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The investigation of Li containing materials is of crucial importance for a number of material applications, including aerospace engineering. The aluminum alloy (AA) 2050, containing 2.7–5.0 at.% of Li, exhibits a lower density as well as excellent mechanical properties, due to the variety of phases that can form, including metastable ones. The high chemical composition sensitivity and near-atomic spatial resolution in three-dimensions of atom probe tomography make it a powerful technique to accurately assess solute partitioning and phase composition in particular at structural defects such as dislocations and grain boundaries. However, Li quantification is not straightforward due to its fast diffusion. Here, specimen preparation and Li electric field driven migration during atom probe analysis are investigated separately to better address the challenges associated with the experimental investigation of Li content in Li containing Al-alloys. We demonstrate a striking effect of Ga on Li distribution, which turns out to be significantly more critical than the electric field induced migration of Li during the atom probe tomography analysis. • Ga+ ion implantation totally depletes microstructural defects from Li. • Laser-induced atomic transport does not induce GB in-situ delithiation in AA2050. • Electric-field assisted migration enriches further Li-rich GBs. [ABSTRACT FROM AUTHOR]

Highlights: [•] Overview/opinion on the topic of spatial reconstruction for atom probe tomography is provided. [•] The standard reconstruction methodology is discussed, including limitations and their causes. [•] Computational methods to simulate field evaporation processes are reviewed. [•] Reconstruction issues in the far and near field are reviewed and discussed. [ABSTRACT FROM AUTHOR]

Abstract: In the atom probe microanalysis of steels, inconsistencies in the measured compositions of solutes (C, N) have often been reported, as well as their appearance as molecular ions. Here we propose that these issues might arise from surface migration of solute atoms over the specimen surface. Surface migration of solutes is evidenced by field-ion microscopy observations, and its consequences on atom probe microanalysis are detailed for a wide range of solute (P, Si, Mn, B, C and N). It is proposed that directional walk driven by field gradients over the specimen surface and thermally activated is the prominent effect. [Copyright &y& Elsevier]

Abstract: State-of-the art atom probe tomography (APT) combined with transmission electron microscopy (TEM) were used to investigate the microstructure at different stages of the ageing process of an alloy of composition (at%) Al–1.68%Cu–4.62%Li–0.33%Mg–0.1%Ag. These alloys were shown to exhibit a complex microstructure of T1 plates and several metastable phases, including θ′ and S. We will highlight the early stages of clustering, precipitate interactions and possible solute segregation at the matrix/precipitate interfaces and detail the chemical composition of the different phases. [Copyright &y& Elsevier]

Abstract: Key to the integrity of atom probe microanalysis, the tomographic reconstruction is built atom by atom following a simplistic protocol established for previous generations of instruments. In this paper, after a short review of the main reconstruction protocols, we describe recent improvements originating from the use of exact formulae enabling significant reduction of spatial distortions, especially near the edges of the reconstruction. We also show how predictive values for the reconstruction parameters can be derived from electrostatic simulations, and finally introduce parameters varying throughout the analysis. [Copyright &y& Elsevier]

Abstract: In the interaction between ultrafast laser pulses and a field emitter both optical and thermal processes are involved. In this paper, these physical process, and their timescales, are experimentally explored. Simple models are proposed to explain the observed experimental behaviour, and the influence of various parameters are investigated. In the case of optical processes, it is shown that the optical field is greatly enhanced at the tip apex, and that field evaporation could be induced by an optical non-linear effect called optical rectification. In the case of thermal processes, it is shown that the temperature rise because of light absorption can be determined and that the cooling process of the tip surface can be studied by pump probe measurements. [Copyright &y& Elsevier]

Partitioning of C during tempering of quenched Fe-9.6Ni-0.5C-0.6Mn-0.6Mo-0.7Cr-0.1V (at.%) steel is determined by atom probe tomography and the resulting microstructure is described. The precipitated austenite size, together with its C and Ni content control its thermal stability and these can vary differently with tempering time and temperature. Thus, both austenite and strong carbide formers compete for the available C early in the process. Due to widely different transport kinetics, C likely plays a dominant role early but is either fully consumed or its role diminishes by dilution, and Ni partitioning eventually takes over as the austenite stability-controlling species. Unlabelled Image [ABSTRACT FROM AUTHOR]

Electron microscopy and atom probe tomography were combined to investigate the crystallography and chemistry of a single twin boundary (TB) in a rare-earth-free ferromagnetic MnAl Heusler alloy. The results establish a significant segregation of Mn along the twin boundaries. An enrichment of approx. ~8 at.% Mn was measured along the twin boundary with a confined depletion outside the twin boundary, suggesting short range solute diffusion occurring during massive transformation. [ABSTRACT FROM AUTHOR]

