Atom Probe Tomography Measurements Research Articles

Influence of natural aging and ramping before artificial aging on the microstructure of two different 6xxx alloys

In this study the effect of a ramp up to artificial aging (AA) temperature in combination with natural aging (NA) was analyzed for two different industrial aluminium alloys. The alloys differ in their Mg/Si ratio as well as their copper (Cu) content. Both alloys have, however, a comparable amount of solute and dispersoid forming elements. The peak aged conditions of the alloys were investigated. NA prior to AA reduces the strength as compared with directly aged regardless of ramping. The hardness up to peak is very similar for conditions with and without the ramp if NA is performed, but for the Cu rich alloy a clear increase in peak hardness could be measured when ramped from the supersaturated solid solution (SSSS). The peak strength plateau is shorter for the ramped samples regardless of the NA time. Investigations by (scanning) transmission electron microscopy found longer but fewer precipitates when ramped from the SSSS and an increased precipitate number density when ramped after NA. Fewer hybrid-type precipitates and less overaged precipitates, e.g. B’, and more L/C could be found for all ramped conditions. Atom probe tomography measurements showed that more solute is left in the matrix between the precipitates when ramped directly from the SSSS. This was related to a lower precipitate number density.

Characterization of structure and hardness at aging of the A319 type aluminum alloy with Sn trace addition

The effect of 0.1 wt% (0.02 at%) Sn trace addition on the structure and phase composition of Al-Si-Cu based alloy has been studied using thermodynamic calculations (Thermo-Calc software) and experimental techniques (transmission electron microscopy (TEM), atom probe tomography (APT), hardness and specific electrical conductivity measurements). The Vickers hardness measurement made after aging at 175 °C revealed that the peak hardness of the Sn-containing alloy is about 22% higher than that of the Sn-free alloy (135 HV vs 115 HV). Moreover, the peak hardness is achieved in a much shorter aging time (4 h vs 16 h). Considerably finer precipitates of the θ′ phase (average length less than 60 nm and thickness 3–5 nm) with a substantially higher number density are observed in the Sn-containing alloy. Tiny, rounded particles most of which are associated with the θ′ phase precipitates are also observed. Atom probe tomography analysis confirmed that the observed nanoparticles consist of tin. Quantitative APT analysis revealed that the number density of the copper-containing precipitates is at least twice that of the tin-containing ones (7.6·104 vs 3·104 µm-3). APT data also revealed a clear evidence of noticeable dissolution of Sn (average concentration from 0.05 to 0.10 at%) and silicon (average concentration from 2.2 to 2.8 at%) at the core of the θ′ phase precipitates, which is consistent with the fact that Sn and to a lesser extent Si act as catalysts in the nucleation of this phase. The data obtained using the proxigram technique suggest that Sn atoms show no tendency to segregate at the coherent θ′/(Al) heterophase interface upon aging, whereas localized Sn segregation is observed at the semi-coherent interface. Both Sn-free and Sn-containing alloys in their peak aged states have been subjected to uniaxial compression tests at an elevated temperature of 250 °С. The results suggest that the yield strength of the Sn-containing alloy is about 1.5 times that of the Sn-free alloy (250 MPa vs 175 MPa). The revealed difference is very big and testifies to a high potential of the new alloys as materials for new-generation engine parts.

Influence of the Ti Content on the Grain Stability and the Recrystallization Behavior of Nb‐Alloyed High‐Strength Low‐Alloyed Steels

To achieve higher strength and good hardenability and at the same time use the positive effects of thermomechanical controlled processing, the concept of Nb and Ti microalloyed steels is increasingly used for high‐strength low‐alloy (HSLA) steels with higher C contents. Herein, how the addition of Ti affects the grain growth and static recrystallization behavior of a Nb microalloyed HSLA steel with a C content of 0.23 wt% is investigated. For this reason, alloys with varying Ti and constant Nb content are produced and investigated by means of annealing and double‐hit deformation experiments. Atom probe tomography measurements reveal that the Nb concentration in the matrix decreases with increasing Ti content. Therefore, the static recrystallization behavior is steadily inhibited with decreasing Ti content, as more dissolved Nb is available for the formation of strain‐induced NbC precipitates. The annealing experiments show that the combined addition of Ti and Nb is most effective against grain coarsening, regardless of whether the Ti content is 90 or 180 ppm. To use the positive properties of Ti against grain coarsening and Nb to inhibit recrystallization, a middle content must be chosen when alloying Ti to HSLA steels with higher C content.

