014 SGR 1638. M.d.l.M. thanks the CSIC Jae-Predoc program. J.A. and M.d.l.M. thank funding from Spanish MINECO MAT2014-51480-ERC.

Microalloying of low carbon steel with niobium (Nb) and titanium (Ti) is standardly applied in high-strength low-alloy (HSLA) steels enabling austenite conditioning during thermo-mechanical controlled processing (TMCP), which results in pronounced grain refinement in the finished steel. The metallurgical effects of microalloying elements are related solute drag and precipitate particle pinning, both acting on the austenite grain boundary thereby delaying or suppressing recrystallization of the deformed grain. In that respect it is important to better understand the precipitation kinetics as well as the precipitation sequence in a typical Nb-Ti-microalloyed steel. Various characterization methods have been utilized in this study for tracing microalloy precipitation after simulating different austenite TMCP conditions in a Gleeble apparatus. Atom probe tomography (APT), scanning transmission electron microscopy in a focused ion beam equipped scanning electron microscope (STEM-on-FIB) and electrical resistivity measurements provide complementary information on the precipitation status and are correlated with each other. It will be demonstrated that accurate electrical resistivity measurements can monitor the general consumption of solute microalloys (Nb) during hot working which was complemented by APT measurements of the steel matrix. On the other hand, STEM revealed that a large part of Nb-containing particles during hot working are co-precipitated with titanium during cooling from the austenitizing temperature. Precipitates that form during cooling or isothermal holding can be distinguished from strain-induced precipitates by corroborating STEM measurements with APT results. APT specifically allows obtaining detailed information about the chemical composition of precipitates as well as the distribution of elements inside the particle. Electrical resistivity measurement, on the contrary, provides macroscopic information on the progress of precipitation and can be calibrated by APT. The current paper highlights the complementarity of these methods and shows first results within the framework of a larger study on strain-induced precipitation.

The fundamental understanding of quantum dot (QD) growth mechanism is essential to improve QD based optoelectronic devices. The size, shape, composition, and density of the QDs strongly influence the optoelectronic properties of the QDs. In this article, we present a detailed review on atomic-scale characterization of droplet epitaxy quantum dots by cross-sectional scanning tunneling microscopy (X-STM) and atom probe tomography (APT). We will discuss both strain-free GaAs/AlGaAs QDs and strained InAs/InP QDs grown by droplet epitaxy. The effects of various growth conditions on morphology and composition are presented. The efficiency of methods such as flushing technique is shown by comparing with conventional droplet epitaxy QDs to further gain control over QD height. A detailed characterization of etch pits in both QD systems is provided by X-STM and APT. This review presents an overview of detailed structural and compositional analysis that have assisted in improving the fabrication of QD based optoelectronic devices grown by droplet epitaxy.

Two groups of martensitic alloys were examined for changes induced by deep cryogenic treatment (DCT). The first group was a range of binary and ternary compositions with 0.6 wt % carbon, and the second group was a commercial AISI D2 tool steel. X-ray diffraction showed that DCT made two changes to the microstructure: retained austenite was transformed to martensite, and the dislocation density of the martensite was increased. This increase in dislocation density was consistent for all alloys, including those that did not undergo phase transformation during DCT. It is suggested that the increase in dislocation density may be caused by local differences in thermal expansion within the heterogeneous martensitic structure. Then, samples were tempered, and the cementite size distribution was examined using small angle neutron scattering (SANS) and atom probe tomography. First principles calculations confirmed that all magnetic scattering originated in cementite and not carbon clusters. Quantitative SANS analysis showed a measurable change in cementite size distribution for all alloys as a result of prior DCT. It is proposed that the increase in dislocation density that results from DCT modifies the cementite precipitation through enhanced diffusion rates and increased cementite nucleation sites Refereed/Peer-reviewed

International audience; A multi-microscopy investigation of a GaN tunnel junction (TJ) grown on an InGaN-based light emitting diode (LED) has been performed. The TJ consists of a heavily Ge-doped n-type GaN layer grown by ammonia-based molecular-beam epitaxy on a heavily Mg-doped p-type GaN thin layer, grown by metalorganic vapor phase epitaxy. A correlation of atom probe tomography, electron holography and secondary ion mass spectrometry has been performed in order to investigate the nm-scale distribution of both Mg and Ge at the TJ. Experimental results reveal that Mg segregates at the TJ interface, and diffuses into the Ge-doped layer. As a result, the dopant concentration and distribution differ significantly from the nominal values. Despite this, electron holography reveals a TJ depletion width of ~7 nm, in agreement with band diagram simulations using the experimentally determined dopant distribution.

