Atom Probe Tomography Analysis Research Articles

Role of Zn on the rapid age-hardening in Mg-Ca-Zn alloys

Rapid age-hardening is critical for developing bake-hardenability in heat-treatable wrought magnesium alloys. This study investigates the role of Zn on the rapid age-hardening in Mg-0.3Ca-0.6Zn (at.%) alloy using positron annihilation lifetime spectroscopy (PALS), scanning transmission electron microscopy (STEM), and atom probe tomography (APT). Although trace Zn addition into the Mg-0.3Ca alloy accelerates the age-hardening response, PALS analysis indicates that it cannot be explained by excess quenched-in vacancies, which play a critical role in accelerating the kinetics in aluminum alloys. Instead, smaller trapping sites, i.e., open spaces, were detected in both samples. APT analysis reveals that the number density of Ca-Zn co-clusters in the Mg-0.3Ca-0.6Zn alloy increases in the early stage of aging, while that of Ca clusters tends to decrease in the Mg-0.3Ca alloy. Therefore, the rapid age-hardening in the Mg-0.3Ca-0.6Zn alloy is attributed to the formation of a large number of Ca-Zn co-clusters facilitated by open spaces.

The effect of ultrafast heating rate on the elemental distribution between phases in a low carbon steel

This work focuses on the effect of heating rate, i.e. conventional heating (CH) and ultrafast heating (UFH) rates on the elemental distribution between phases in a low carbon steel. Microstructural characterization was carried out using an Electron Backscatter Diffraction (EBSD) and Atom Probe Tomography (APT) technique. Nanohardness of individual microconstituents was measured. It is shown that the applied heat treatments result in the formation of multiphase microstructures consisting of a ferritic matrix with embedded martensite and retained austenite. The ferritic matrix of the CH material was fully recrystallized, whereas both recrystallized (coarser) and non-recrystallized (finer) ferritic grains were present in the matrix of the UFH material. APT analysis indirectly confirmed that recrystallized grains after both heat treatments have a lower carbon content, when the non-recrystallized grains after UFH have a higher carbon content. It correlates with the nanohardness results, i.e. non-recrystallized grains show higher hardness, while recrystallized grains have a lower hardness. The segregations of C and Mn atoms at the martensite/ferrite interface were observed after both treatments. It is hypothesized that the segregations are formed under negligible partitioning local equilibrium condition during CH, whereas the solute drag effect results in the formation of interface segregation during UFH.

The effect of hydrogen on the multiscale mechanical behaviour of a La(Fe,Mn,Si)13-based magnetocaloric material

Magnetocaloric cooling offers the potential to improve the efficiency of refrigeration devices and hence cut the significant CO2 emissions associated with cooling processes. A critical issue in deployment of this technology is the mechanical degradation of the magnetocaloric material during processing and operation, leading to limited service-life. The mechanical properties of hydrogenated La(Fe,Mn,Si)13-based magnetocaloric material are studied using macroscale bending tests of polycrystalline specimens and in situ micropillar compression tests of single crystal specimens. The impact of hydrogenation on the mechanical properties are quantified. Understanding of the deformation/failure mechanisms is aided by characterization with transmission electron microscopy and atom probe tomography to reveal the arrangement of hydrogen atoms in the crystal lattice. Results indicate that the intrinsic strength of this material is ~3–6 GPa and is dependent on the crystal orientation. Single crystals under compressive load exhibit shearing along specific crystallographic planes. Hydrogen deteriorates the strength of La(Fe,Mn,Si)13 through promotion of transgranular fracture. The weakening effect of hydrogen on single crystals is anisotropic; it is significant upon shearing parallel to the {111} crystallographic planes but is negligible when the shear plane is {001}-oriented. Atom probe tomography analysis suggests that this is associated with the close arrangement of hydrogen atoms on {222} planes.

