Atom Probe Analysis Research Articles

Microstructure and chemistry of the composite precipitate structure in Al-Li-Cu alloy

A combination of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging and three-dimensional atom probe analyses (3DAP) technologies was employed to accurately characterize the composite precipitate structure. Two types of the composite precipitate structure, i.e. δ′/T1 and δ′/T1/δ′ (T1-Al2CuLi and δ′-Al3Li), were observed. 3DAP analysis further confirmed the existence of this structure and solutes distribution was precise determined by concentration profiles. We also found that the coarsening of T1 was achieved by the nucleation of precursor on one side of matured T1 precipitate uncovered by δ′ precipitate and the coarsening of the inward T1 precipitate within the composite structure may would be depressed by the sideward δ′ precipitate.

Formation and crystallization behavior of Fe-based amorphous precursors with pre-existing α-Fe nanoparticles—Structure and magnetic properties of high-Cu-content Fe-Si-B-Cu-Nb nanocrystalline alloys

Structure, crystallization behavior, and magnetic properties of as-quenched and annealed Fe81.3Si4B13Cu1.7 (Cu1.7) alloy ribbons and effects of Nb alloying have been studied. Three-dimensional atom probe and transmission electron microscopy analyses reveal that high-number-density Cu-clusters and Pre-existing Nano-sized α-Fe Particles (PN-α-Fe) are coexistence in the melt-spun Cu1.7 amorphous matrix, and the PN-α-Fe form by manners of one-direction adjoining and enveloping the Cu-clusters. Two-step crystallization behavior associated with growth of the PN-α-Fe and subsequent nucleation and growth of newly-formed α-Fe is found in the primary crystallization stage of the Cu1.7 alloy. The number densities of the Cu-clusters and PN-α-Fe in melt-spun Fe81.3−xSi4B13Cu1.7Nbx alloys are gradually reduced with enriching of Nb, and a fully amorphous structure forms at 4 at.% Nb, although smaller Cu-clusters still exist. After annealing, 2 at.% Nb coarsens the average size (Dα-Fe) of the α-Fe grains from 14.0 nm of the Nb-free alloy to 21.6 nm, and 4 at.% Nb refines the Dα-Fe to 8.9 nm. The mechanisms of the α-Fe nucleation and growth during quenching and annealing for the alloys with large quantities of PN-α-Fe as well as after Nb alloying have been discussed, and an annealing-induced α-Fe growth mechanism in term of the barrier co-contributed by competitive growth among the PN-α-Fe and diffusion-suppression effect of Nb atoms has been proposed. A coercivity (Hc) ∝ Dα-Fe3 correlation has been found for the nanocrystalline alloys, and the permeability is inverse with the Hc.

Radiation tolerance and microstructural changes of nanocrystalline Cu-Ta alloy to high dose self-ion irradiation

Nanocrystalline materials are known to possess excellent radiation resistance due to high fraction of grain boundaries that act as defect sinks, provided they are microstructurally stable at such extreme conditions. In this work, radiation response of a stable nanocrystalline Cu-Ta alloy is studied by irradiating with 4 MeV copper ions to doses (close to the surface) of 1 displacements per atom (dpa) at room temperature (RT); 10 dpa at RT, 573 and 723 K; 100 and 200 dpa at RT and 573 K. Nanoindentation results carried out for samples irradiated till 100 dpa at RT and 573 K show exceptionally low radiation hardening behavior compared to various candidate materials from literature. Results from microstructural characterization, using atom probe analysis and transmission electron microscopy, show a stable nanocrystalline microstructure with minimal grain growth and a meagre swelling in samples irradiated to 100 dpa (~0.2%) and 200 dpa at RT, while no voids in those at 573 K. This radiation tolerance is partly attributed to the stability of Ta nanoclusters due to phase separating nature of the alloy. Additionally, the larger tantalum particles are observed to undergo ballistic dissolution at doses greater than 100 dpa and are eventually precipitated as nanoclusters, replenishing the sink strength, which enhanced material's radiation tolerance when exposed to high irradiation doses and elevated temperatures.

Thermoelectric properties of half-Heusler high-entropy Ti2NiCoSn1-xSb1+x (x = 0.5, 1) alloys with VEC>18

ABSTRACT A new set of half-Heusler high-entropy alloys Ti2NiCoSn1-xSb1+x (x = 0.5, 1), with a valence electron count higher than 18, were investigated for thermoelectric applications. Vacuum arc melting was employed for synthesis. Atom probe analysis confirmed single-phase at atomic level. The alloys were subsequently ball milled for 1 h followed by spark plasma sintering for consolidation. In 1 h BM cases, the alloy with x = 0.5 exhibited a low lattice thermal conductivity of 2.48 Wm−1K−1, and a ZT of 0.29 at 873 K.

