Atom Probe Tomography Analysis Research Articles

Surface microstructure and mechanical properties of Ti-6Al-4V/Ag nanocomposite prepared by FSP

The surface microstructure and mechanical properties of Ti-6Al-4V/Ag nanocomposite processed by friction stir processing (FSP) are studied in this work. Scanning electron microscope (SEM) results show the gradient grain size of surface layer. The pile-up and tangle of dislocations and the deformation twins are observed by transmission electron microscopy (TEM). Atom probe tomography (APT) analyses reveal the presence of Ag-enriched nanoparticles in both the interiors and boundaries of α-Ti grains. Severe plastic deformation via FSP is responsible for considerable dislocations pile-up and tangle in the deformation bands. The enhancement in hardness and elastic modulus is attributed to the grain refinement and formation of much more deformation twinning. Moreover, Ag agglomeration could accelerate the recrystallization process and prevent grain coarsening, resulting in the higher hardness and the larger elastic modulus. This study confirms FSP is effective on surface modification of titanium alloys for improving the surface mechanical properties.

Atomic Resolution Imaging of the Cathode-Electrolyte Interface on a LiMn2O4 Electrode

Understanding the nature and composition of the cathode-electrolyte interface (CEI) layer in lithium-ion cells is a key element for the development of safer and longer lasting batteries. The CEI layer is a passivation layer formed, after electrolyte decomposition, on the surface of a positive electrode in lithium-ion batteries. Analogous to the solid electrolyte interface (SEI) on the anode, it is usually thinner (~5 nm vs 10-100 nm) and composed of poor ion-conductors that cause an additional resistance to the Li+ ions path [1]–[3]. Different surface sensitive techniques have been previously used to study the composition of the CEI layer and a model for its formation has been proposed [4]. However, its structure and composition are not fully understood yet and experimental techniques with sub-nanometer resolution are needed for an accurate characterization of this layer. Atom probe tomography (APT) is probably the best candidate, being a powerful technique for three-dimensional analysis of sub-nanometer structures and compositional variations that can achieve atomic resolution, along with chemical sensitivity for Li [5], [6]. In this work we successfully investigated for the first time the CEI layer formation on the surface of a LiMn2O4 (LMO) electrode using the APT technique. A commercial LMO cathode (from MTI) was calendared for ten days in a cell using standard 1M LiPF6 electrolyte in EC:DMC, and lithium metal as a counter and reference electrode. Three-electrode impedance measurements were performed at the OCV with regular intervals to observe and resolve the growth of the CEI layer on the cathode from the growth of the SEI layer on the lithium metal anode. After disassembling the cell, we prepared the sample for APT analysis and determined with atomic resolution the composition and structure of the CEI. A mixed layered- and mosaic-structure was observed, consisting of two inner Mn-rich layers adsorbed on the LMO surface and an external mosaic structure, mainly composed by LiF, Li2O and Li-carboxylates.

Atomic-scale investigations of isothermally formed bainite microstructures in 51CrV4 spring steel

Atomic-scale investigation was performed on 51CrV4 steel, isothermally held at different temperatures within the bainitic temperature range. Transmission electron microscopy (TEM) analysis revealed three different morphologies: lower, upper, and inverse bainite. Atom Probe Tomography (APT) analysis of lower bainite revealed cementite particles, which showed no evidence of partitioning of substitutional elements; only carbon partitioned into cementite to the equilibrium value. Carbon in the bainitic ferrite was found to segregate at dislocations and to form Cottrell atmospheres. The concentration of carbon remaining in solution measured by APT was more than expected at the equilibrium. Upper bainite contained cementite as well. Chromium and manganese were found to redistribute at the cementite-austenite interface and the concentration of carbon in the ferritic matrix was found to be lower than the one measured in the case of lower bainite. After isothermal treatments close to the bainite start temperature, another austenite decomposition product was found at locations with high concentration of Mn and Cr, resembling inverse bainite. Site-specific APT analysis of the inverse bainite reveals significant partitioning of manganese and chromium at the carbides and at the ferrite/martensite interfaces, unlike what is found at isothermal transformation products at lower temperatures.

Quantification Challenges for Atom Probe Tomography of Hydrogen and Deuterium in Zircaloy-4.