Origins of the hydrogen signal in atom probe tomography: case studies of alkali and noble metals

Atom probe tomography (APT) analysis is being actively used to provide near-atomic-scale information on the composition of complex materials in three-dimensions. In recent years, there has been a surge of interest in the technique to investigate the distribution of hydrogen in metals. However, the presence of hydrogen in the analysis of almost all specimens from nearly all material systems has caused numerous debates as to its origins and impact on the quantitativeness of the measurement. It is often perceived that most H arises from residual gas ionization, therefore affecting primarily materials with a relatively low evaporation field. In this work, we perform systematic investigations to identify the origin of H residuals in APT experiments by combining density-functional theory (DFT) calculations and APT measurements on an alkali and a noble metal, namely Na and Pt, respectively. We report that no H residual is found in Na metal samples, but in Pt, which has a higher evaporation field, a relatively high signal of H is detected. These results contradict the hypothesis of the H signal being due to direct ionization of residual H2 without much interaction with the specimen’s surface. Based on DFT, we demonstrate that alkali metals are thermodynamically less likely to be subject to H contamination under APT-operating conditions compared to transition or noble metals. These insights indicate that the detected H-signal is not only from ionization of residual gaseous H2 alone, but is strongly influenced by material-specific physical properties. The origin of H residuals is elucidated by considering different conditions encountered during APT experiments, specifically, specimen-preparation, transportation, and APT-operating conditions by taking thermodynamic and kinetic aspects into account.

Atom probe analysis of electrode materials for Li-ion batteries: challenges and ways forward.

The worldwide development of electric vehicles as well as large-scale or grid-scale energy storage to compensate for the intermittent nature of renewable energy generation has led to a surge of interest in battery technology. Understanding the factors controlling battery capacity and, critically, their degradation mechanisms to ensure long-term, sustainable and safe operation requires detailed knowledge of their microstructure and chemistry, and their evolution under operating conditions, on the nanoscale. Atom probe tomography (APT) provides compositional mapping of materials in three dimensions with sub-nanometre resolution, and is poised to play a key role in battery research. However, APT is underpinned by an intense electric field that can drive lithium migration, and many battery materials are reactive oxides, requiring careful handling and sample transfer. Here, we report on the analysis of both anode and cathode materials and show that electric-field driven migration can be suppressed by using shielding by embedding powder particles in a metallic matrix or by using a thin conducting surface layer. We demonstrate that for a typical cathode material, cryogenic specimen preparation and transport under ultra-high vacuum leads to major delithiation of the specimen during the analysis. In contrast, the transport of specimens through air enables the analysis of the material. Finally, we discuss the possible physical underpinnings and discuss ways forward to enable shielding from the electric field, which helps address the challenges inherent to the APT analysis of battery materials.

Chemical characterization of Mg0.25Mn0.75-H(D) nanocomposites by Atom Probe Tomography (APT)

Hydrogen (H)- and deuterium (D)-loaded Mg0.25Mn0.75 nanocomposites were prepared by a ball-milling process and Atom Probe Tomography (APT) analyses were carried out in order to investigate the resultant nanostructure. Due to the immiscibility of the Mg−Mn system, non-uniform and interconnected Mg-rich domains with a thickness of several to 10 nm formed in both of the H- and D-loaded composites. Such complex morphology could be attributed to the severe cold-rolling effects induced by ball-milling. The inhomogeneous distribution of D observed by the APT analysis is likely the result of severe lattice distortion and multiple defects, which are cold-rolling effects. While the detection of D in the D-loaded composite was hampered significantly by the desorption of D during specimen preparation, D remained around the boundary of the Mg-rich domains/Mn-rich matrix. This suggests that the modulated intermixing of immiscible Mg and Mn atoms together with the ball-milling effect results in a disordered atomic arrangement around the boundary. This is assumed to be one of the contributing factors in the destabilization of Mg hydride observed in our previous works.

Quantitative Analysis of Nitrogen by Atom Probe Tomography Using Stoichiometric γ'-Fe4N Consisting of 15N Isotope.

The nitrogen deficiency in steels measured by atom probe tomography (APT) is considered to arise from the obscurement of singly charged dimer nitrogen ions (N2+) by the iron-dominant peak (56Fe2+) at 28 Da. To verify this by quantifying the amount of N2+ ions, γ′-Fe4N consisting of the 15N isotope was prepared on iron substrates by plasma nitriding using a nitrogen isotopic gas (15N2). Although considerable amounts of 15N2+ were observed at 30 Da without overlap with any iron peak, the observed nitrogen concentrations of γ′-Fe4N were clearly lower than the stoichiometric composition (19–20 at%), using both pulsed voltage and pulsed laser atom probes. The origin of the missing nitrogen, excluding nitrogen obscured by other ion species, was predicted to be the occurrence of neutral nitrogen or nitrogen gas molecules in field evaporation. The generation rate of iron nitride ions (FeN2+) for 15N was significantly lower than that for 14N in γ′-Fe4N, which affected the amount of the missing nitrogen. The isotope effect suggests that the isotopic ratio cannot always be determined from only one ion species among the multiple species observed in the APT analysis. We discuss the mechanism of the isotope effect in FeN2+ formation by field evaporation.

