Atom Probe Tomography Measurements Research Articles

How precisely are solute clusters in RPV steels characterized by atom probe experiments?

Atom probe tomography (APT) is a powerful microscopy technique to characterize nano-sized clusters of the alloying elements in the bulk of reactor pressure vessel (RPV) steels. These clusters are known to dominantly determine the evolution of mechanical properties under irradiation. The results are conventionally summarized as the overall number density N and the average diameter D of the solute clusters identified in the material. Here, we illustrate that these descriptors are intrinsically imprecise because they are steered by the parameters involved in the measurement and data processing, some of which are directly under the control of the operators, but some others not. Consequently, a direct comparison between data derived at different laboratories is compromised, and key trends such as the evolution with dose, are masked. This study relies on a state-of-the-art physical model for neutron irradiation in steels to make reliable estimates of the microstructure before the measurement is performed, which allows the prediction of the population of solute clusters that are not seen by APT. We mimic APT measurements from simulated microstructures, performing a detailed study of the effects of the parameters of the analysis. We show that the values of N and D reported in the scientific literature can be matched by the predictions of our theoretical model only if specific sets of parameters are used for each laboratory that issued the measurements. We also show that if, on the contrary, all studied cases are analyzed in a consistent way, the scatter of N and D values is reduced. Specifically, we find that the average diameter D is nearly a constant value with dose, independently of the material's chemical compositions, while N increases with dose, but is also influenced by other variables. The approach we developed and used proves to have added value as complement to APT experiments in reactor pressure vessel steels.

Adapting Conductivity and Mechanical Properties through Layer Thickness Variation in Copper Niobium Laminated Metallic Composites

Cu/Nb laminated metallic composites (LMCs) exhibit extraordinary mechanical strength while electrical conductivity is maintained at a high level. In this research, Cu‐based LMCs with varying volume fraction of niobium are produced via accumulative roll bonding (ARB) and additionally cold‐rolled and heat‐treated to identify the role of these heterogeneous phase boundaries on the overall sheet properties. Deformation structures in these layered materials are studied through scanning transmission electron microscopy investigations. The existence of an interface‐affected zone at the phase boundaries and mechanical intermixing is attributed to decreasing electrical and increasing mechanical properties, especially in very thin layered LMCs. Atom probe tomography measurements show that mechanical alloying occurs due to severe plastic deformation during ARB processing of the laminates. An additional subsequent heat treatment leads to chemical demixing at the interfaces of the phase boundaries.

State-of-the-Art and Future Directions of fs-Laser Assisted Specimen Preparation Techniques for Atom Probe Tomography Measurements

State-of-the-Art and Future Directions of fs-Laser Assisted Specimen Preparation Techniques for Atom Probe Tomography Measurements

Chromium agglomeration induced by Fe+ ion irradiation of Fe-10at%Cr

Fe-Cr alloys serve as model alloys for the investigation of radiation induced effects in ferritic-martensitic steels which are candidate structural materials for future fusion reactors. In this work the effect of Cr segregation and/or agglomeration in 490 keV Fe+ ion irradiated Fe-10at%Cr alloys in the form of thin films is investigated. The irradiations took place at 300 °C at doses ranging from 0.5 to 20 displacements per atom (dpa). Polarized Neutron Reflectivity (PNR) measurements were used for the determination of the solute Cr concentration in the Fe-Cr matrix. Cr depletion from the Fe-Cr matrix up to 2.4 at% was found. This is related to solute Cr decrement as the accumulated dose increases. After the damage of 4 dpa, solute Cr reaches the asymptotic value of 8.4 at%, close to that of the thermodynamic equilibrium in Fe-Cr. Atom Probe Tomography (APT) measurements showed that after irradiation Cr accumulates into clusters the majority of which is co-located with oxygen.

Atomic-scale investigation of interactions between Cu solute and defects in Zr alloys

The effects of dilute Cu solute on dislocations and grain boundaries in Zr alloys have been investigated at the atomic scale by atom probe tomography measurements, molecular dynamics, and molecular statics simulations. We demonstrate that Cu can segregate at dislocations and form nanoclusters in Zr alloys to retard the dislocation motion at high-stress creep. The solid solution strengthening of Cu is stronger than Sn and Nb at the same concentration and 673 K. Moreover, Cu is inclined to segregate at grain boundaries in Zr alloys, which is expected to reduce the Coble creep rate. As such, minor Cu alloying is an effective strategy to tailor the properties of dislocations and grain boundaries for enhancing the creep performance of Zr alloys.