Quantitative analysis of mixed niobium-titanium carbonitride solubility in HSLA steels based on atom probe tomography and electrical resistivity measurements

Solubility of mixed niobium-titanium carbonitrides in commercially relevant High-Strength Low-Alloy (HSLA) steel was investigated by combined use of electrical resistivity measurements and APT after interrupted quenching from soaking temperatures between 950 and 1250 °C. Increasing electrical resistivity of the bulk material towards higher soaking temperatures was proportional to the nominal niobium addition which was varied between 0.002 and 0.022–0.043–0.085 wt.-%. Correlative APT analysis of the solutes in the steel matrix showed good agreement with electrical resistivity. Investigating numerous precipitate particles, APT also derived a precise composition for mixed niobium-titanium-carbonitrides which constitute the steel microstructure after casting/before soaking. The scavenging of microalloy elements by insoluble titanium nitrides was corrected by means of combined APT analysis of such precipitate and a quantitative image analysis for the estimation of the total volume fraction of titanium nitrides.For the first time, solute and precipitate composition together were used for solubility calculations of such mixed carbonitrides to derive an experimental solubility product. This was compared to solubility products of well-established simple carbides and nitrides and theoretical calculations of the solubility of multicomponent carbonitrides. The large discrepancy between experimentally derived and modelled solubility emphasizes the necessity of a robust methodology for the quantification of microalloy precipitation in HSLA steels.

3D nanoscale elemental mapping of precipitates in steel: Evaluation of analytical electron tomography and comparison to atom probe tomography

To identify different types of precipitates and their composition, analytical electron tomography analysis on a needle-shaped sample was performed. Three-dimensional chemical maps from each element in a high alloyed steel are acquired and the resulting elemental maps are jointly reconstructed. Since analytical electron tomography data suffers from noise, total generalized variation regularization is used to improve the reconstruction quality compared to conventional reconstruction techniques. We analyse the influence of regularization parameters on these reconstructions in terms of elemental quantification, and compare these results to atom probe tomography measurements done on another sample of the same material.

Microstructural, Chemical, and Crystallographic Investigations of Dynamic Strain-Induced Ferrite in a Microalloyed QT Steel

Dynamic strain-induced transformation (DSIT) enables the formation of fine-grained ferritic microstructures, which are well suited for cold forming processes in the as-rolled condition. In this work, the formation mechanism, chemical composition, and crystallographic orientation of DSIT ferrite were investigated in a micro-alloyed steel and compared to pre-eutectoid ferrite. High-resolution techniques, such as scanning transmission electron microscopy and atom probe tomography (APT), were used for the investigations. To generate DSIT ferrite and pre-eutectoid ferrite, different experimental routes were applied using a compression deformation dilatometer. The results show a large number of NbC precipitates within DSIT ferrite, and show that the formation of DSIT ferrite is accompanied with C diffusion and the formation of retained austenite. APT measurements revealed that the C- and Mn concentration in DSIT ferrite is higher compared to pre-eutectoid ferrite. The crystallographic orientation of DSIT ferrite was examined using electron backscatter diffraction. The crystallographic orientation of DSIT ferrite after the deformation route revealed that the <111> plane normals are parallel to the compression direction with the <110> directions pointing towards the radial direction of the compressed sample. The results suggest that the formation of DSIT ferrite is a displacive mechanism, accompanied by C diffusion.

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.

High conductivity n-Al0.6Ga0.4N by ammonia-assisted molecular beam epitaxy for buried tunnel junctions in UV emitters

Highly doped n-Al0.6Ga0.4N can be used to form tunnel junctions (TJs) on deep ultraviolet (UVC) LEDs and markedly increase the light extraction efficiency (LEE) compared to the use of p-GaN/p-AlGaN. High quality Al0.6Ga0.4N was grown by NH3-assisted molecular beam epitaxy (NH3 MBE) on top of AlN on SiC substrate. The films were crack free under scanning electron microscope (SEM) for the thickness investigated (up to 1 µm). X-ray diffraction reciprocal space map scan was used to determine the Al composition and the result is in close agreement with atom probe tomography (APT) measurements. By varying the growth parameters including growth rate, and Si cell temperature, n-Al0.6Ga0.4N with an electron density of 4×1019 /cm3 and a resistivity of 3 mΩ·cm was achieved. SIMS measurement shows that a high Si doping level up to 2×1020 /cm3 can be realized using a Si cell temperature of 1450 °C and a growth rate of 210 nm/hr. Using a vanadium-based annealed contact, ohmic contact with a specific resistance of 10−6 Ω·cm2 was achieved as determined by circular transmission line measurement (CTLM). Finally, the n-type AlGaN regrowth was done on MOCVD grown UVC LEDs to form UVC TJ LED. The sample was processed into thin film flip chip (TFFC) configuration. The emission wavelength is around 278 nm and the excess voltage of processed UV LED is around 4.1 V.