Nanostructure Characteristics of Al3Sc1-xZrx Nanoparticles and Their Effects on Mechanical Property and SCC Behavior of Al-Zn-Mg Alloys.

The Nanostructure characteristics of Al3Sc1−xZrx nanoparticles and their effects on the mechanical properties and stress corrosion cracking (SCC) behavior of Al–Zn–Mg alloys were investigated by 3D atom probe analyses, high-angle annular-dark-field scanning transmission electron microscopy methods, electron back scattered diffraction techniques, electrochemical measurements, slow strain rate tests and quantitative calculations. The results show that adding small amounts of scandium (0.10 percent by weight) and zirconium into Al–Zn–Mg extrusion bars can precipitate Al3Sc1−xZrx nanoparticles with a number density of (7.80 ± 3.83) × 1021 per cubic meter. Those particles, with a low lattice misfit with matrix (1.14 ± 0.03 percent) and stable core-shell L12-nanostructure in aged Al–Zn–Mg alloys, can increase the yield strength by 161 ± 7 MPa via strong Orowan strengthening (the theoretical calculated value is 159 MPa) and weak Hall-Petch strengthening (the theoretical calculated value is 6 MPa). Moreover, Al3Sc1−xZrx nanoparticles can change the fracture mechanism of alloys in 3.5% NaCl solution from intergranular cracks to transgranular failure, and decrease the proportion of high-angle grain boundaries from 87% to 31%, thus reducing the microchemistry differences around the grain boundaries and anodic dissolution kinetics, and improving intergranular SCC resistance and ductility. This study offers a new approach to the simultaneous improvement in mechanical property and corrosion performance of high strength alloys.

Novel Applications of FIB-SEM-Based ToF-SIMS in Atom Probe Tomography Workflows

Atom probe tomography (APT) is used to quantify atomic-scale elemental and isotopic compositional variations within a very small volume of material (typically <0.01 µm3). The small analytical volume ideally contains specific compositional or microstructural targets that can be placed within the context of the previously characterized surface in order to facilitate a correct interpretation of APT data. In this regard, careful targeting and preparation are paramount to ensure that the desired target, which is often smaller than 100 nm, is optimally located within the APT specimen. Needle-shaped specimens required for atom probe analysis are commonly prepared using a focused ion beam scanning electron microscope (FIB-SEM). Here, we utilize FIB-SEM-based time-of-flight secondary ion mass spectrometry (ToF-SIMS) to illustrate a novel approach to targeting <100 nm compositional and isotopic variations that can be used for targeting regions of interest for subsequent lift-out and APT analysis. We present a new method for high-spatial resolution targeting of small features that involves using FIB-SEM-based electron deposition of platinum "buttons" prior to standard lift-out and sharpening procedures for atom probe specimen manufacture. In combination, FIB-ToF-SIMS analysis and application of the "button" method ensure that even the smallest APT targets can be successfully captured in extracted needles.

A method to study the electric field distribution on sample surfaces in atom probe analysis

We report the development of a nondestructive method to estimate the electric field (EF) distribution on a nano‐sized sample surface in atom probe (AP) analysis. The simulated EF distribution on an ideal hemisphere indicates that the largest EF exists on the geometrical top of the ideal hemisphere and that EF decreases as the emitting area getting away from the sample apex. To estimate the EF distribution on a real sample surface, the sample apex is determined via comparing the field ion microscopy (FIM) signal intensity of {113} planes on the symmetrical sample surface. A series of contour maps showing the intensity of the evaporated ions (eg, H+) was obtained by applying various EFs on the sample surface. A plot of relative EFs with respect to the emitting angle can thus be extracted.

Crack initiation mechanisms during very high cycle fatigue of Ni-based single crystal superalloys at high temperature

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.