Knowledge of solute interaction with the interface during the transformation of austenite into ferrite is fundamental in predicting its kinetics in multicomponent steel. This interaction notably translates in segregation, or depletion, of the solutes at the transformation interface. Here, this segregation was successfully quantified by atom probe tomography (APT) in four ternary Fe-X-C systems involving substitutional solutes commonly found in modern steel grades (X = Cr, Mn, Ni, Mo). Controlled decarburization was used to grow a uniform, planar and incoherent ferrite layer at the surface of fully austenitic samples. In the case of Fe-Cr-C and Fe-Mo-C, the interfacial concentrations permitted the evaluation of the binding energy of each substitutional solute to the interface, which was found to be comparable to its respective grain boundary binding energy. In the case of Fe-Mn-C and Fe-Ni-C, undesirable motion of the interface during the quench of the samples could not be avoided, preventing a reliable estimation of their binding energy since temperature and interface velocity were unknown. [ABSTRACT FROM AUTHOR]

Abstract: Pulsed laser atom probe tomography was employed to study 300nm thick In0.25Ga0.75N layers grown on a GaN buffer layer. The unique three-dimensional data provided by atom probe tomography revealed a complex microstructure in the InGaN formed by indium-rich columns and plates adjacent to strongly indium-depleted regions. This complex indium-related domain structure is likely to represent the early stages of spinodal decomposition occurring during layer growth. The interface with the buffer layer is characterized by the absence of this microstructure and a generally more homogeneous indium distribution. Also, a significantly reduced InN mole fraction was found in that region. Both are believed to be a result of the interfacial misfit strain, which suppresses phase separation and impedes indium incorporation during growth, which is termed the compositional pulling effect. [Copyright &y& Elsevier]

Abstract: In this work, the importance of optimising experimental conditions for the analysis of reactor pressure vessel (RPV) steels using atom probe tomography is explored. The quality of the resultant atom probe data is assessed in terms of detection efficiency, noise levels and mass resolution. It is demonstrated that artefacts can exist even when experimental conditions have been optimised. In particular, it is shown that surface diffusion of some minority species, including P and Si, to major poles prior to field evaporation can be an issue. The effects were most noticeable during laser pulsing. The impact of surface migration on the characterisation of dislocations and grain boundaries is assessed. The importance of selecting appropriate regions of the reconstructed data for subsequent re-analysis is emphasised. [Copyright &y& Elsevier]

Abstract: Atom probe tomography uses time-of-flight mass spectrometry to identify the chemical nature of atoms from their mass-to-charge-state ratios. Within a mass spectrum, ranges are defined so as to attribute a chemical identity to each peak. The accuracy of atom probe microanalysis relies on the definition of these ranges. Here we propose and compare several automated ranging techniques, tested against simulated mass spectra. The performance of these metrics compare favourably with a trial of users asked to manually range a simplified simulated dataset. The optimised automated ranging procedure was then used to precisely evaluate the very low iron concentration (0.003–0.018at%) in a zirconium alloy to reveal its behaviour in the matrix during corrosion; oxygen is injected into solution and has the effect of increasing the local iron concentration near the oxide–metal interface, which in turn affects the corrosion properties of the metal substrate. [Copyright &y& Elsevier]

Carbon fibres in structural batteries are multifunctional by acting both as structural reinforcement and as lithium (Li)-ion battery electrode. The relationship between the microstructure and mechanical capabilities of carbon fibres are well established, but much remains unexplored regarding their electrochemical properties. Specifically needed is a nanoscale understanding of how Li atoms distribute and interact in the carbon fibres. Atom probe tomography (APT) is uniquely positioned to provide subnanometre resolution in three dimensions. However, it has previously been hampered by undesirable Li migration during analysis. Here, we show that APT is successfully used to analyse electrochemically cycled polyacrylonitrile-based carbon fibres, through electrostatic shielding by means of conductive coating. We measure ∼1.5 at% Li in the carbon fibres after full delithiation, and thus identify trapped Li to constitute a substantial part of the initial capacity fade. After lithiation, Li accounts for ∼9 at% and according to frequency distribution analysis tend to agglomerate on the atomic scale. With nearest neighbour analysis, Li agglomeration is shown independent of heteroatom dopants such as nitrogen. Thus, the agglomeration is more likely induced by differing accessibility for Li in the crystalline and amorphous domains in the carbon fibre. The method used in this study can inform APT experiments on other type of Li-containing carbon electrodes. The findings of the study can be used to guide design of novel carbon fibres for structural batteries with enhanced electrochemical properties. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] The softening and strengthening contributions in pre-deformed and aged Al–Mg–Cu alloys containing 3 wt.%Mg and 0.5 wt.%Cu are evaluated by a combination of microscopy, mechanical testing and modelling. A refined phenomenological model for the work hardening response, accounting for the separate effects of recovery and precipitation, is shown to be suitable for an unambiguous determination of the precipitation hardening contribution in these alloys. Significantly, it is found that the mechanical response of these alloys is not strongly impacted by Cu content (in the low Cu content regime), pre-deformation level or aging temperature meaning that the alloys are robust with respect to variations in composition. This is interesting from the perspective of alloy design concepts based on 'recycling friendly' compositions in applications that include paint-baking. [ABSTRACT FROM AUTHOR]