Atom probe specimen preparation methods for nanoparticles

Revealing the position of materials with chemical selectivity at atomic scale within functional nanoparticles is essential to understand and control their performance and cutting-edge atom probe tomography is a powerful tool to undertake this task. In this paper, we demonstrate three effective methods to prepare the needle-shaped specimens required for atom probe tomography measurements from nanoparticles of different sizes and provide examples of how atom probe can be used to provide data that is critical to their functionality. Samples measured include lithium-ion batteries (LIBs) cathode nanoparticles (300 – 500 nm), nickel-doped silicon dioxide (Ni@SiO2) catalytic nanoparticles (100 – 200 nm) and tin-doped copper (Sn@Cu) catalytic nanoparticles (<100 nm). The methods presented can be used to address the ongoing challenge of specimen preparation from particle samples for atom probe measurement, and they provide high quality data. These methods will broaden the application of atom probe tomography and will provide alternative option for researchers to assess the performance/structure of their functional nanomaterials.

Towards predictive simulations of spinodal decomposition in Fe-Cr alloys

Simulations of spinodal decomposition in an Fe-36 wt%Cr alloy at 773 K are performed by solving the non-linear Cahn–Hilliard equation, and the results are compared with atom probe tomography measurements. The influence of gradient energy coefficient, atomic mobilities and initial structure on the kinetics of spinodal decomposition is studied. It is shown that a proper initial structure, accounting for the thermal history above the miscibility gap, is crucial and enables predictive simulations of spinodal decomposition in Fe-Cr alloys.

Grain boundary segregation in Ni-base alloys: A combined atom probe tomography and first principles study

Grain boundary engineering (GBE) plays an important role in the design of new polycrystalline materials with enhanced mechanical properties. This approach has been shown to be very effective in design of Ni-base alloys, where grain boundary segregation is expected to play a central role in defining their mechanical behavior. In the present work, we apply a powerful combination of advanced experimental and theoretical methods to reveal the grain boundary chemistry of the 725 Ni-base alloy at the atomic level. The methods of investigation comprise atom probe tomography (APT) measurements and density functional theory (DFT) calculations. We also propose a way to cross-validate DFT and APT results in a DFT-based model approach for evaluation of the interfacial excess as a function of the heat treatment history of the material and its chemistry. Both theoretical and experimental methods are applied to a detailed analysis of the GB chemistry of three modifications of the 725 alloy and the results of this investigation are presented and discussed in detail.

Investigation of Cu Diffusivity in Fe by a Combination of Atom Probe Experiments and Kinetic Monte Carlo Simulation

In the present study, the diffusion coefficient of Cu in Fe was experimentally estimated from the precipitation kinetics down to 390°C. At this temperature, diffusion couples, which is a typical method to obtain diffusion coefficients, cannot be applied. The matrix Cu concentration and the number density of Cu precipitates in Fe–Cu alloy, which were the main parameters used to estimate the diffusion coefficient, were directly obtained using atom probe tomography. The temperature dependency of the diffusion coefficient of Cu in Fe estimated in the present study was more reliable than that obtained in a previous study, which also reported the diffusion coefficient of Cu in Fe from precipitation kinetics. This indicated that our estimation of the diffusion coefficient of Cu in Fe with atom probe tomography measurements yielded greater accuracy. In addition, the estimated diffusion coefficient of Cu in Fe tended to deviate to higher values from the extrapolated diffusion coefficient of Cu in Fe, which was obtained by diffusion couples, with decreasing temperature. This deviation is discussed by employing a kinetic Monte Carlo simulation.

Compensating Local Magnifications in Atom Probe Tomography for Accurate Analysis of Nano-Sized Precipitates.