Significant Reduction of Phase-Transition Hysteresis for Magnetocaloric (La 1-xCe x) 2Fe 11Si 2H y Alloys by Microstructural Manipulation

First-order magnetostructural phase transitions are inevitably accompanied by large hysteresis, which evokes non-ignorable energy losses a main challenge for the utilization of giant magnetocaloric effect (MCE) materials in the emerging magnetic cooling technology. In this work, we present a novel approach to reduce the hysteresis and simultaneously to remain the giant entropy change for La-Fe-Si-based MCE alloys by microstructural manipulation. The microstructure evolution is comprehensively studied by high angle annular dark field-scanning transmission electron microscope (HAADF-STEM), three-dimensional atom probe (3DAP) and geometric phase analysis (GAP). For the LaFe13-xSix system via co-doping of Ce and H atoms, we have observed the appearance of nano-sized grains (5 ~ 50 nm) that is totally different from the conventional compositions. Such refinement can be ascribed to the release of internal stresses caused by the inhomogeneous distribution of hydrogen atoms. With the increase of Ce content for (La1-xCex)2Fe11Si2Hy alloys, the value of hysteresis losses can be significantly reduced from 48.3 to 0.6 J kg-1. More importantly, the magnetostructural transition keeps the obvious first-order type, which leads to a large adiabatic temperature change of 2.2 K in 1.3 T upon 105 magnetic cycles, as well as a high refrigeration capacity of 89.4 J kg-1.

Direct Observation of Nanoparticulate Goethite Recrystallization by Atom Probe Analysis of Isotopic Tracers

Goethite (α-FeOOH) is dispersed throughout the earth's surface, and its propensity to recrystallize in aqueous solutions determines whether this mineral is a source or sink for critical trace elements in the environment. Under reducing conditions, goethite commonly coexists with aqueous Fe(II) (Fe(II)aq), which accelerates recrystallization by coupled electron transfer and atom exchange. Quantifying the amount of the mineral phase that exchanges its structural Fe(III) atoms with Fe(II)aq is complicated by recrystallization models with untested assumptions of whether, and to what extent, the recrystallized portion of the mineral continues to interact with the solution. Here, we reacted nanoparticulate goethite with 57Fe-enriched Fe(II)aq and used atom probe tomography (APT) to resolve the three-dimensional distribution of Fe isotopes in goethite at the sub nm scale. We found that the 57Fe tracer isotope is enriched in the bulk structure (tens of nanometers deep), with some samples having 57Fe penetration throughout at a level that is similar to the isotopic composition of Fe(II)aq. This suggests that some particles undergo near-complete recrystallization. In other cases, however, the distribution of 57Fe is more heterogeneous and generally concentrates near the particle periphery. Nanoparticle encapsulation and subsequent APT can hence capture hidden recrystallization mechanisms which are critical to predicting mineral reactivity in aqueous solutions.

Atomic-scale characterization of diffusion kinetics in Ru/Al multilayer thin films

The microstructure properties of reactive multilayers play a key role in their chemical reactions. Ru/Al multilayers are investigated in as-deposited and annealed conditions using atom probe tomography to probe the interdiffusion and early phase formation. Performing atom probe analysis was accompanied with difficulties stemming from weak adhesion of the bi-layers and reconstruction artifacts. Nonetheless, it was possible to obtain valuable qualitative information concerning diffusion kinetics. Asymmetric interdiffusion during deposition is found. Grain boundary diffusion is shown to have a major contribution on enhancing mass-transport across the layers as well as on the nucleation and growth of the RuAl6 intermetallic phase.

An atom probe analysis of self-assembled monolayers: A novel approach to investigate mixed and unmixed self-assembled monolayers (SAMs) on gold

Atom probe tomography was used to analyze self-assembled monolayer of octanethiol and (1H,1H,2H,2H) perfluoro decanethiol (PFDT) in a mixed and unmixed state on a gold surface. The first part of this report, we have analyzed the two SAMs with a direct comparison, whereas in the second part, a binary mixture of these two SAMs (with a 1:1 ratio) are analyzed. Au was taken as a metal substrate in both cases. It has been observed that, although having similar structure and belongs from the same SAMs group, the sequence of evaporation of both chains appeared significantly different molecular fractions in the first case, while in the mixed phase condition the two SAMs chains were arranged in a patchy domain by forming a small nanoscale cluster on the gold surface. Primarily, this study gives an overall understanding of evaporation sequence and surface morphology of the SAMs in an unmixed and mixed condition on the gold substrate, which may lead to the further design process in various applications.

Influence of Deep Cryogenic Treatment on Microstructure and Properties of 7A99 Ultra-High Strength Aluminum Alloy