Methods are presented determining the Gibbs triple junction excess ( $${\varGamma }^{\text{TJ}}$$ ) of solute segregation in polycrystalline materials from single atom counting in 3D volumes. One method bases on cumulative profile analysis, while two further methods use radial integration of solute atoms. The methods are demonstrated and compared on simulated model volumes which include three grain boundaries joining together at a triple junction with set values for Gibbs grain boundary and triple junction excess. An experimental technique that provides 3D volumes with single atom detection and spatial resolution close to atomic scale is atom probe tomography. An atom probe tomography volume of a $$\hbox {CoSi}_2$$ thin film that contains three grain boundaries and a triple junction has been acquired. Ti segregation is found qualitatively at the grain boundaries and triple junction. The quantification of the Ti excess at the investigated $$\hbox {CoSi}_2$$ triple junction reveals for the three introduced methods positive Gibbs triple junction excess values. It demonstrates that there is an excess of Ti at $$\hbox {CoSi}_2$$ triple junctions and provides opportunities for its quantification.

This study reports on the effect of interrupted quenching on the microstructure and mechanical properties of plates made of the AlZnMg(Cu) alloy AA7050. Rapid cooling from the solution heat treatment temperature is interrupted at temperatures between 100 and 200 &deg, C and continued with a very slow further cooling to room temperature. The final material&rsquo, s condition is achieved without or with subsequent artificial ageing. The results show that an improvement in the strength&ndash, toughness trade-off can be obtained by using this method. Interrupted quenching at 125 &deg, C with peak artificial ageing leads to a yield strength increase of 27 MPa (538 MPa to 565 MPa) compared to the reference material at the same fracture toughness level. A further special case is the complete omission of an artificial ageing treatment with interrupted quenching at 200 &deg, C. This heat treatment exhibits an 20% increase in fracture toughness (35 to 42 MPa m&minus, 1/2) while retaining a sufficient yield strength of 512 MPa for industrial applications. A detailed characterization of the relevant microstructural parameters like present phases, phase distribution and precipitate-free zones is performed using transmission electron microscopy and atom probe tomography.

International audience; The localization of Ti at grain boundaries after the phase formation of CoSi 2 on Si(111) from a Co-Ti bilayer is examined. For the chemical analysis at the atomic scale, atom probe tomography (APT) is applied. The structure of grain boundaries is analyzed by high resolution transmission electron microscopy (HR TEM). A polycrystalline CoSi 2 film has been formed after annealing. Inside the grains of CoSi 2 , Ti is determined to be present in low concentration of 0.2 at %. An excess of Ti is measured at the grain boundaries. The Ti excess varies for different grain boundaries. HR TEM shows different distances between defects for varying misorientation. The distances between defects are compared with theoretical misfits at the grain boundaries. A relationship between the Ti excess at grain boundaries determined by APT and the defect densities of grain boundaries analyzed by HRTEM is proposed.

Aluminium alloys and intermetallic are being widely investigated as potential aerospace materials. In this study, metal flux has been employed as a synthesize method for different intermetallic phases. The considerable potential of aluminium liquid is demonstrated as a powerful synthesis solvent of important intermetallic phases such as $ M{g_2}Si $ , $ A{l_2}Mg $ and CaMgSi. The mechanical properties of the synthesized system have been estimated through the hardness analysis using Nano-indentation hardness test. The microstructure evolution and the phase analyses were examined using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The interactions between intermetallic phases and the eutectic microstructure of molten flux is rather complex and yet to be fully understood. Thus, tracing of local chemistry on an atomic scale is crucial. The atom probe tomography technique is utilized to characterize the intermediate reaction steps of the flux-grown intermetallic phases. The study proposed a direct approach to investigate the involved reactions during the formation of the synthesized intermetallic phase.