Bainite Formation in a Carbon-Free Fe–Cr–N System

In order to promote the understanding of bainitic transformation in nitrogen alloyed steels, the microstructure, transformation kinetics, and diffusion/precipitation characteristics of an isothermally heat-treated carbon-free system Fe–5Cr–N were investigated. Quenching dilatometry was used to perform heat treatments at temperatures between 400 °C and 500 °C and simultaneously receive information about the transformation kinetics. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) investigations indicate a plate- to lenticular-like, fully ferritic microstructure with nanometer scale CrN. In order to analyze the diffusion and precipitation behavior of nitrogen, atom probe tomography measurements were performed. On the one hand, nitrogen segregation was found, and on the other hand, the presence of CrN precipitates and preliminary stages of CrN nitrides were validated.

Analyzing the Precipitation Effects in Low-Alloyed Copper Alloys Containing Hafnium and Chromium

Copper alloys containing chromium and hafnium combine elevated mechanical strength and high electrical and thermal conductivity. For the simultaneous enhancement of both material properties, precipitation hardening is the utilized mechanism. Therefore, the aim is to analyze the influence of chromium and hafnium in binary and ternary low-alloyed copper alloys and to compare the precipitation processes during temperature exposure. Atom probe tomography (APT) and differential scanning calorimetry (DSC) measurements enable to understand the precipitation sequence in detail. CuCr0.7 starts to precipitate directly, whereas CuHf0.7 is highly influenced by prior diffusion facilitating cold rolling. Within the ternary alloy, hafnium atoms accumulate at the shell of mainly Cr-containing precipitates. Increasing the local hafnium concentration results in the formation of intermetallic CuHf precipitates at the sites of mainly Cr-containing precipitates. Indirect methods are utilized to investigate the materials’ properties and show the impact of cold rolling prior to an aging treatment on binary alloys CuCr and CuHf. Finally, ternary alloys combine the benefits of facilitated precipitation processes and decelerated growing and coarsening, which classifies the alloys to be applicable for usage at elevated temperatures.

Growth and Electrical Characterization of Hybrid Core/Shell InAs/CdSe Nanowires.

Core-only InAs nanowires (NWs) remain of continuing interest for application in modern optical and electrical devices. In this paper, we utilize the II-VI semiconductor CdSe as a shell for III-V InAs NWs to protect the electron transport channel in the InAs core from surface effects. This unique material configuration offers both a small lattice mismatch between InAs and CdSe and a pronounced electronic confinement in the core with type-I band alignment at the interface between both materials. Under optimized growth conditions, a smooth interface between the core and shell is obtained. Atom probe tomography (APT) measurements confirm substantial diffusion of In into the shell, forming a remote n-type doping of CdSe. Moreover, field-effect transistors (FETs) are fabricated, and the electron transport characteristics in these devices is investigated. Finally, band structure simulations are performed and confirm the presence of an electron transport channel in the InAs core that, at higher gate voltages, extends into the CdSe shell region. These results provide a promising basis toward the application of hybrid III-V/II-VI core/shell nanowires in modern electronics.

Reconciliation between simulated and measured microstructures of metastable FeCr alloys: A phase field approach

This work presents a novel phase field method for modeling the microstructure induced by the thermal decomposition of immiscible metastable binary alloys. This approach bypasses the problem associated with the modeling of the nucleation process introducing a random distribution of critical precipitates as an initial condition for the Cahn–Hilliard equation, extensively used to model the microstructure in the spinodal regime. The comparison between simulated and measured microstructures obtained from atom probe tomography measurements performed on ultra pure Fe0.81Cr0.19 samples justifies the validity of our method. In particular, we succeeded modeling the evolution of the chemical composition of α′ clusters produced during annealing. Such an evolution has been many times reported from atom probe tomography measurements but never explained. Moreover, these simulations allowed proposing a physical mechanism to explain the increase of the yield strength, observed in many annealed FeCr samples.

Quantitative Mapping of Chemical Defects at Charged Grain Boundaries in a Ferroelectric Oxide.