Grain boundary segregation and toughness of friction-stir-welded high-phosphorus weathering steel

Friction stir welding (FSW) of a high-P (0.3 wt%) weathering steel was performed at temperatures above A3 and below A1. Three-dimensional atom probe tomography analysis revealed that the grain boundaries in the stir zone formed below A1 (which had a considerably finer grain size) had higher P and lower C concentrations than those of the grain boundaries in the stir zone formed above A3. The effect of grain size on non-equilibrium grain boundary segregation during cooling after FSW was analyzed using the diffusion equation, assuming grain boundaries acted as sink sites for solutes. The fast diffusion of C is postulated to result in an almost equilibrium segregated state, such that its segregation decreases with decreasing grain size. In contrast, P diffuses only an extremely small distance, presumably leading to the grain-size-independent segregation of P. Therefore, the experimental result that the grain boundary concentration of P in the stir zone formed below A1 was larger than that in the stir zone formed above A3 is assumed to stem from the site competition effect between P and C and/or the enhanced P diffusion to grain boundaries due to dynamic recrystallization. Moreover, the segregation heat treatment was performed after the FSW to investigate the effect of the grain boundary segregation of P on the toughness. An investigation into the correlation between grain boundary segregation and toughness revealed that when the grain boundary concentration is less than approximately 7 at%, the effect of P segregation on intergranular fracture is relatively small and the toughness can be effectively improved by grain refinement.

Strengthening mechanisms of solid solution and precipitation at elevated temperature in fire-resistant steels and the effects of Mo and Nb addition

Fire–resistant properties of Mo-/Nb-added structural steels were evaluated using hot tension tests and constant-load tests. Hot tension tests showed that an increase in the Mo and Nb contents led to a slow decrease in the strength as the holding time increased at a high temperature (600 °C), enhancing the fire resistance. Transmission electron microscopy (TEM) and atom probe tomography analyses revealed that this improved fire resistance stemmed from the annihilation of dislocations at high temperatures, which was suppressed by the solid solution of Mo atoms and fine precipitation of Ti/Nb-enriched MC carbides. Meanwhile, an increase in Ti content decreased fire resistance via precipitation of coarse TiC particles, i.e., in-situ TEM observations showed that dislocations moved easily along the surface of coarse particles by climb, indicating that coarse particles did not play a role in disturbing dislocation movement during deformation at high temperatures. Next, constant-load tests were carried out with a constant load of 50% of the yield strength, while the temperature was increased linearly until failure. The results were in good agreement with the results of the hot tension tests; i.e., the failure temperature increased with an increase in the Mo and Nb contents, indicating improvement in fire resistance.

Reliability and failure analysis in power GaN-HEMTs during S-band pulsed-RF operating

This paper reports a reliability study on two technologies of AlGaN/GaN high-electron mobility transistors (AlGaN/GaN HEMTs) (Device “A” and Device “B”). A failure analysis study is conducted on devices stressed under real operating conditions for radar applications. The devices underwent pulsed-RF long ageing tests and after 11,000 h show a degradation in RF and DC performances (Drop of drain current and RF output power, pinch-off shift, decrease of the maximum of transconductance, lateral translation of transconductance, and increase of gate-lag and drain-lag). Hot electron effects are supposed to be the origin of the observed degradations and trapping phenomena within the passivation or GaN layers. Photon emission microscopy (PEM), Optical Beam Induced Resistance Change (OBIRCH), Electron Beam Induced Current (EBIC) measurements concur with this hypothesis. The three techniques reveal a non-uniform response and an inhomogeneous distribution along the gate fingers, in addition to the presence of some localized spots localized on the gate edge either on the drain side or on the source side. Spectral PEM analysis of these spots identifies a native defect that could be related to crystallographic defects such as dislocations or impurities. Atom probe tomography (APT) analysis on the two technologies of AlGaN/GaN HEMTs supports this hypothesis. APT results show the presence of some chemical impurities like carbon and oxygen. These impurities are in relatively significant concentrations in device “A” which could explain the high level of gate-lag and drain-lag in this device compared to device “B”.