A 10Cr creep resistant martensitic steel with 108 ppm B was normalized at 1100 °C for 1 h and air cooled. Fine (Cr,Fe)2B borides were observed on the majority of prior austenite grain boundaries, all of which were high angle boundaries, as revealed by EBSD-based reconstruction of parent austenite grains. Some high angle boundaries including twin boundaries were boride-free. Segregation of boron to austenite grain boundaries during slow cooling from 1100 °C led to boride nucleation and growth. Their size increased with decreasing cooling rate. Borides were verified by atom probe tomography, auger spectroscopy, transmission and scanning electron microscopy.

The authors acknowledge the XTMS beamline members, the Brazilian Synchrotron Light Source (LNLS) and the Metals Characterization and Processing group (CPM) at the Brazilian Nanotechnology National Laboratory (LNNano) for the support with the diffraction experiments; Villares Metals S.A. for the donation of the materials; and CNPq-SWE 02766/2014-4, FAPESP (2014/20844-1), FAPESP (2016/13466-6) for the PhD funding. APT was conducted at ORNL's Center for Nanophase Materials Sciences (CNMS), which is a U.S. DOE Office of Science User Facility. The maximization of stable reverted austenite at room temperature through inter-critical tempering is a widely used method to reduce hardness in supermartensitic stainless steels. Nevertheless, partial martensitic transformation might occur due to insufficient compositional stabilization. In this work, we conducted a time-resolved triple-step inter-critical tempering, specially designed to obtain maximum austenite stability and minimum hardness through the progressive suppression of the martensitic transformation. The mechanism behind the progressive increase in stable reverted austenite was the generation of a meta-equilibrium state, which imposed a limit in both high temperature austenite reversion and room temperature austenite stabilization. Such limit corresponded to the high temperature volume fraction of austenite, obtained right before cooling from the first cycle. This effect was associated to the Ni-rich fresh martensite laths acting as local Ni compositional pockets, providing site-specific austenite reversion; and to the suppression of any additional nucleation at the Ni-poor matrix as the T0 temperature for austenite reversion was strongly increased. The softening mechanism was mainly controlled by the carbon arrest effect by the precipitation of Ti (C, N), which was completed after the first tempering cycle. Nevertheless, maximizing reverted austenite and suppressing fresh martensite at room temperature did not result in additional hardness reductions. authorsversion published

The multiphase microstructure of carbide-free bainitic steels comprises bainitic ferrite laths, retained austenite with different morphologies, a minor fraction of carbides and so-called martensite-austenite areas, which partially transform during the last cooling step. While the other constituent received much attention, little is known about the structure of the martensite-austenite constituent in carbide-free bainitic steels. Thus, in this study, it was structurally and chemically investigated by high-resolution techniques such as transmission electron microscopy and atom probe tomography after preceded unambiguous identification by electron backscatter diffraction in conventional as well as transmission mode. The results, ranging from carbon segregation to cementite precipitation in the martensitic part, indicate strong auto-tempering during final cooling which is followed by aging. Also, some kind of structural modulation in the austenite belonging to the martensite-austenite areas was observed. Atom probe tomography revealed a heterogeneous carbon distribution, further supporting the findings by transmission electron microscopy.

Phosphorus- and boron-doped silicon nanocrystals (Si NCs) embedded in silicon oxide matrix can be fabricated by plasma-enhanced chemical vapour deposition (PECVD). Conventionally, SiH4 and N2O are used as precursor gasses, which inevitably leads to the incorporation of ≈10 atom % nitrogen, rendering the matrix a silicon oxynitride. Alternatively, SiH4 and O2 can be used, which allows for completely N-free silicon oxide. In this work, we investigate the properties of B- and P-incorporating Si NCs embedded in pure silicon oxide compared to silicon oxynitride by atom probe tomography (APT), low-temperature photoluminescence (PL), transient transmission (TT), and current–voltage (I–V) measurements. The results clearly show that no free carriers, neither from P- nor from B-doping, exist in the Si NCs, although in some configurations charge carriers can be generated by electric field ionization. The absence of free carriers in Si NCs ≤5 nm in diameter despite the presence of P- or B-atoms has severe implications for future applications of conventional impurity doping of Si in sub-10 nm technology nodes.