Polar discontinuities, as well as compositional and structural changes at oxide interfaces can give rise to a large variety of electronic and ionic phenomena. In contrast to earlier work focused on domain walls and epitaxial systems, here, w e investigate the potential relation between polar discontinuities and the local chemistry at grain boundaries in polycrystalline ferroelectric ErMnO3 . Using orientation mapping and different scanning probe microscopy techniques, w e demonstrate that the polycrystalline material develops charged grain boundaries with enhanced electronic conductance. By performing atom probe tomography measurements, w e find an enrichment of erbium and a depletion of oxygen at all grain boundaries. The observed compositional changes translate into a charge that exceeds possible polarization-driven effects, demonstrating that structural phenomena rather than electrostatics determine the local chemical composition and related changes in the electronic transport behavior. The study shows that the charged grain boundaries behave distinctly different from charged domain walls, giving additional opportunities for property engineering at polar oxide interfaces. This article is protected by copyright. All rights reserved.

Thermal ageing effect on solute segregation and precipitation in the heat-affected-zone of dissimilar metal welds for nuclear power plants

Solute segregation in the carbon-depleted (CDZ) heat-affected zone of dissimilar metal welds (DMW) designed for nuclear power plant EPRTM reactors has been investigated by atom probe tomography (APT) and related to the mechanical properties (toughness test). The analysis of grain boundaries (GBs) by APT allowed the quantification of the P segregation and other solute elements, in all the analyzed samples, before and after long-term ageing heat treatments. The post-weld stress-relief heat treatment applied to the samples before the ageing heat treatments leads to important GBs segregation levels. Following thermal ageing treatments allow a relative increase of GB segregation. P segregation increases with the various thermal ageing conditions. The experimental data were used to model P segregation in high-angle GBs. The simulations show that P segregation is kinetically limited by bulk diffusion, explaining the observed P segregation increase with temperature. P segregation in GBs in the CDZ of the DMW should remain low compared to the expected equilibrium segregation level during the entire reactor service. In addition to solute segregation in extended defects, APT measurements revealed the presence of Cu-rich clusters in GBs, dislocations, and grains in all the aged samples. These clusters were not observed in the sample before ageing.

Correlating laser energy with compositional and atomic-level information of oxides in atom probe tomography

Atom probe tomography (APT) is a 3D analysis technique that offers unique chemical accuracy and sensitivity with sub-nanometer spatial resolution. There is an increasing interest in the application of APT to complex oxides materials, giving new insight into the relation between local variations in chemical composition and emergent physical properties. However, in contrast to the field of metallurgy, where APT is routinely applied to study materials at the atomic level, complex oxides and their specific field evaporation mechanisms are much less explored. Here, we perform APT measurements on the hexagonal manganite ErMnO3 and systematically study the effect of different experimental parameters on the measured composition and structure. We demonstrate that both the mass resolving power (MRP) and compositional accuracy can be improved by increasing the charge-state ratio (CSR) working at low laser energy (< 5 pJ) for a given fixed detection rate. Furthermore, we observe a substantial preferential retention of Er atoms, which is suppressed at higher CSRs. We explain our findings based on fundamental field evaporation concepts, expanding the knowledge about the impact of key experimental parameters and the field evaporation process in complex oxides in general.

Atom probe tomography of hyper-doped Ge layers synthesized by Sb in-diffusion by pulsed laser melting

Hyper-doped Ge layers synthesized by Sb in-diffusion by pulsed laser melting have been analyzed by Atom Probe Tomography (APT). The 1D concentration profiles obtained by APT under low field conditions are consistent with those obtained by secondary ion mass spectrometry. However, the 3D distribution of Sb indicates the occurrence of Sb segregation along line defects, outside of which Sb is randomly distributed. The APT measurements indicate a dependence of the measured composition on the electric field. By analyzing the mass spectra and the multiple detection events, we show that low-field conditions should be adopted in order to obtain an accurate composition measurement.

Focused Ion Beam induced hydride formation does not affect Fe, Ni, Cr-clusters in irradiated Zircaloy-2

Room temperature focused ion beam (FIB) milling is known to potentially promote the formation of hydrides in zirconium and its alloys. We used atom probe tomography (APT) to determine the composition of irradiated and as-produced Zircaloy-2 fuel cladding. We consistently found ∼ 50 at% hydrogen in all room temperature FIB-milled specimens run in voltage pulsing APT measurements. Crystallographic analysis of APT data however showed slightly better agreement with δ-hydride (ZrH2, FCC, ∼ 60–66.7 at% H) than γ-hydride (ZrH, FCT, ∼ 50 at% H). Electron energy loos spectroscopy (EELS) measurements prior to APT analyses confirmed the presence of δ-hydride. Hence, APT gives a systematic underestimation of hydrogen for Zr-hydride. Milling at cryogenic temperatures was found to not cause such hydride formation. However, we did not find significant differences in the clusters formed by segregation of the alloying elements Fe, Cr and Ni to irradiation induced a-loops whether the material was identified as α-Zr or hydride. Therefore, analyzing irradiation-induced redistribution of alloying elements in Zr fuel cladding using APT does not rely on FIB preparation at cryogenic temperatures. However, in conjunction with voltage pulsing APT cryo-FIB can be worthwhile if one aims at investigating hydrogen distribution or hydrides.