Effect of the microstructural characteristics of die-cast ADC12 alloy controlled by Na and Cu on the mechanical properties of the alloy

The mechanical properties of a die-cast Al–Si–Cu alloy (DC ADC12) were improved using new processing techniques that induce grain refinement and precipitation hardening. Several gravity-cast ADC12 (GC ADC12) samples with different microstructural sizes were also evaluated to better understand the material properties of DC ADC12. The material properties of DC ADC12 indicate that the alloy has both advantages (fine grains) and disadvantages (defects). The mechanical properties of ADC12 can be improved by adding Na before the T5 process. The grain refinement of ADC12 was achieved by introducing Na using benign and inexpensive NaHCO3 rather than metallic Na. The Na elements including sodium oxide retained in the melt cause fine spherical eutectic structures to form in the spherical α-Al matrix by interrupting the Si phase growth. The phenomenon can be detected via a three-dimensional atom probe tomography analysis. Precipitation hardening of the die-cast Al alloy was achieved by subjecting it to T5 treatment and a large amount of Cu, replacing the T6 treatment, for 3 h at 175 °C, because the die casting process can be replaced with the solid solution process in T6 owing to its high cooling rate. Precipitation hardening occurred in the grain-refined DC ADC12-Na sample during the T5 process, which was verified via in situ observation of its microstructure using laser microscopy during the heating and cooling processes. Al2MgCu, CuAl2, and fine Si particles, which were different from the precipitates obtained using the T6 process, were produced in the spherical α-Al phase. The demonstrated techniques resulted in remarkable improvements in the ultimate tensile strength and fracture strain of DC ADC12.

Effect of boron segregation on bainite nucleation during isothermal transformation

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

Unprecedented age-hardening and its structural requirement in a severely deformed Al-Cu-Mg alloy

A gradient nanostructured surface layer was produced in an Al-Cu-Mg alloy (Al 2024) by means of a surface sliding friction treatment carried out at liquid nitrogen temperature. After aging treatment, unprecedented age-hardening was achieved at the surface layer, where microhardness values of >320 HV, more than twice that achievable by conventional precipitation hardening (150 HV), and well above the previously reported maximum value (280 HV) for this alloy, were achieved. Transmission electron microscopy and atom probe tomography analysis revealed that the formation of a nanograin structure at the deformed surface layer resulted in enhanced segregation of Cu and Mg during aging to narrowly spaced grain boundaries, with a corresponding suppression of precipitation. The enhanced grain boundary segregation instead of precipitation is considered to be responsible for the substantial age-hardening, although the underlying hardening mechanisms resulting from grain boundary segregation require further investigation.

Unprecedented Age-Hardening and its Structural Requirement in a Severely Deformed Al-Cu-Mg Alloy

A gradient nanostructured surface layer was produced in an Al-Cu-Mg alloy (Al 2024) by means of surface sliding friction treatment carried out at liquid nitrogen temperature. After aging treatment, unprecedented age-hardening was achieved at the surface layer, where microhardness values of >320 HV, more than twice that achievable by conventional precipitation hardening (150 HV), and well above the previously reported maximum value (280 HV) for this alloy, were achieved. Transmission electron microscopy and atom probe tomography analysis revealed that the formation of a nanograin structure at the deformed surface layer resulted in enhanced segregation of Cu and Mg during aging to narrowly spaced grain boundaries, with a corresponding suppression of precipitation. The enhanced grain boundary segregation instead of precipitation is considered to be responsible for the substantial age-hardening, although the underlying hardening mechanisms resulting from grain boundary segregation require further investigation.

Position artifacts in 3D reconstruction of plate‐shaped precipitates in steels depending on the analysis direction of atom probe tomography

Atom probe tomography (APT) analysis often produces artifacts in atomic position owing to the difference in evaporation fields in materials. The atomic density profiles of grain boundaries, plate‐shaped low‐field precipitates (cementite), and high‐field precipitates (vanadium carbide) in steels were investigated via two analyses in directions perpendicular and parallel to the boundaries and platelets. Although the perpendicular analysis indicated better quantitative performances in the composition analyses of segregating atoms at the grain boundary and the constituent atoms of the precipitates, an abnormal atomic density was observed around the interfaces of the precipitates. Geometric calculation using a two‐radius model based on the local magnification effect suggested that a spike structure at the front interface and a dip structure at the rear interface for lamellar cementite correspond to an overlap and a gap in the evaporated atoms from the cementite and matrix, respectively. This indicates that even in the perpendicular analysis, the precipitate constituent atoms and matrix atoms mixed in the depth direction. We discuss the origin and impact of the position artifacts in the perpendicular analyses of low‐ and high‐field plate‐shaped precipitates and also discussed the application limits of the model.