Tempering-induced carbon redistribution in martensitic steels from clustering to carbide precipitation is relevant to various steel processing operations and resulting property combinations. In low carbon steels with martensite start temperatures well above room temperature, carbon redistribution is already initiated during quenching. The extent of autotempering varies locally across the as-quenched microstructure, even within a single prior austenite grain, depending on the sequential transformation of martensite laths. To overcome this practical limitation in the study of carbon redistribution, we systematically track the tempering-induced microstructure evolution within a single martensite lath. This is achieved through interrupted tempering treatments and microstructure analyses by electron channeling contrast imaging and atom probe tomography, both correlated with crystallographic orientation information from electron backscatter diffraction. The results show plate-shaped carbon clusters parallel to {100} martensite lattice planes in the as-quenched state, where the maximum carbon compositions remain below ε- or η-carbide compositions. The driving force for initial cluster formation is discussed in relation to spinodal decomposition - analogous to room temperature aged Fe-Ni-C alloys after martensitic transformation during cryogenic quenching. Upon tempering, direct evidence for cluster dissolution and simultaneous cementite nucleation at carbon-rich dislocation cores is obtained. A crucial role is ascribed to the dislocation network which serves as a carbon diffusion path from clusters to cementite nucleation sites. All color figures for web only. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Recently 1:12 magnets of Sm-(Fe,V) have shown promising coercivities and the potential to be alternative rare-earth-lean permanent magnets. In this work, we investigated the effects of partial substitution of Cu, Mo and Ti for V in the magnets prepared by hot compaction and hot deformation of mechanically milled powders. The microstructure of the Sm-Fe-(V,Cu) and Sm-Fe-(V,Ti) hot-deformed magnets consisted in fine grains with sizes between 50 and 150 nm. The Sm-Fe-(V,Cu) magnet showed the best performance with μ 0 H c = 0.96 T, μ 0 M r = 0.49 T, (BH) max = 42 kJ m − 3 and T C = 362 ∘ C. Atom probe tomography of this magnet revealed the presence of a thin Sm 17.5 Fe 71.5 V 8 Cu 3 intergranular phase of 3-6 nm surrounding the 1:12 nanograins. The addition of a small amount of Cu, not only improved the magnetic properties but also hindered the grain growth during hot deformation. Micromagnetic simulations of the magnetization reversal agreed with the experimental values of coercivity. The presence of the intergranular phase reduces the number of grains that switch simultaneously. [ABSTRACT FROM AUTHOR]