Analysis and understanding of the role of hydrogen in metals is a significant challenge for the future of materials science, and this is a clear objective of recent work in the atom probe tomography (APT) community. Isotopic marking by deuteration has often been proposed as the preferred route to enable quantification of hydrogen by APT. Zircaloy-4 was charged electrochemically with hydrogen and deuterium under the same conditions to form large hydrides and deuterides. Our results from a Zr hydride and a Zr deuteride highlight the challenges associated with accurate quantification of hydrogen and deuterium, in particular associated with the overlap of peaks at a low mass-to-charge ratio and of hydrogen/deuterium containing molecular ions. We discuss possible ways to ensure that appropriate information is extracted from APT analysis of hydrogen in zirconium alloy systems that are important for nuclear power applications.

Asymmetric atomic diffusion and phase growth at the Al/Ni and Ni/Al interfaces in the Al-Ni multilayers obtained by magnetron deposition

The interface morphology and interdiffusion of the Al-Ni multilayers were studied by transmission electron microscopy (TEM) and atom probe tomography (APT). Asymmetric atomic diffusion and phase growth at the interfaces formed by Al atoms deposited on solid Ni (Al/Ni) and Ni deposited on Al (Ni/Al) were observed. Due to the blocking effect of interface, Ni diffused only into the upper Al layer after annealing at 573 K for 3 h leading to slow reaction rate. APT analysis further reveals that Al could diffuse across the Ni/Al interface into the inner Ni layers and distribute heterogeneously. Some Al atoms diffused into the particle and located on the same crystal plane with Ni, which helps to interpret spontaneous and rapid phase transformation of this nanostructured reactive multilayer.

Study on complex precipitation kinetics in Cr- and Cu-added nitriding steels by atom probe tomography

Cr- and Cu-added steel sheets enable hardening of the surface region (the diffusion layer) by nitriding and simultaneously strengthening of the inner region due to thermally activated Cu precipitation during nitriding. Atom probe tomography (APT) analysis revealed that precipitate pairs, each composed of a Cu particle and CrN particle, were generated in the nitrided surface region. The influence of the complex precipitation of Cu and CrN on the precipitation kinetics and hardening properties was investigated by experimentally varying the initial state of Cu atoms in the steel by pre-aging before nitriding. It was found that Cu particles and CrN particles acted as heterogeneous nucleation sites for CrN and Cu, respectively, and the complex precipitation adversely affected the hardening of the nitrided surface region depending on the initial state of Cu. In addition, it was found that there were two types of precipitate pair forms (type-A/B), which were considered to be determined by the relation between the {001}-facet-size of the Cu particle and the size of the CrN nucleus in heterogeneous nucleation.

Enhancing Element Identification by Expectation-Maximization Method in Atom Probe Tomography.

This paper describes an alternative way to assign elemental identity to atoms collected by atom probe tomography (APT). This method is based on Bayesian assignation of label through the expectation-maximization method (well known in data analysis). Assuming the correct shape of mass over charge peaks in mass spectra, the probability of each atom to be labeled as a given element is determined, and is used to enhance data visualization and composition mapping in APT analyses. The method is particularly efficient for small count experiments with a low signal to noise ratio, and can be used on small subsets of analyzed volumes, and is complementary to single-ion decomposition methods. Based on the selected model and experimental examples, it is shown that the method enhances our ability to observe and extract information from the raw dataset. The experimental case of the superimposition of the Si peak and N peak in a steel is presented.

Visualizing the iron atom exchange front in the Fe(II)-catalyzed recrystallization of goethite by atom probe tomography

The autocatalytic redox interaction between aqueous Fe(II) and Fe(III)-(oxyhydr)oxide minerals such as goethite and hematite leads to rapid recrystallization marked, in principle, by an atom exchange (AE) front, according to bulk iron isotopic tracer studies. However, direct evidence for this AE front has been elusive given the analytical challenges of mass-resolved imaging at the nanoscale on individual crystallites. We report successful isolation and characterization of the AE front in goethite microrods by 3D atom probe tomography (APT). The microrods were reacted with Fe(II) enriched in tracer 57Fe at conditions consistent with prior bulk studies. APT analyses and 3D reconstructions on cross-sections of the microrods reveal an AE front that is spatially heterogeneous, at times penetrating several nanometers into the lattice, in a manner consistent with defect-accelerated exchange. Evidence for exchange along microstructural domain boundaries was also found, suggesting another important link between exchange extent and initial defect content. The findings provide an unprecedented view into the spatial and temporal characteristics of Fe(II)-catalyzed recrystallization at the atomic scale, and substantiate speculation regarding the role of defects controlling the dynamics of electron transfer and AE interaction at this important redox interface.