Advanced semiconductor devices offer a metrology challenge due to their small feature size, diverse composition and intricate structure. Atom probe tomography (APT) is an emerging technique that provides 3D compositional analysis at the atomic-scale; as such, it seems uniquely suited to meet these challenges. However, the semiconductor industry has demanding requirements against which the techniques in use are evaluated. This article explores the use of APT in the semiconductor industry, showing the potential of the technique, the obstacles that occur in practise, and possible future developments.

International audience; The early stages of precipitation of the γ ' phase in a model Co based superalloy (Co-9.1Al-7W (at.%)) have been investigated at 900 °C using electron microscopy and atom probe tomography. Nucleation, growth and coarsening stages have been studied with a focus on the temporal evolution of the precipitate composition in the light of recent theoretical developments on phase separation in multicomponent alloys. The experimental data have been confronted to the theories of nucleation and coarsening recently developed for such alloys, which are valid for non-ideal and non-dilute systems, and predict the temporal evolution of both the matrix and precipitate compositions. The rate constant for the mean size evolution of the particles, as derived from experiments, has been compared to the one predicted by the mentioned coarsening theory that accounts for a more accurate description of the thermodynamics of the phases, as compared with more classical approaches. From this comparison the γ/γ' interfacial energy was derived and found equal to ~48 mJ/m 2. the exponents for the temporal evolution of average particles size, number of particles per unit volume were found identical to those for binary alloys during the coarsening regime, as expected, and the temporal evolutions of compositions in both γ and γ' phases were found to evolve as predicted by theory. Indeed, the W content in the particles, measured from atom probe tomography (APT) experiments, was found to significantly decrease with time and the observed evolution is remarkably well described by the theory and therefore is shown to originate from the competition between diffusion and capillarity.

Hydrogen embrittlement can cause a dramatic deterioration of the mechanical properties of high-strength metallic materials. Despite decades of experimental and modelling studies, the exact underlying mechanisms behind hydrogen embrittlement remain elusive. To unlock understanding of the mechanism and thereby help mitigate the influence of hydrogen and the associated embrittlement, it is essential to examine the interactions of hydrogen with structural defects such as grain boundaries, dislocations and stacking faults. Atom probe tomography (APT) can, in principle, analyse hydrogen located specifically at such microstructural features but faces strong challenges when it comes to charging specimens with hydrogen or deuterium. Here, we describe three different workflows enabling hydrogen/deuterium charging of site-specific APT specimens: namely cathodic, plasma and gas charging. We discuss in detail the caveats of the different approaches in order to help future research efforts and facilitate further studies of hydrogen in metals. Our study demonstrates successful cathodic and gas charging, with the latter being more promising for the analysis of the high-strength steels at the core of our work.

Grain boundary engineering plays a major role for controlling the properties of modern high-performance materials. Especially Mo and its alloys have advantageous high-temperature structural properties as well as a number of attractive functional properties. However, depending on the processing state, technically pure Mo is prone to intercrystalline failure at low temperatures. The addition of B and/or C is known to improve interface cohesion, allowing for a targeted improvement of mechanical properties through segregation engineering. In this work, the early stages of crack initiation of technically pure and B micro-doped Mo are investigated by scanning electron microscopy on the tension-loaded surface after three-point-bending of mm-sized specimens. Increased grain boundary cohesion is evident from a drastically reduced relative length of separated interfaces in the B-doped material. The presence of B at the grain boundaries is confirmed via atom probe tomography experiments.