Crack initiation mechanisms in the very high cycle fatigue (VHCF) regime of nickel-based single-crystal (SX) superalloys have been investigated at high temperature (1000 °C) and under fully reversed conditions (R = −1). For all Ni-based SX alloys tested, a discernible area called rough zone has been identified around the crack initiation site on the fracture surface and is for the first time described. By means of electron microscopy and atom probe analyses, localized and severe plastic activity occurring in the rough zone is evidenced. The high dislocation density provided by the slip bands induces single-phase recrystallized grains and phases precipitation (intermetallics or carbides) via the redistribution of interacting solutes in the rough zone. A crack initiation mechanism based on these observations and on the stress intensity parameter determined around the rough zone – which has been demonstrated to be a constant threshold dependent on the material – is finally proposed. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Abstract Grain boundary segregation, embrittlement and phase nucleation are interconnected phenomena that are often treated separately, which is partly due to limitations of the current models to predict grain boundary segregation in non-ideal solid solutions. Here, a simple model is introduced to predict grain boundary segregation in solid solutions by coupling available bulk thermodynamic data with a mean-field description of the grain boundary character. The model is confronted with experimental results obtained in Fe-Mn alloys analysed by atom probe tomography. This model successfully predicts a first order transition or a discontinuous jump in the composition of the grain boundary which kinetically implies the formation of spinodal Mn fluctuations that tend to grow further with time within the segregated region. The increase in solute concentration at the grain boundary leads to an increase of the enthalpy of the boundary and to its embrittlement at lower temperatures. Once austenite is formed, the amount of segregated solute Mn on the grain boundaries is drastically reduced and the toughness of the grain boundary is increased. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract In this study, the effect of rhenium (Re) addition on microstructural evolution of a new low-density Co-Ni-Al-Mo-Nb based superalloy is presented. Addition of Re significantly influences the γ′ precipitate morphology, the γ/γ′ lattice misfit and the γ/γ′ microstructural stability during long term aging. An addition of 2 at.% Re to a Co-30Ni-10Al-5Mo-2Nb (all in at.%) alloy, aged at 900 °C for 50 h, reduces the γ/γ′ lattice misfit by ∼ 40% (from +0.32% to +0.19%, measured at room temperature) and hence alters the γ′ morphology from cuboidal to round-cornered cuboidal precipitates. The composition profiles across the γ/γ′ interface by atom probe tomography (APT) reveal Re partitions to the γ phase ( K R e = 0.34) and also results in the partitioning reversal of Mo to the γ phase ( K M o = 0.90) from the γ′ precipitate. An inhomogeneous distribution of Gibbsian interfacial excess for the solute Re ( Γ R e , ranging from 0.8 to 9.6 atom.nm−2) has been observed at the γ/γ′ interface. A coarsening study at 900 °C (up to 1000 h) suggests that the coarsening of γ′ precipitates occurs solely by evaporation–condensation (EC) mechanism. This is contrary to that observed in the Co-30Ni-10Al-5Mo-2Nb alloy as well as in some of the Ni-Al based and high mass density Co-Al-W based superalloys, where γ′ precipitates coarsen by coagulation/coalescence mechanism with an extensive alignment of γ′ along <100> directions as a sign of microstructural instability. The γ′ coarsening rate constant (K r) and γ/γ′ interfacial energy are estimated to be 1.13 × 10−27 m3/s and 8.4 mJ/m2, which are comparable and lower than Co-Al-W based superalloys. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract A medium-Mn steel (Fe-12Mn-3Al-0.05C wt%) was designed using Thermo-Calc® simulations to balance the fraction and stacking fault energy of reverted austenite. Intercritical annealing for 0.5, 8 and 48 h was carried out at 585 °C to investigate the microstructural evolution. X-ray diffraction (XRD), electron backscatter diffraction (EBSD), 3-dimensional EBSD, energy-dispersive spectroscopy via scanning-transmission electron microscopy (STEM-EDS) and atom probe tomography (APT) enable characterization of phase fraction, grain area, grain morphology and alloy partitioning. An increase in annealing time from 0.5 h to 48 h increases the amount of ultrafine-grained (UFG) reverted austenite from 3 to 40 vol %. EBSD and TEM reveal multiple morphologies of UFG austenite (equiaxed, rod-like and plate-like). In addition, most of the remaining microstructure consists of recovered α′-martensite that resembles the cold-rolled state, as well as a relatively small fraction of UFG ferrite (i.e., only a small amount of martensite recrystallization occurs). Multi-scale characterization results show that the location within the cold-rolled microstructure has a strong influence on boundary mobility and grain morphology during austenite reversion. Results from APT reveal Mn-decoration of dislocation networks and low-angle lath boundaries in the recovered α′-martensite, but an absence of Mn-decoration of defects in the vicinity of austenite grains, thereby promoting recovery. STEM-EDS and APT reveal Mn depletion zones in the ferrite/recovered α′-martensite near austenite boundaries, whereas gradients of C and Mn co-partitioning are visible within some of the austenite grains after annealing for 0.5 h. Relatively flat C-enriched austenite boundaries are present even after 8 h of annealing and indicate certain boundaries possess low mobility. At later stages the growth of austenite followed the local equilibrium (LE) model such that the driving force between two equilibrium phases moves the mobile interface, as confirmed by DICTRA simulations (a Thermo-Calc® diffusion module). The sequence of austenite reversion is: (i) formation of Mn- and C-enriched face-centered-cubic nuclei from decorated dislocations and/or particles; (ii) co-partitioning of Mn and C and (iii) growth of austenite controlled by the LE mode. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract Austenite (γ) reversion in a cold-rolled 17.6 wt.