Local magnification artifacts in atom probe tomography (APT) caused by multiphase materials with heterogeneous evaporation behavior are a well-known problem. In particular, the analysis of the exact size, shape, and composition of small precipitates is, therefore, not trivial. We performed numerical simulations of APT measurements to predict the reconstructed morphology of precipitates with contrasting evaporation thresholds. Based on a statistical approach that avoids coarse graining, the simulated data are evaluated to develop a model for the calculation of the original size of the precipitates. The model is tested on experimental APT data of precipitates with a higher and lower evaporation field in a ferritic alloy. Accurate sizes, proven by a complementary investigation by transmission electron microscopy, are obtained. We show further, how the size information can be used to obtain compositional information of the smallest precipitates and present a new methodology to determine a correct in-depth scaling of the APT reconstruction in case no complementary geometric information about the specimen exists or if no lattice planes are visible in the reconstruction.

Examining the creep strengthening nanoprecipitation in novel highly reinforced heat resistant steels

In this work, the creep strengthening nanoprecipitation in novel heat resistant steels strengthened by a high density of stable nanoprecipitates has been studied. The results show that the high number density of MX nanoprecipitates within the martensitic laths in these steels comes from the great amount of MX former elements present in their chemical compositions and the high dislocation density generated by the martensitic transformation that takes place during cooling after austenitization. These dislocations act as nucleation sites for these nanoprecipitates during the tempering applied after the austenitization and quenching. Atom probe tomography measurements show that these nanoprecipitates are rich in Nb, V, Cr and N for the HDSN1 and HDSN2 steels and V, Cr and N for the HDSN3 steel. The distribution of the nanoprecipitates within the martensitic laths suggests that the creep strengthening produced by the Nb, V, Cr and N nanoprecipitates in the HDSN1 and HDSN2 steels is more effective at pinning dislocations at high temperature than that obtained in the HDSN3 steel by the V, Cr and N nanoprecipitates. However, the high amount of W in the HDSN3 steel enhances the solid solution strengthening at high temperatures, resulting in a similar high temperature strength for all HDSN steels.

Segregation-assisted spinodal and transient spinodal phase separation at grain boundaries

Segregation to grain boundaries affects their cohesion, corrosion, and embrittlement and plays a critical role in heterogeneous nucleation. In order to quantitatively study segregation and low-dimensional phase separation at grain boundaries, here, we apply a density-based phase-field model. The current model describes the grain-boundary thermodynamic properties based on available bulk thermodynamic data, while the grain-boundary-density profile is obtained using atomistic simulations. To benchmark the performance of the model, Mn grain-boundary segregation in the Fe–Mn system is studied. 3D simulation results are compared against atom probe tomography measurements conducted for three alloy compositions. We show that a continuous increase in the alloy composition results in a discontinuous jump in the segregation isotherm. The jump corresponds to a spinodal phase separation at grain boundary. For alloy compositions above the jump, we reveal an interfacial transient spinodal phase separation. The transient spinodal phenomenon opens opportunities for knowledge-based microstructure design through the chemical manipulation of grain boundaries. The proposed density-based model provides a powerful tool to study thermodynamics and kinetics of segregation and phase changes at grain boundaries.

(Invited) Engineering SiGeSn Semiconductors for MIR and THz Opto-electronic Devices

Sn-containing group IV semiconductors (Si)GeSn represent a versatile platform to implement a variety of Si-compatible photonic, optoelectronic, and photovoltaic devices operating from SWIR to MIR wavelengths. This class of semiconductors provides two degrees of freedom, strain and composition, to tailor the band structure and lattice parameter thus enabling a variety of heterostructures and low-dimensional systems on a Si substrate.[1]In this presentation, the recent progress in the epitaxial growth and opto-electronic properties of metastable (Si)GeSn semiconductors with Sn contents above 15 at.% will be discussed. The growth of the (Si)GeSn multi-layer heterostructure is performed in a chemical vapor deposition (CVD) reactor on a Si wafer. By reducing the growth temperature, the Sn content in the alloy is increased, while preserving a high degree of crystal purity for the heterostructure in the topmost Sn-rich layer. Atom probe tomography (APT) measurements will be discussed to address the abruptness of the interfaces and the compositional profile across the GeSn multi-layer heterostructure.[2-4]By tailoring the strain relaxation in Ge0.83Sn0.17 room-temperature photoluminescence (PL) emission wavelength above 4.0 μm upon is achieved.[2,5] The absence of defect- and impurity-related emission and limited carrier losses into non-radiative recombination channels will be addressed using temperature-dependent PL measurements. Direct band gap absorption will be shown using transmission measurements performed at room-temperature, with energies closely matching the PL data. These observations will be discussed in the light of photocurrent measurements on GeSn photodetectors operating up to ~4.6 μm at room-temperature. The integration of p-i-n heterostructures in GeSn photodetectors will be investigated by correlating structural and opto-electronics properties of the fabricated devices.In addition, strategies to further extend the operational wavelength range of (Si)GeSn devices toward THz wavelengths will be discussed.[1] S. Wirths, et al., Prog. Cryst. Growth Charact. Mater. 62, 1 (2016)[2] S. Assali, et al., Appl. Phys. Lett. 112, 251903 (2018).[3] S. Assali, et al., J. Appl. Phys. 125, 025304 (2019).[4] S. Assali, et al., Appl. Phys. Lett. 114, 251907 (2019)[5] S. Assali, et al., arxiv:2004.13858