Correlative Atom Probe Tomography and Transmission Electron Microscopy Analysis of Grain Boundaries in Thermally Grown Alumina Scale.

We employed correlative atom probe tomography (APT) and transmission electron microscopy (TEM) to analyze the alumina scale thermally grown on the oxide dispersion-strengthened alloy MA956. Segregation of Ti and Y and associated variation in metal/oxygen stoichiometry at the grain boundaries and triple junctions of alumina were quantified and discussed with respect to the oxidation behavior of the alloy, in particular, to the formation of cation vacancies. Correlative TEM analysis was helpful to avoid building pragmatically well-looking but substantially incorrect APT reconstructions, which can result in erroneous quantification of segregating species, and highlights the need to consider ionic volumes and detection efficiency in the reconstruction routine. We also demonstrate a cost-efficient, robust, and easy-handling setup for correlative analysis based solely on commercially available components, which can be used with all conventional TEM tools without the need to modify the specimen holder assembly.

Analysis of Natural Rutile (TiO2) by Laser-assisted Atom Probe Tomography.

Since the introduction of laser-assisted atom probe, analysis of nonconductive materials by atom probe tomography (APT) has become more routine. To obtain high-quality data, a number of acquisition variables needs to be optimized for the material of interest, and for the specific question being addressed. Here, the rutile (TiO2) reference material 'Windmill Hill Quartzite,' used for secondary ion mass spectrometry U-Pb dating and laser-ablation inductively coupled plasma mass spectrometry, was analyzed by laser-assisted APT to constrain optimal running conditions. Changes in acquisition parameters such as laser energy and detection rate are evaluated in terms of their effect on background noise, ionization state, hit-multiplicity, and thermal tails. Higher laser energy results in the formation of more complex molecular ions and affects the ionization charge state. At lower energies, background noise and hit-multiplicity increase, but thermal tails shorten. There are also correlations between the acquisition voltage and several of these metrics, which remain to be fully understood. The results observed when varying the acquisition parameters will be discussed in detail in the context of utilizing APT analysis of rutile within geology.

Interpreting the Presence of an Additional Oxide Layer in Analysis of Metal Oxides–Metal Interfaces in Atom Probe Tomography

Atom Probe Tomography (APT) analysis of specimens embedded with metal oxide/metal leads to nonintuitive observations of a very thin layer of oxide at the interface due to oxygen migration under the...

Investigation by Mössbauer spectroscopy and atom probe tomography of the phase transformation of Nd-Fe-B alloys after high-energy ball milling

The main goal of this paper is to investigate the behavior of mechanically milled Nd2Fe14B powders. The effect of the milling on the magnetic properties of the Nd2Fe14B intermetallic is measured and is compared to the structural analysis performed by Mössbauer spectroscopy and atom probe tomography (APT). The high energy ball milling of Nd2Fe14B powders for up to 12 h induces the progressive disappearance of the hard magnetic behavior. Mössbauer spectroscopy and APT analyses have shown the decomposition of the Nd2Fe14B phase in the early stages of milling into an amorphous, disordered Nd-Fe-B phase that is rich in neodymium and in an α-Fe phase. The corresponding microstructure is very characteristic, with the formation of submicrometric pure iron particles, surrounded by an amorphous shell enriched in neodymium and in boron. By using APT, each step of the process can be identified. This high-resolution technique allows us obtaining a 3D mapping in real space of the chemical species (Nd, Fe, and B) on the atomic scale. Three-dimensional reconstructions of the nanostructure of the analyzed samples are realized from the data and show the formation of the Nd2Fe14B nanocomposite that is composed of α-Fe soft magnetic nanosized crystallites embedded in the Nd2Fe14B hard magnetic matrix. These results confirm the decomposition of the initial Nd2Fe14B phase. After 2 h of high-energy mechanical milling, Fe particles with diameters of up to 100 nm are observed. By increasing the milling time, the element intermixing and grain size reduction of the decomposition products can be observed.