The hardness, toughness, wear resistance, and fatigue behavior of materials can be improved through a deep cryogenic treatment (DCT). During this treatment, low temperatures (−100 °C to −196 °C) are maintained and then increased to room or higher. In this work, an indirect-extrusion plate of 7A99 ultra-high strength aluminum alloy was subjected to a T6 (peak aging) treatment and a T6-DCT treatment. The influence of the T6-DCT treatment on the mechanical properties, grain morphologies, precipitates, and atom–cluster distribution was investigated via tensile testing, electron backscatter diffraction, transmission electron microscopy, and three-dimensional atom probe analysis. The tensile strength (maximum: 705 deep cryogenic treatment), yield strength (maximum: 678 MPa), and elongation (maximum: 14%) of the T6-DCT-treated alloy were higher than those of the T6-treated alloy. Moreover, the T6-DCT treatment resulted in (i) grain size refinement and increased uniformity of the microstructure (homogeneous distribution of η’-MgZn2- and η-phase precipitates), and (ii) reduced segregation degree of Zn, Mg, and Cu atoms in the matrix (fraction of small atom clusters (sizes: 10–20 nm, 20–50 nm) increased, fraction of large clusters (size: &gt;1,000 nm) decreased). Therefore, DCT can refine the precipitates and increase the uniformity of the precipitate distribution, thereby improving the strength and plasticity of the alloy.

Atom Probe Tomography for 3D Structural and Chemical Analysis of Individual Proteins.

Determination of the 3D structure of proteins and other biomolecules is a major goal in structural biology, to provide insights to their biological function. Such structures are historically unveiled experimentally by X-ray crystallography or NMR spectroscopy, and in recent years using cryo-electron microscopy. Here, a method for structural analysis of individual proteins on the sub-nanometer scale using atom probe tomography is described. This technique offers a combination of high-resolution analysis of biomolecules in 3D, and the chemical sensitivity of mass spectrometry. As a model protein, the well-characterized antibody IgG is used. IgG is encapsulated in an amorphous solid silica matrix via a sol-gel process to provide the requisite support for atom probe analysis. The silica synthesis is tuned to resemble physiological conditions. The 3D reconstructions show good agreement with the protein databank IgG crystal structure. This suggests that the silica-embedding strategy can open the field of atom probe tomography to the analysis of biological molecules. In addition to high-resolution structural information, the technique may potentially provide chemical information on the atomic scale using isotopic labeling. It is envisaged that this method may constitute a useful complement to existing tools in structural biology, particularly for the examination of proteins with low propensity for crystallization.

Ti2NiCoSnSb - a new half-Heusler type high-entropy alloy showing simultaneous increase in Seebeck coefficient and electrical conductivity for thermoelectric applications

A new single phase high entropy alloy, Ti2NiCoSnSb with half-Heusler (HH) structure is synthesized for the first time by vacuum arc melting (VAM) followed by ball-milling (BM). The BM step is necessary to obtain the single phase. Local electrode atom probe (LEAP) analysis showed that the elements are homogeneously and randomly distributed in the HH phase without any clustering tendency. When the BM was carried out for 1 hour on the VAM alloy, microcrystalline alloy is obtained with traces of Sn as secondary phase. When BM was carried out for 5 h, single HH phase formation is realized in nanocrystalline form. However, when the BM samples were subjected to Spark plasma sintering (SPS), secondary phases were formed by the decomposition of primary phase. Nanostructuring leads to simultaneous increase in S and σ with increasing temperature. The micro (1 h BM-SPS) and nanocrystalline (5 h BM-SPS) alloys exhibited a power factor (S2σ) of 0.57 and 1.02 mWm−1K−2, respectively, at 860 K. The microcrystalline sample had a total thermal conductivity similar to bulk TiNiSn sample. The nanocrystalline alloy exhibited a ZT of 0.047 at 860 K. The microcrystalline alloy showed a ZT to 0.144 at 860 K, in comparison to the nanocrystalline alloy.

Relationship between the microstructure and properties of a peak aged Cu–Ni–Co–Si alloy

ABSTRACTThe microstructure and properties of a Cu–Ni–Co–Si alloy were investigated through high-resolution transmission electron microscopy and three-dimensional atom probe analysis after aging at 500°C for various times. Characterisation along different diffraction directions showed that precipitate morphology was disk shaped. The concentration profiles for Ni, Co, and Si were flattened in the precipitate interiors at levels of approximately 30, 35, and 35 at.-%, respectively. The co-segregation of Co was more significant than that of Ni and Si owing to the extremely low solubility in the matrix at room temperature. Theoretical calculations excellently agreed with the experimental data, which demonstrated that solid solution scattering and the Orowan bypass mechanism were the key reinforcement mechanisms for electrical and mechanical properties, respectively.HighlightsThe crystallography and morphology of precipitates in a peak aged Cu–Ni–Co–Si alloy are investigated in detail at different diffraction directions. Disk-shaped (Ni, Co)2Si precipitates are confirmed through 3DAP characterization.The distribution maps of Ni, Co, and Si alloying elements are investigated. Meanwhile, the segregation of the elements in the alloy are analyzed.The strengthening mechanism and conductivity curve of Cu–1.82 Ni–1.62 Co–0.86 Si alloy (wt-%) aged at 500°C for 2 h (peak aged state) are discussed, and theoretical calculations are in excellent agreement with the experimental data.