Irradiation effects on binary tungsten alloys at elevated temperatures: Vacancy cluster formation, precipitation of alloying elements and irradiation hardening

Irradiation responses of binary W alloys were investigated systematically from the perspective of the binding energy of an alloying element with a W self-interstitial atom (W-SIA). Plates of W, W-0.3 at.% Cr, W-5 at.% Re, W-2.5 at.% Mo and W-5 at.% Ta alloys were irradiated at 1073 K with 6.4 MeV Fe ions to 0.26 dpa at the damage peak, where the binding energy of alloying element with W-SIA is in order of Cr > Re > Mo > Ta. The formation of vacancy-type defects (vacancies and vacancy clusters) was studied by using positron lifetime measurement. The precipitation of alloying elements was studied by using atom probe tomography (APT) and the hardness changes in the irradiated volumes were measured by the nanoindentation technique. The formation of vacancy-type defects was strongly suppressed by the addition of Cr and Re, while Ta and Mo had no noticeable suppression effect. The APT measurements showed fine Cr- and Re-rich precipitates in W-0.3 at.% Cr and W-5 at.% Re alloys, respectively, where the density of precipitates in the latter was clearly lower than that in the former. The distributions of Mo and Ta were uniform even after irradiation. Irradiation hardening was observed for all materials but that of W-5 at.% Re alloy was significantly smaller than the hardening of W, W-2.5 at.% Mo and W-5 at.% Ta alloys. These observations suggest that the irradiation hardening of W, W-2.5 at.% Mo, and W-5 at.% Ta alloys were mainly caused by vacancy-type defects. It was concluded that an alloying element with moderate binding energy with a W-SIA effectively suppresses vacancy formation without significantly enhanced precipitation and consequently mitigates irradiation hardening.

The evolution of precipitates in an Al–Zn–Mg alloy

The precipitation sequence in Al–Zn–Mg alloys has been subject to many revisions over the years as more structural details of the early-stage Guinier-Preston (GP) zones and precipitates have been uncovered. To further investigate this, a 7003 aluminium alloy naturally aged for one year was subjected to artificial ageing at 140 °C to investigate the evolution of early-stage precipitates. Scanning transmission electron microscopy was coupled with atom probe tomography and hardness measurements to characterise the precipitate structure and chemistry of the alloy at different stages in the heat treatment. The naturally aged condition contained a dense population of GPI zones, co-existing with a smaller distribution of η′ precipitates. After artificial ageing for 10 min, the hardness decreased from 121 HV to 88 HV. The number densities of clusters were similar in the two conditions, while the average size of the clusters had increased during the first 10 min of artificial ageing. The average Zn/Mg ratio of the clusters decreased from 2.0 to 1.7 and the clusters no longer exhibited the GPI zone atomic structure, indicating that the GPI zones had dissolved. Concurrently, the fraction of precipitates having η′ structure increased from 2% to 18%. During further ageing, the hardness increased again, reaching a peak after 5 h. In this condition, η′ was the dominant phase, co-existing with η1, η2 and T′ phases. Atomically resolved scanning transmission electron microscopy images in addition to precession electron diffraction patterns of the disputed T′ phase are presented and compared to literature.

Strong charge carrier scattering at grain boundaries of PbTe caused by the collapse of metavalent bonding

Grain boundaries (GBs) play a significant role in controlling the transport of mass, heat and charge. To unravel the mechanisms underpinning the charge carrier scattering at GBs, correlative microscopy combined with local transport measurements is realized. For the PbTe material, the strength of carrier scattering at GBs depends on its misorientation angle. A concomitant change in the barrier height is observed, significantly increasing from low- to high-angle GBs. Atom probe tomography measurements reveal a disruption of metavalent bonding (MVB) at the dislocation cores of low-angle GBs, as evidenced by the abrupt change in bond-rupture behavior. In contrast, MVB is completely destroyed at high-angle GBs, presumably due to the increased Peierls distortion. The collapse of MVB is accompanied by a breakdown of the dielectric screening, which explains the enlarged GB barrier height. These findings correlate charge carrier scattering with bonding locally, promising new avenues for the design of advanced functional materials.