Effect of Nb and Fe on damage evolution in a Zr-alloy during proton and neutron irradiation

Detailed analysis was carried out on proton and a neutron irradiated Nb-containing Zr-alloy to study the evolution of dislocation loop size and densities as well as the formation and evolution of irradiation-induced precipitation/clustering. The results obtained here have been contrasted against previously published work on a Nb-free Zr-alloy [1, 2] to investigate the mechanistic reason for the improved resistance to irradiation-induced growth of Nb-containing Zr alloys. The combined use of bright field scanning transmission electron microscopy, ultra-high-resolution energy dispersive spectroscopy and atom probe tomography analysis provides evidence of evenly distributed radiation-induced Nb clusters that have formed during the early stage of proton irradiation and Fe-rich nano-rods near Fe-containing second phase particles. The former seems to have a profound effect on <a> loop and subsequent <c> loop formation, keeping <a> loop size small but number density high while <c> loops seem to initially form at similar dose levels compared to a Nb-free alloy but <c> loop line density does not increase during further irradiation. It is hypothesized that the formation of the Nb nano-precipitates/clusters significantly hinders mobility and growth of <a> loops, resulting in a small size, high number density and limited ability of <a> loops to arrange along basal traces compared to Nb-free Zr-alloys. It is suggested that it is the limited <a> loop arrangement that slows down <c> loop formation and the root cause for the high resistance of Nb-containing Zr-alloys to irradiation-induced growth.

Interfacial structure and bonding mechanism of AZ31/carbon-fiber-reinforced plastic composites fabricated by thermal laser joining

Multi-material joining is attracting the attention of the automotive industry given its potential to realize lighter vehicles, and therefore fuel savings and reduced emissions. The aim of the present study was to understand the bonding mechanisms whereby metal and plastic/composites are joined and to improve the multi-material joint strength. In this study, the effect of thermal oxidation of Mg alloy sheets on the strengths of Mg–CFRP (carbon-fiber-reinforced plastic) lap joints prepared using laser-assisted metal and plastic joining technique was investigated. Characterization techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), micro-computed tomography (MCT), x-ray photoelectron spectroscopy (XPS), and atom probe tomography (APT) were used to study the underlying mechanisms of the thermal oxidation. The formation of bubbles, mechanical interlocking and chemical reactions at the joint interface were found to be the three key factors influencing the strength of joints. Thermal oxidation increased the joint strength significantly through the suppression of bubble formation, CFRP decomposition and the creation of mechanical interlocking effects at the joint interface. Moreover, MgCO3, MgO1+x, and Mg(OH)2 phases were detected by XPS analysis at the joints prepared using thermally oxidized Mg alloy sheets. The presence of the high O/Mg ratio phases was also confirmed by APT analysis. The formation of these phases confirmed the occurrence of chemical reactions between the MgO and CFRP matrix at the nanometer level, which are regarded as contributing to the increase in the joint strength.

Dendritic Oxide Growth in Zerovalent Iron Nanofilms Revealed by Atom Probe Tomography

Atom probe tomography (APT) analysis of chemically pure nanofilms of zerovalent iron (Fe(0), or ZVI) and their thermal oxide nano-overlayers reveals the presence of dendritic iron oxide features that extend from the oxide nano-overlayer surface into the ZVI bulk. The dendrites are observed by APT to be in the 5 nm × 10 nm size range and form quickly under natural atmospheric conditions. Their growth into the ZVI layer is, within the limit of our three month long study, self-limiting (i.e., their initial growth appears to quickly discontinue). The atomistic views presented here shed first light on the atmospheric corrosion process of Fe(0)-bearing engineered nanostructures and their surfaces in the limit of low bulk impurities. Possible roles of the newly identified oxidized iron dendrites are also discussed in the context of passivation processes limiting technological applications of Fe(0).