% Mn steel was tracked by means of dilatometry and in-situ magnetic measurements during slow continuous annealing. A splitting of the γ-reversion into two stages was observed to be a result of strong elemental partitioning between γ and α′-martensite during the low temperature stage between 390 and 575 °C. Atom probe tomography (APT) results enable the characterization of the Mn-enriched reversed-γ and the Mn-depleted remaining α′-martensite. Because of its lower Mn content, the reversion of the remaining α′-martensite into austenite takes place at a higher temperature range between 600 and 685 °C. APT results agree with partitioning predictions made by thermo-kinetic simulations of the continuous annealing process. The critical composition for γ-nucleation was predicted by thermodynamic calculations (Thermo-Calc) and a good agreement was found with the APT data. Additional thermo-kinetic simulations were conducted to evaluate partitioning-governed γ-growth during isothermal annealing at 500 °C and 600 °C. Si partitioning to γ was predicted by DICTRA and confirmed by APT. Si accumulates near the moving interface during γ-growth and homogenizes over time. We used the chemical composition of the remaining α′-martensite from APT data to calculate its Curie temperature (T Curie) and found good agreement with magnetic measurements. These results indicate that elemental partitioning strongly influences not only γ-reversion but also the T Curie of this steel. The results are important to better understand the thermodynamics and kinetics of austenite reversion for a wide range of Mn containing steels and its effect on magnetic properties. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract We explore the effects of the elemental site occupancy in γ′-A 3 B (L1 2) intermetallic phases and their partitioning across the γ/γ′ interface in a class of multicomponent W-free Co-based superalloys. Atom probe tomography and first principles density functional theory calculations (DFT) were used to evaluate the Cr site occupancy behavior in the γ′ phase and its effect on the γ/γ′ partitioning behavior of other solutes in a series of Co-30Ni-10Al-5Mo-2Ta-2Ti-XCr alloys, where x is 0, 2, 5, and 8 at.% Cr, respectively. The increase in Cr content from 0 to 2 to 5 at.% leads to an inversion of the partitioning behavior of the solute Mo from the γ′ phase (K M o > 1) into the γ matrix (K M o < 1). At 5 at.% Cr, the Cr also has a preference to replace the excess anti-site Co atoms from the B-sites. At 8 at.% Cr, the Cr develops an additional preference to replace Co atoms from the A-sites. These compositional changes in the phases and the site partitioning behavior in the γ′ phase are accompanied by an overall decrease in the lattice misfit (δ) across the γ/γ′ interfaces as measured by high-resolution X-ray diffraction at room temperature. The reduction in misfit triggers a change in morphology of the γ′ phase from cuboidal (δ ∼ +0.48% at 0 at.% Cr) to round-cornered (δ ∼ +0.34% at 5 at.% Cr) to spheroidal shaped (δ ∼ +0.19% at 8 at.% Cr) precipitates. We also observed an increase in the solvus temperature from 1066 °C to 1105 °C when adding 5 at.% Cr to the alloy. These results on the effects of Cr in Co-base superalloys enable tuning the microstructure of these alloys and widening the alloy spectrum for designing improved high temperature alloys. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract Harmonic alloys processed by powder metallurgy are constituted by a core of coarse grains embedded in an interconnected small grains shell. They have attracted attention due to their excellent strength combined with large ductility, the two properties being rather antagonist from the classical metallurgy point of view. In contrast, conventional β -Ti alloys are currently vastly studied owing their excellent properties especially for biomedical applications. In the present study, we explore at the micron scale the deformation mechanisms operating both in standard and harmonic-structured β -Ti-25Nb-25Zr alloys using transmission electron microscopy (TEM). Although we show some similarities, deformation mechanisms appear significantly different due to the activation of martensitic transformation in conventional samples. The combined use of automated crystal orientation in TEM and in-situ TEM straining reveals that deformation bands nucleate and grow according to a mechanism involving both martensitic transformation and twinning. The comparison between in-situ and post-mortem experiments shows globally a good agreement and highlights a strain relaxation mechanism between martensite and twin. More importantly, a cross-glide mechanism similar to what is observed in dilute solid solutions is proposed to explain the dynamics of dislocation motion. Stress estimations derived from the observations of dislocation curvature between pinning points, show a reasonable good agreement with macroscopic values. The observation of deformation mechanisms operating both in core and shell structures of the harmonic-structured alloy allows us to propose a scenario of plastic deformation in the early stages. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract The aging response of two Al-3Mg alloys with Cu addition < 1 wt% has been tracked under simulated automotive paint bake conditions (∼ 20 min, 160 and 200 °C) to quantify the processes controlling hardening. The decomposition of the solid solution, observed by atom probe tomography, has been interpreted using a novel pair correlation function approach and incorporated into a model for prediction of precipitation hardening. It is shown that the hardening is controlled by clusters/Guinier-Preston-Bagaryatsky (GPB) zones similarly to what has been previously observed in much higher Cu containing 2XXX-series alloys. Interestingly, it is shown that very small additions of Cu (< 0.1 at%) can be used to catalyze a high number density of strengthening particles owing to the high enrichment in Mg compared to particles found in more conventional high Cu/low Mg alloys. This allows hardening during the first hour of aging that is as high as that obtainable in these high Cu alloys. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Zircaloy-4 (Zr-1.5%Sn-0.2%Fe-0.1%Cr wt%) was electrochemically charged with deuterium to create deuterides and subsequently analysed with atom probe tomography and scanning transmission electron microscopy to understand zirconium hydride formation and embrittlement. At the interface between the hexagonal close packed (HCP) α-Zr matrix and a face centred cubic (FCC) δ deuteride (ZrD 1.5–1.65 ), a HCP ζ phase deuteride (ZrD 0.25–0.5 ) has been observed. Furthermore, Sn is rejected from the deuterides and segregates to the deuteride/α-Zr reaction front. [ABSTRACT FROM AUTHOR]