Atom Probe Microscopy of Strengthening Effects in Alloy 718

Polycrystalline Ni-based superalloys for aerospace and power generation applications are often precipitation hardened to achieve strengthening at elevated temperatures. Here, atom probe microscopy has become an essential tool to study the complex morphology of nanoscale precipitates. This study focuses on Alloy 718, which is hardened by semi-coherent, ordered γ' (Ni3(Al, Ti)) and γ″ (Ni3(Nb)) particles. According to previous research, these particles often occur as duplets or triplets with a stacking sequence dependent on prior processing. This creates various interfaces with a strong impact on the mechanical properties, highlighting the importance of quantitative studies which are challenging with electron microscopy. We present atom probe data reconstruction and analysis approaches particularly suited for precipitation hardened superalloys. While voltage atom probe allows for an accurate reconstruction, the acquired data volume is often limited. Laser-assisted atom probe provides statistically significant data, but the loss of crystallographic information requires correlation with voltage-mode datasets. We further describe an advanced iso-surface method where initially arbitrarily chosen concentration thresholds of Al + Ti for γ' and Nb for γ″ particles are optimized. Recognizing the importance of the precipitate stacking order, the different types of precipitate interfaces are quantified, and these methods may be applicable to other engineering alloys.

Fabrication of Atom Probe Tomography Specimens from Nanoparticles Using a Fusible Bi-In-Sn Alloy as an Embedding Medium.

We propose a new method for preparing atom probe tomography specimens from nanoparticles using a fusible bismuth-indium-tin alloy as an embedding medium. Iron nanoparticles synthesized by the sodium borohydride reduction method were chosen as a model system. The as-synthesized iron nanoparticles were embedded within the fusible alloy using focused ion beam milling and ion-milled to needle-shaped atom probe specimens under cryogenic conditions. An atom probe analysis revealed boron atoms in a detected iron nanoparticle, indicating that boron from the sodium borohydride reductant was incorporated into the nanoparticle during its synthesis.

The Laplace Project: An integrated suite for preparing and transferring atom probe samples under cryogenic and UHV conditions.

We present sample transfer instrumentation and integrated protocols for the preparation and atom probe characterization of environmentally-sensitive materials. Ultra-high vacuum cryogenic suitcases allow specimen transfer between preparation, processing and several imaging platforms without exposure to atmospheric contamination. For expedient transfers, we installed a fast-docking station equipped with a cryogenic pump upon three systems; two atom probes, a scanning electron microscope / Xe-plasma focused ion beam and a N2-atmosphere glovebox. We also installed a plasma FIB with a solid-state cooling stage to reduce beam damage and contamination, through reducing chemical activity and with the cryogenic components as passive cryogenic traps. We demonstrate the efficacy of the new laboratory protocols by the successful preparation and transfer of two highly contamination- and temperature-sensitive samples—water and ice. Analysing pure magnesium atom probe data, we show that surface oxidation can be effectively suppressed using an entirely cryogenic protocol (during specimen preparation and during transfer). Starting with the cryogenically-cooled plasma FIB, we also prepared and transferred frozen ice samples while avoiding significant melting or sublimation, suggesting that we may be able to measure the nanostructure of other normally-liquid or soft materials. Isolated cryogenic protocols within the N2 glove box demonstrate the absence of ice condensation suggesting that environmental control can commence from fabrication until atom probe analysis.

Thermal stability of nanocrystalline grains in Cu-W films

It has been hypothesized that introduction of proper solute atoms, which preferentially segregates to grain boundaries, can stabilize the nanoscale grain size at elevated temperatures. To explore this possibility, we synthesized pure Cu and Cu-3 at. % W nanocrystalline films using magnetron sputtering. Scanning electron microscopy and x-ray diffraction showed that while grains in pure Cu coarsen rapidly when annealed at 673 K, Cu-3% W maintains a stable microstructure when annealed at the same temperature, achieving grain size of 37 nm after 120 min annealing. Nanoindentation measurements gave results consistent with the grain size evolution: upon annealing at 673 K, the hardness of the pure Cu film decreases to 40% of its initial value after 20 min, while the Cu-3% W film retains more than 90% of its initial hardness even after 120 min. Atom probe analysis shows W segregation in the Cu-3% W film. Results of this study indicate that solute segregation can be used as a method to fabricate nanocrystalline materials that are stable against grain coarsening at elevated temperatures.