Double-shell structure of Al3(Zr,Sc) precipitate induced by thermomechanical treatment of Al–Zr–Sc alloy cable

A spheroidal Al3(Zr,Sc) precipitate with a double-shell structure, comprising a Sc-enriched core enveloped by a Zr-enriched inner shell and a Sc-enriched outer shell (∼9 nm in thickness), appears in an Al–0.2Zr–0.1Sc alloy cable after thermomechanical treatment. The average diameter of the spheroidal Al3(Zr,Sc) precipitate is approximately 80 nm. The double-shelled Al3(Zr,Sc) precipitate presents three different interfaces and is semi-coherent with the Al matrix. Atom probe tomography (APT) analyses further show that the outer shell of Al3(Zr,Sc) precipitate is Sc element enrichment. The electrical conductivity of Al–0.2Zr–0.1Sc alloy cable increases by 6.5 MS/m within the aging time from 0.2 to 100 h at 350 °C, with double-shelled Al3(Zr,Sc) precipitate.

Metal flux synthesis and atom probe tomography analyses of different intermetallic al–mg phases

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

Interdiffusion in as-deposited Ni/Ti multilayer thin films analyzed by atom probe tomography

The nanoscale design of metallic multilayer thin films is one crucial factor that greatly influences the kinetics, often inducing unusual phase transformations. Metastable or amorphous phases may directly form in as-deposited films of certain thicknesses, which is common for Ni/Ti multilayers. Atom probe tomography and X-ray diffraction analyses are performed here to study the interdiffusion and structural changes as a function of the bilayer thickness in Ni/Ti multilayers. The films are deposited by DC magnetron sputtering with near 50:50 compositions. Multilayers with 5 nm bilayer thickness are found to be highly intermixed, with compositions up to ∼25 at.% for both diffuser metals, inducing amorphization reactions during deposition.

Characterization of (Ti,Mo,Cr)C nanoprecipitates in an austenitic stainless steel on the atomic scale

Nanometer sized (Ti,Mo,Cr)C (MX-type) precipitates that grew in a 24% cold worked Ti-stabilized austenitic stainless steel (grade DIN 1.4970, member of the 15-15Ti austenitic stainless steels) after heat treatment were fully characterized with transmission electron microscopy (TEM), probe corrected high angle annular dark field scanning transmission electron microscopy (HR-HAADF STEM), and atom probe tomography (APT). The precipitates shared the cube-on-cube orientation with the matrix and were facetted on {111} planes, yielding octahedral and elongated octahedral shapes. The misfit dislocations were believed to have Burgers vectors a/6<112> which was verified by geometrical phase analysis (GPA) strain mapping of a matrix-precipitate interface. The dislocations were spaced five to seven atomic planes apart, on average slightly wider than expected for the lattice parameters of steel and TiC. Quantitative atom probe tomography analysis of the precipitates showed that precipitates were significantly enriched in Mo, Cr and V, and that they were hypostoichiometric with respect to C. These findings were consistent with a reduced lattice parameter. The precipitates were found primarily on Shockley partial dislocations originating from the original perfect dislocation network. These novel findings could contribute to the understanding of how TiC nanoprecipitates interact with point defects and matrix dislocations. This is essential for the application of these Ti-stabilized steels in high temperature environments or fast spectrum nuclear fission reactors.

Phase and microstructures in sputter deposited nanocrystalline Fe–Cr thin films

A series of FexCr1-x thin films (where X = 0 to 1) were magnetron sputter deposited. Their phase and microstructures were studied by transmission electron microscopy (TEM) and atom probe tomography (APT). The elemental films adopted the BCC structure with in-plane grain sizes (in the tens of nanometers). The Fe-49Cr (at.%) film grew as the high temperature σ-phase, with similarly shaped but even more refined grain sizes. Films of Fe-16Cr, Fe-24Cr, and Fe-72Cr deposited as the A2 structure. The Fe-24Cr and Fe-72Cr films revealed abnormal grain sizes that ranged from tens of nanometers to hundreds of nanometers. It is proposed that the thermodynamic driving force to phase separation contributed to the increased adatom mobility that resulted in varied grain sizes. These films also revealed a peculiar ‘star-burst’ contrast pattern within their grains which has been revealed to be from a modest crystallographic orientation shift contributed to the early onset of phase separation evident in the APT analysis. In the lower solute limit, the Fe-5Cr film short-range ordering of Cr (confirmed by APT) was noted and confirms prior modeling and diffuse neutron diffraction results of its presence at this composition. Upon depositing this film at 673 K, the short-range ordering was lost.