We report the enhancement of fracture toughness and strength of a cobalt‑tantalum-based metallic glass thin film with increasing boron content. The improvement of the mechanical performance is attributed to the formation of a compositionally lamellar compared to uniform glass microstructure, which becomes thinner with increasing boron content as revealed by transmission electron microscopy. Compositional variations across the lamellar structure are revealed by atom probe tomography. Cobalt- and boron-rich regions alternate sequentially, whereas tantalum exhibits slight variations across the lamellae. Our results can be utilized in future design efforts for metallic glass thin films with outstanding mechanical performance. [ABSTRACT FROM AUTHOR]

Mo-Si-B alloys are potential creep resistant materials for accessing harsh loading scenarios beyond Ni-based superalloys due to their excellent mechanical performance at ultra-high temperatures (> 1200 °C). Here, we report on the fabrication through laser additive manufacturing of a Mo rich Mo-Si-B alloy with and without dispersion of oxide (La 2 O 3 ) particles. The major phase in the solidified material is dendritic α-Mo. The inter-dendritic regions contain a mixture of the Mo 5 Si 3 (T1) + Mo 5 SiB 2 (T2) phases, and not the expected equilibrium Mo 3 Si + Mo 5 SiB 2 (T2) phases. This combination of phases is shown to yield improved high temperature creep resistance but was only accessible through by addition of Nb, W or Ti that substitute Mo in the intermetallic phases. Whereas here it is attributed to the large undercooling in the small melt pool produced during laser processing. We show that this phase mixture, upon annealing, is stable at 1200 °C for 200 h. We also demonstrate successful dispersion of oxide particles mainly in the inter-dendritic regions leading to a high indentation fracture toughness of ∼18 MPa√m at room temperature. Toughening originates from crack trapping in the ductile α-Mo and the formation of micro-cracks and crack deflection in the vicinity of oxide particles. [ABSTRACT FROM AUTHOR]

We investigated the thermodynamics and kinetics of carbide precipitation in a cold-rolled Fe-7Mn-0.1C-0.5Si medium manganese steel during low temperature tempering. The material was annealed up to 24 h at 450 °C in order to follow the kinetics of precipitation. Using thermodynamics and kinetics simulations, we predicted the growth of M 23 C 6 carbides according to the local-equilibrium negligible partition (LENP) mode where carbide growth is controlled by the diffusion of carbon, while maintaining local chemical equilibrium at the interface. Atom-probe tomography (APT) measurements performed on samples annealed for 1, 6 and 24 h at 450 °C confirmed that LENP is indeed the mode of carbide growth and that Mn segregation is necessary for the nucleation. Additionally, we observed the heterogeneous nucleation of transition carbides with a carbon content between 6 and 8 at% at segregated dislocations and grain boundaries. We describe these carbides as a complex face-centered cubic transition carbide type (CFCC-TmC phase) obtained by the supersaturation of the FCC structure by carbon that will act as precursor to the more stable γ-M 23 C 6 carbide that forms at the dislocations and grain boundaries. The results suggest that the addition of carbon does not directly favor the formation of austenite, since Mn is consumed by the formation of the carbides and the nucleation of austenite is thus retarded to later stages of tempering as every FCC nucleus in the initial stages of tempering is readily converted into a carbide nucleus. We propose the following sequence of transformation: (i) carbon and Mn co-segregation to dislocations and grain boundaries; (ii) formation of FCC transition carbides; (iii) growth controlled according to the LENP mode and (iv) austenite nucleation and growth. [ABSTRACT FROM AUTHOR]

Boron is an essential solute element for improving the grain boundary strength in several high temperature metallic alloys especially in Ni- and Co-base superalloys although the detailed strengthening mechanisms are still not well understood. In superalloys, boron leads to the formation of borides and precipitate depleted zones around the grain boundaries and alters the bond strength among the grains directly. In this paper, we explore in detail the role of the boron content in ternary γ/γ′ Co-9Al-9W alloys. Local as well as bulk mechanical properties were evaluated using nanoindentation and compression testing and correlated to near-atomic scale microstructure and compositions obtained from electron microscopy and atom probe tomography. The alloy variant with low B content (0.005 at.% B) reveals an increase in yield strength at room temperature and 600 °C and atom probe tomography investigations show that solute B segregates to the grain boundaries. However, in the bulk B exclusively partitions to the γ′ phase. Additionally, the γ′/γ′ grain boundaries are depleted in W and Al with the concentration locally shifted towards the γ composition forming a very thin γ layer at the γ′/γ′ grain boundaries, which supports dislocation mobility in the γ′/γ′ grain boundary region during deformation. Higher content of B (0.04 at.% B) promotes formation of W-rich borides at the grain boundaries that leads to undesirable precipitate depleted zones adjacent to these borides that decrease the strength of the alloy drastically. However, it was also found that a subsequent annealing heat treatment eliminates these detrimental zones by re-precipitating γ′ and thus elevating the strength of the alloy. This result shows that, if a precipitate depleted zone can be avoided, B significantly improves the mechanical properties of polycrystalline Co-base superalloys by strengthening the γ′ phase and by improving grain boundary cohesion. [ABSTRACT FROM AUTHOR]