Low temperature bainite is a mixture of ferrite and austenite with a high dislocation density and nanoscale precipitates produced by isothermal transformation of the austenite in high-carbon high-silicon steels. The mass balance for carbon is systematically unsuitable when considering only ferrite and austenite forming the structure, but no attempt has been made to evaluate the amount of carbon located at linear defects and precipitates. Additionally, bainitic ferrite has been recently shown to have a tetragonal crystal structure, allowing greater amounts of carbon in solid solution than those expected by the paraequilibrium phase boundaries. In order to quantify the contribution of all the carbon sinks, we have followed the evolution of carbon in ferrite and austenite, along with the precipitation of cementite and η–carbide, during the isothermal bainitic transformation at 220 and 250 °C by means of in-situ synchrotron high energy X-ray diffraction and complementary transmission electron microscopy (TEM) and atom probe tomography (APT) analyses. This is the first time that the mass balance for carbon is successfully achieved by considering all the transformation products together with an estimation of the carbon segregated to linear defects., APT was conducted at ORNL's Center for Nanophase Materials Sciences (CNMS), which is a U.S. DOE Office of Science User Facility. The synchrotron experiments were conducted in December 2014 under the MA2305 grant at the ESRF in Grenoble which is fully acknowledged. The authors thank the support of LabEx DAMAS (ANR-11-LABX-0008-01) from Lorraine. The authors would also like to show their gratitude to Dr. J.C. Hell for his valuable assistance when conducting the experiments at ESRF. This research was supported by the Spanish Ministerio de Economia y Competitividad (MINECO) in the form of a Coordinate Project (MAT2016-80875-C3-1-R); and the Research Fund for Coal and Steel under the Contract RFSR-CT- 2014-00019.

Transmission electron microscopy (TEM), second ion mass spectrum (SIMS) and atom probe tomography (APT) techniques are used to study the Li ion distribution in the oxide formed on the rolling surface (SN) of Zircaloy-4 corroded in lithiated water with 0.01 M LiOH at 633 K/18.6 MPa. The results showed that the Li ions segregated in the grain boundaries and subgrain boundaries in the oxide film, but nearly no Li ions were found in the oxide around the interface between the oxide and matrix. Finally, we discussed the mechanism of the LiOH influence on the corrosion resistance of Zircaloy-4.

Correlation between off-axis electron holography and atom probe tomography (APT) provides morphological, chemical and electrical information about Mg doping (p-type) in gallium nitride (GaN) layers that have been grown at different temperatures at a nanometric scale. APT allows access to the three-dimensional distribution of atoms and their chemical nature. In particular, this technique allows visualisation of the Mg-rich clusters observed in p-doped GaN layers grown by metal-organic chemical vapour deposition. As the layer growth temperature increases, the cluster density decreases but their size indicted by the number of atoms increases. Moreover, APT reveals that threading dislocations are decorated with Mg atoms. Off-axis electron holography provides complementary information about the electrical activity of the Mg doping. As only a small fraction of dopant atoms are ionised at room temperature, this fraction is increased by annealing the specimen to 400 °C in situ in a transmission electron microscope (TEM). A strong reduction of the dopant electrical activity is observed for increases in the layer growth temperature. The correlation of APT with TEM-based techniques was shown to be a unique approach in order to investigate how the growth temperature affects both the chemical distribution and electrical activity of Mg dopant atoms.

The incorporation of impurities during the chemical synthesis of nanomaterials is usually uncontrolled and rarely reported because of the formidable challenge in measuring trace amounts of often light elements with sub‐nanometer spatial resolution. And yet, these foreign elements (introduced by doping, for example) influence functional properties. We demonstrate how the hydrothermal growth and a partial reduction reaction on hollow TiO2 nanowires leads to the introduction of parts per millions of boron, sodium, and nitrogen. This doping explains the presence of oxygen vacancies and reduced Ti states at the surface, which enhance the functional properties of TiO2. Our results were obtained on model metal oxide nanomaterials and they shed light on a general process that leads to the uncontrolled incorporation of trace impurities in TiO2, thereby, having a strong effect on applications in energy‐harvesting., Impure thoughts: Precise characterization of impurities is critical for understanding and enhancing the functional properties of nanomaterials. The structure–property relationships of partially reduced hollow TiO2 nanowires are characterized using scanning transmission electron microscopy and atom probe tomography. Oxygen vacancies and undesired boron, sodium, and nitrogen trace impurities are detected on the surface.