The effect of heat treatments on mechanical properties of M789 steel fabricated by laser powder bed fusion

The microstructural evolution during the aging heat treatment of a newly developed steel (designed to be employed in the laser powder bed fusion), known as M789, was evaluated. The electron-backscatter diffraction (EBSD) analysis suggests that the as-printed sample contains a fully martensitic phase with random crystallographic texture. The samples were then solutionized at 1000 °C for 1 h, followed by aging at a temperature range of 400–600 °C and holding times from 40 to 120 min. The maximum hardness and tensile strength were achieved at aging temperatures of 450 °C and 500 °C for 2 h, with the latter temperature generated the highest amount of Ni3Ti precipitates (η-phase) with no traces of reverted austenite. Then, the heat-treated sample underwent transmission electron microscopy (TEM) and atom probe tomography (APT) analyses, which revealed the nano-scale, plate-like and spherical precipitates with number densities of 1.11 × 1024 m−3 and 0.93 × 1024 m−3, respectively. These precipitates contributed to the increase in tensile strength, from 1019 MPa (as-printed) to 1798 MPa (heat-treated), a net increase of 779 MPa. As a consequence of enhanced strength, the elongation decreased from 16% to 9%. Generally, the results of all experiments are consistent with simulations; the errors were identified to be less than 5%.

Atom Probe Tomography Investigations of Ag Nanoparticles Embedded in Pulse-Electrodeposited Ni Films.

Atomic mapping of nanomaterials, in particular nanoparticles, using atom probe tomography (APT) is of great interest, as their properties strongly depend on shape, size, and composition. However, APT analyses of nanoparticles are extremely challenging, and there is an urgent need for developing robust and universally applicable sample preparation methods. Herein, we explored a method based on pulse electrodeposition to embed Ag nanoparticles in a Ni matrix and prepare APT specimens from the resulting composite film. By systematically varying the duty cycle during pulse electrodeposition, the dispersion and number density of the nanoparticles within the matrix was significantly enhanced as compared to DC electrodeposition. Several Ag nanoparticles were analyzed with APT from such samples. Shape distortions and biased compositions were observed for the Ag nanoparticles after applying a standard data reconstruction protocol. Numerical simulations of the field evaporation process showed that such artifacts were caused by a difference in the evaporation field of Ni and Ag and a local magnification effect. We expect such detrimental effects to be mitigated by a careful selection of the matrix material, matching the evaporation field of the nanoparticles. Furthermore, we anticipate that the method presented herein can be extended to a wider range of nanomaterials.

Structural and compositional analysis of (InGa)(AsSb)/GaAs/GaP Stranski\u2013Krastanov quantum dots

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

A new mechanism of surface phase formation on precipitation-hardening alloy under ion irradiation

Understanding surface modification of materials by Ga-ion milling has attracted growing research interests. This research reports that ion beam milling can induce rapid formation of θ-phase on the surface of a precipitation-hardening Al-Cu alloy. Atom probe tomography analysis reveals that an Al-Cu alloy tip develops a surface Cu-rich layer after electro-polishing. Cu-rich solute clusters in the 1-week naturally aged alloy sample are effective to assist the nucleation for rapid formation of surface θ-phase under the ion milling. The growth of the surface θ-phase is via a combination reaction of the surface Cu-rich layer with the Al matrix, rather than a simple decomposition of the Al-Cu alloy under the Ga ion irradiation. Partition and segregation behaviours of Ga with respect to the surface θ-phase are addressed by this investigation.

Quantitative APT analysis of solid transmutants in ferritic-martensitic steels after irradiation in SINQ targets: Experimental validation of neutronic calculations

The properties of materials applied in fission and fusion reactors and targets of spallation neutron sources can be greatly degraded by radiation-induced transmutants. A precise prediction of the production of transmutants is highly important for the safety assessment of these facilities. The prediction is normally made using neutronic calculations. Experimental validation of the calculation results is essentially necessary, but usually very challenging. In the present work, a quantitative analysis of transmutants in ferritic-martensitic (FM) steels F82H, Eurofer 97, and ODS Eurofer after irradiation at 9.6 and 19.6–21 dpa in the SINQ targets was performed using Atom Probe Tomography (APT). The results indicate that solid transmutants Ti, Sc, Ca, K, Cl, and S can be precisely measured and a gas element Ar could also be detected. The difference between the measured concentrations and that calculated with the MCNPX code is less than 30% for most of detectable nuclides of the transmutants. This work demonstrates that the neutronic calculations applied to STIP irradiation experiments are reliable.