We report on the strengthening and strain hardening mechanisms in an aged high-Mn lightweight steel (Fe-30.4Mn-8Al-1.2C, wt.%) studied by electron channeling contrast imaging (ECCI), transmission electron microscopy (TEM), atom probe tomography (APT) and correlative TEM/APT. Upon isothermal annealing at 600 °C, nano-sized κ-carbides form, as characterized by TEM and APT. The resultant alloy exhibits high strength and excellent ductility accompanied by a high constant strain hardening rate. In comparison to the as-quenched κ-free state, the precipitation of κ-carbides leads to a significant increase in yield strength (∼480 MPa) without sacrificing much tensile elongation. To study the strengthening and strain hardening behavior of the precipitation-hardened material, deformation microstructures were analyzed at different strain levels. TEM and correlative TEM/APT results show that the κ-carbides are primarily sheared by lattice dislocations, gliding on the typical face-centered-cubic (fcc) slip system {111}<110>, leading to particle dissolution and solute segregation. Ordering strengthening is the predominant strengthening mechanism. As the deformation substructure is characterized by planar slip bands, we quantitatively studied the evolution of the slip band spacing during straining to understand the strain hardening behavior. A good agreement between the calculated flow stresses and the experimental data suggests that dynamic slip band refinement is the main strain hardening mechanism. The influence of κ-carbides on mechanical properties is discussed by comparing the results with that of the same alloy in the as-quenched, κ-free state. [ABSTRACT FROM AUTHOR]

We investigated a high-purity cold-rolled martensitic Fe-9wt%Mn alloy. Tensile tests performed at room temperature after tempering for 6 h at 450 °C showed discontinuous yielding. Such static strain ageing phenomena in Fe are usually associated with the segregation of interstitial elements such as C or N to dislocations. Here we show by correlative transmission electron microscopy (TEM)/atom probe tomography (APT) experiments that in this case Mn segregation to edge dislocations associated with the formation of confined austenitic states causes similar effects. The local chemical composition at the dislocation cores was investigated for different tempering temperatures by APT relative to the adjacent bcc matrix. In all cases the Mn partitioning to the dislocation core regions matches to the one between ferrite and austenite in thermodynamic equilibrium at the corresponding tempering temperature. Although a stable structural and chemical confined austenitic state has formed at the dislocation cores these regions do not grow further even upon prolonged tempering. Simulation reveals that the high Mn enrichment along the edge dislocation lines (25 at.%Mn at 450 °C) cannot be described merely as a Cottrell atmosphere formed by segregation driven by size interaction. Thermodynamic calculations based on a multiscale model indicate that these austenite states at the dislocation cores are subcritical and defect-stabilized by the compression stress field of the edge dislocations. Phenomenologically, these states are the 1D equivalent to the so-called complexions which have been extensively reported to be present at 2D defects, hence have been named linear complexions. [ABSTRACT FROM AUTHOR]

Abstract: In-depth analysis of pulsed laser atom probe tomography (APT) data on the field evaporation of the III–V semiconductor material GaSb reveals strong variations in charge states, relative abundances of different cluster ions, multiplicity of detector events and spatial correlation of evaporation events, as a function of the effective electric field at the specimen surface. These variations are discussed in comparison with the behaviour of two different metallic specimen materials, an Al-6XXX series alloy and pure W, studied under closely related experimental conditions in the same atom probe instrument. It is proposed that the complex behaviour of GaSb originates from a combination of spatially correlated evaporation events and the subsequent field induced dissociation of cluster ions, the latter contributing to inaccuracies in the overall atom probe composition determination for this material. [Copyright &y& Elsevier]