Phase Transition Kinetics in Austempered Ductile Iron (ADI) with Regard to Mo Content.

The phase transformation to ausferrite during austempered ductile iron (ADI) heat treatment can be significantly influenced by the alloying element Mo. Utilizing neutron diffraction, the phase transformation from austenite to ausferrite was monitored in-situ during the heat treatment. In addition to the phase volume fractions, the carbon enrichment of retained austenite was investigated. The results from neutron diffraction were compared to the macroscopic length change from dilatometer measurements. They show that the dilatometer data are only of limited use for the investigation of ausferrite formation. However, they allow deriving the time of maximum carbon accumulation in the retained austenite. In addition, the transformation of austenite during ausferritization was investigated using metallographic methods. Finally, the distribution of the alloying elements in the vicinity of the austenite/ferrite interface zone was shown by atom probe tomography (APT) measurements. C and Mn were enriched within the interface, while Si concentration was reduced. The Mo concentration in ferrite, interface and austentite stayed at the same level. The delay of austenite decay during Stage II reaction caused by Mo was studied in detail at 400 °C for the initial material as well as for 0.25 mass % and 0.50 mass % Mo additions.

In situ assessment of isochronal phase transformations in a lamellar Ti–5Al–5Mo–5V–3Cr–1Zr alloy using synchrotron X-ray diffraction

Phase transformations in a lamellar Ti–5Al–5Mo–5V–3Cr–1Zr (Ti-55531) alloy upon heating at continuous rates of 5, 20 and 100 °C/min has been studied by in situ high energy X-ray diffraction (HEXRD), aided by scanning and transmission electron microscopy, atom probe tomography and hardness measurements. During slow heating, the β phase has shown a behavior suggestive of additional transformations other than the continuous α → β decomposition observed at higher heating rates. Such behavior has been attributed to the precipitation of an embryonic isothermal ω phase. APT analyses have shown that no elemental partitioning occurs between the β matrix and the lamellar α phase, but it does between β and grain boundary α, suggesting that ω phase precipitates with solute rejection in regions outside lamellar colonies embedded in an enriched matrix. Above 500 °C, diffusion becomes significantly enhanced and α decomposition takes place.

Removal of hydrocarbon contamination and oxide films from atom probe specimens

Many materials science phenomena require joint structural and chemical characterization at the nanometer scale to be understood. This can be achieved by correlating electron microscopy (EM) and atom probe tomography (APT) subsequently on the same specimen. For this approach, specimen yield during APT is of particular importance, as significantly more instrument time per specimen is invested as compared to conventional APT measurements. However, electron microscopy causes hydrocarbon contamination on the surface of atom probe specimens. Also, oxide layers grow during specimen transport between instruments and storage. Both effects lower the chances for long and smooth runs in the ensuing APT experiment. This represents a crucial bottleneck of the method correlative EM/APT. Here, we present a simple and reliable method based on argon ion polishing that is able to remove hydrocarbon contamination and oxide layers, thereby significantly improving APT specimen yield, particularly after EM.

Effect of cold rolling on phase separation in 2202 lean duplex stainless steel

The effect of plastic deformation (cold rolling) on the thermal aging of a lean 2202 duplex stainless steel at intermediate temperatures (280 °C–450 °C) was studied using a combination of atom probe tomography and thermoelectric power measurements. In the undeformed condition, spinodal decomposition of ferrite was shown to be the prime aging process with an activation energy of 206 kJ/mol. Plastic deformation enhanced the aging and the austenite partly transformed into martensite. When no precipitation or phase transformation was observed in austenite, both ferrite and martensite decomposed. The spinodal decomposition kinetics of ferrite increased with increasing deformation rate. The FeCr-rich martensite also decomposed at 450 °C, via a nucleation and growth process, owing to a decrease of the Cr content in the matrix. Chromium nitrides, G-Phase, and Cu-rich precipitates were observed in the decomposed martensite.