Effect of interference laser treatment on the surface region homogeneity of a biomedical [formula omitted]-Ti alloy

This study provides a detailed insight into the microstructure and chemical composition of a surface region of a novel, metastable Ti-29Nb-13Ta-4.6Zr (TNTZ) alloy with a low Young’s modulus modified by Direct Laser Interference Patterning (DLIP) using an Nd:YAG (1064 nm) with 10 ns pulse duration. Scanning Electron Microscopy (SEM), Transmission Electron Microscopy, and Scanning Transmission Electron Microscopy (STEM) were employed to investigate the microstructure of the cross-section of modified TNTZ. For chemical composition analysis of the surface region, X-ray Photoelectron Spectroscopy (XPS) and Atom Probe Tomography (APT) were applied. The chemical composition analysis was divided into two stages: surface analysis (XPS and APT) and analysis of a bulk (APT at the depth of 300 nm). Through the interference phenomenon during DLIP treatment, we observed significant microstructure evolution in the surface region of TNTZ alloy — grain refinement on the tops of the grooves while the microstructure at the bottoms was unattached. APT measurements at the depth of 300 nm revealed a noticeable local chemical heterogeneity of the substrate alloy, that has not been considered before when designing the metastable β-Ti systems. The applied laser treatment homogenizes the remelted zones and thus provides an additional factor influencing the surface properties.

Deuterium Distribution in Fe/V Multi-Layered Films.

The recent progress of Atom Probe Tomography (APT) has opened up atomic-scale elemental analysis including hydrogen species. For APT measurements, the use of deuterium is highly recommended, due to its low mobility compared to the fast and quantum mechanically tunneling isotope hydrogen. In addition, deuterium can be distinguished from hydrogen originating from the APT analysis chamber. To date, however, APT studies on materials with high D concentrations are scarce. In this study, the D concentration profile in a Fe/V multi-layered film sample was investigated, and spanned a wide concentration range. The mean hydrogen isotope concentration was alternatively quantified by electromotive force (EMF) measurements on a similar Fe/V film, thus verifying the APT results. The reduction found in the D concentration at the Fe/V interface results from local alloying at the Fe/V interfaces which accompanies a change in the available volume in the V lattice. Even at the same Fe concentration, the shape of the observed D depth profile was asymmetric at high D2 pressures. This indicates a stress impact caused by the deposition sequence.

Numerical simulation of precipitation kinetics in multicomponent alloys

• Partial off-diagonal diffusion, involved in the precipitation kinetics equation, improves computational efficiency meanwhile keeping computational accuracy. • The ability to handle sophisticated alloy chemistry and compositional sensitivity of dimensional properties of precipitates, by quantitative equations. • Voronoi construction visualizes the spatial distribution of precipitates in a characteristic cell. • The diffusion distance in precipitation kinetics equation is depicted in a more realistic way by Voronoi construction. A universal numerical model based on the particle size distribution (PSD) approach has been developed for the simulation of precipitation kinetics in multicomponent alloys during isothermal ageing. Nucleation was implemented utilizing the classical nucleation theory (CNT). Growth and coarsening were modeled by a single growth kinetics equation, which is constructed based on the interfacial diffusion flux balance and the capillarity effect. Only partial off-diagonal terms in the diffusion matrix (diffusion of individual components in the matrix) were taken into account in the calculations to minimize the computational cost while coupling with CALPHAD to extract thermodynamics equilibrium around the interface. A new feature of the model is the incorporation of a more realistic spatial site distribution via a Voronoi construction in the characteristic cell, for the purpose of modifying the diffusion distance. Computational predictions of the precipitate dimensions and the precipitation kinetics were compared with the atom probe tomography (APT) measurements on ternary Ni-Al-Cr alloys isothermally aged at 873 K. It is found that the temporal evolution of the dimensions and composition of the precipitates is well captured, as is the dependence on changes in the alloy composition. The new modification with Voronoi construction demonstrates that the overall precipitation kinetics depends on the density and the spatial site distribution of precipitates. The ability to handle sophisticated alloy chemistries by quantitative equations, the compositional sensitivity of microstructural characteristics emerging from the simulation results, and the ability to visualize the spatial distribution of precipitates make the work very promising for multicomponent alloy design and optimization.