A systematic study of the compression creep properties of a single-crystalline Co-base superalloy (Co-9Al-7.5W-2Ta) was conducted at 950, 975 and 1000°C to reveal the influence of temperature and the resulting diffusion velocity of solutes like Al, W and Ta on the deformation mechanisms. Two creep rate minima are observed at all temperatures indicating that the deformation mechanisms causing these minima are quite similar. Atom-probe tomography analysis reveals elemental segregation to stacking faults, which had formed in the γ′ phase during creep. Density-functional-theory calculations indicate segregation of W and Ta to the stacking fault and an associated considerable reduction of the stacking fault energy. Since solutes diffuse faster at a higher temperature, segregation can take place more quickly. This results in a significantly faster softening of the alloy, since cutting of the γ′ precipitate phase by partial dislocations is facilitated through segregation already during the early stages of creep. This is confirmed by transmission electron microscopy analysis. Therefore, not only the smaller precipitate fraction at higher temperatures is responsible for the worse creep properties, but also faster diffusion-assisted shearing of the γ′ phase by partial dislocations. The understanding of these mechanisms will help in future alloy development by offering new design criteria. [Display omitted] [ABSTRACT FROM AUTHOR]

Iron- and steelmaking is the largest single industrial CO 2 emitter, accounting for 6.5% of all CO 2 emissions on the planet. This fact challenges the current technologies to achieve carbon-lean steel production and to align with the requirement of a drastic reduction of 80% in all CO 2 emissions by around 2050. Thus, alternative reduction technologies have to be implemented for extracting iron from its ores. The hydrogen-based direct reduction has been explored as a sustainable route to mitigate CO 2 emissions, where the reduction kinetics of the intermediate oxide product Fe x O (wüstite) into iron is the rate-limiting step of the process. The total reaction has an endothermic net energy balance. Reduction based on a hydrogen plasma may offer an attractive alternative. Here, we present a study about the reduction of hematite using hydrogen plasma. The evolution of both, chemical composition and phase transformations was investigated in several intermediate states. We found that hematite reduction kinetics depends on the balance between the initial input mass and the arc power. For an optimized input mass-arc power ratio, complete reduction was obtained within 15 min of exposure to the hydrogen plasma. In such a process, the wüstite reduction is also the rate-limiting step towards complete reduction. Nonetheless, the reduction reaction is exothermic, and its rates are comparable with those found in hydrogen-based direct reduction. Micro- and nanoscale chemical and microstructure analysis revealed that the gangue elements partition to the remaining oxide regions, probed by energy dispersive spectroscopy (EDS) and atom probe tomography (APT). Si-enrichment was observed in the interdendritic fayalite domains, at the wüstite/iron hetero-interfaces and in the oxide particles inside iron. With proceeding reduction, however, such elements are gradually removed from the samples so that the final iron product is nearly free of gangue-related impurities. Our findings provide microstructural and atomic-scale insights into the composition and phase transformations occurring during iron ore reduction by hydrogen plasma, propelling better understanding of the underlying thermodynamics and kinetic barriers of this essential process. [Display omitted] [ABSTRACT FROM AUTHOR]

Oxide dispersion strengthened (ODS) steels are known for their enhanced mechanical performance at high temperatures or under radiation exposure. Their microstructure depends on the manufacturing process, from the nanoparticle addition to the base steel powder, to the processing of the nanoparticle enriched powder. The optimization and control of the processing steps still represent a challenge to establish a clear methodology for the additive manufacturing of ODS steels. Here, we evaluate the microstructure, nanoparticle evolution, and mechanical properties of ODS steels prepared by dielectrophoretic controlled adsorption of 0.08 wt% laser-synthesized yttrium oxide (Y 2 O 3) on an iron-chromium ferritic steel powder (PM2000). The influence of the ODS steel fabrication technique is studied for two standard additive manufacturing techniques, directed energy deposition (DED) and laser powder bed fusion (LPBF). The compressive strength of the ODS steels at 600 °C is increased by 21% and 29% for the DED and LPBF samples, respectively, compared to the DED and LPBF steels manufactured without Y 2 O 3 nanoparticle addition. The Martens hardness is enhanced by 9% for the LPBF ODS steel while no significant change is observed in the DED ODS steel. The microstructure and nanoparticle composition and distribution are evaluated by electron backscatter diffraction, scanning electron microscopy–energy-dispersive X-ray spectroscopy, and atom probe tomography, to compare the microstructural features of DED and LPBF manufactured parts. Smaller grain size and more homogeneous distribution with lower agglomeration of Y-O nanoparticles in the LPBF sample are found to be key factors for enhanced mechanical response at 600 °C. The enhanced mechanical properties of the LPBF-processed sample and the more homogeneous nanoparticle dispersion can be linked to results obtained by finite element methods simulations of the melt pool that show two orders of magnitude faster cooling rates for LPBF than for DED. Therefore, this work presents and validates a complete laser-based methodology for the preparation and processing of an ODS steel, proving the modification of the microstructure and enhancement of the high-temperature strength of the as-built parts. Image, graphical abstract [ABSTRACT FROM AUTHOR]