Atom Probe Tomography Measurements Research Articles

Encapsulation method for atom probe tomography analysis of nanoparticles

Open-space nanomaterials are a widespread class of technologically important materials that are generally incompatible with analysis by atom probe tomography (APT) due to issues with specimen preparation, field evaporation and data reconstruction. The feasibility of encapsulating such non-compact matter in a matrix to enable APT measurements is investigated using nanoparticles as an example. Simulations of field evaporation of a void, and the resulting artifacts in ion trajectory, underpin the requirement that no voids remain after encapsulation. The approach is demonstrated by encapsulating Pt nanoparticles in an ZnO:Al matrix created by atomic layer deposition, a growth technique which offers very high surface coverage and conformality. APT measurements of the Pt nanoparticles are correlated with transmission electron microscopy images and numerical simulations in order to evaluate the accuracy of the APT reconstruction.

Quantitative experimental determination of the solid solution hardening potential of rhenium, tungsten and molybdenum in single-crystal nickel-based superalloys

The solid-solution hardening potential of the refractory elements rhenium, tungsten and molybdenum in the matrix of single-crystal nickel-based superalloys was experimentally quantified. Single-phase alloys with the composition of the nickel solid-solution matrix of superalloys were cast as single crystals, and tested in creep at 980°C and 30–75MPa. The use of single-phase single-crystalline material ensures very clean data because no grain boundary or particle strengthening effects interfere with the solid-solution hardening. This makes it possible to quantify the amount of rhenium, tungsten and molybdenum necessary to reduce the creep rate by a factor of 10. Rhenium is more than two times more effective for matrix strengthening than either tungsten or molybdenum. The existence of rhenium clusters as a possible reason for the strong strengthening effect is excluded as a result of atom probe tomography measurements. If the partitioning coefficient of rhenium, tungsten and molybdenum between the γ matrix and the γ′ precipitates is taken into account, the effectiveness of the alloying elements in two-phase superalloys can be calculated and the rhenium effect can be explained.

Ion-irradiation-induced clustering in W–Re and W–Re–Os alloys: A comparative study using atom probe tomography and nanoindentation measurements

This study examines clustering and hardening in W–2at.% Re and W–1at.% Re–1at.% Os alloys induced by 2MeVW+ ion irradiation at 573 and 773K. Such clusters are known precursors to the formation of embrittling precipitates, a potentially life-limiting phenomenon in the operation of fusion reactor components. Increases in hardness were studied using nanoindentation. The presence of osmium significantly increased post-irradiation hardening. Atom probe tomography analysis revealed clustering in both alloys, with the size and number densities strongly dependent on alloy composition and irradiation temperature. The highest cluster number density was found in the ternary alloy irradiated at 773K. In the ternary alloy, Os was found to cluster preferentially compared to Re. The implications of this result for the structural integrity of fusion reactor components are discussed.

Nonlinear ultrasonic characterization of precipitation in 17-4PH stainless steel

This research is part of a broader effort to develop a nondestructive evaluation technique to monitor radiation damage in reactor pressure vessel steels, the main contributor being copper-rich precipitates. In this work, 17-4PH stainless steel is thermally aged to study the effects of copper precipitates on the acoustic nonlinearity parameter. Nonlinear ultrasonic measurements using Rayleigh waves are performed on isothermally aged 17-4PH. Results showed a decrease in the acoustic nonlinearity parameter with increasing aging time, consistent with evidence of copper precipitation from hardness, thermo-electric power, transmission electron microscopy, and atom probe tomography measurements.

Ni silicide study at the atomic scale: Diffusing species, relaxation and grooving mechanisms

A thin layer of Ge (1nm) deposited between a 30nm Ni film and a (100)Si substrate was used as a marker for the diffusing species during Ni silicide formation. In situ X-ray diffraction heat treatments with temperatures ranging from 150 to 400°C were performed to show the specific steps of silicide formation. Atom probe tomography (APT) enabled the diffusing species during the Ni deposition and during the formation of δ-Ni2Si and NiSi phases to be determined. APT measurements of the redistribution of Ge during the stress relaxation of δ-Ni2Si and during the formation of NiSi highlighted the mechanisms of stress relaxation and morphological degradation of NiSi.

Process-controlled suppression of natural aging in an Al–Mg–Si alloy

In this study natural aging of an Al–Mg–Si alloy was investigated using various quenching processes. Atom probe tomography and electrical resistivity measurements reveal that solute clustering during natural aging can be suppressed by interrupting quenching for 120 s at 160 °C. This phenomenon is elucidated by simulating the excess vacancy annihilation. Reduced frozen-in excess vacancy concentration after the interrupted quenching can explain this experimentally observed suppression of natural aging.

Structural and chemical characterization of the hardening phase in biodegradable Fe–Mn–Pd maraging steels

Abstract

On the Behavior of Yttria/Yttrium during Mechanical Alloying of a Fe - Y<sub>2</sub>O<sub>3</sub> Model Alloy System

Mechanical alloying (MA) is an established way to prepare nanocrystalline materials and metastable solutions of materials, which normally have no mutual solubility. This is also the case for oxide dispersion strengthened (ODS) steels with improved mechanical properties at elevated temperatures. It is known that a small addition of yttria (Y2O3) has a beneficial effect on high temperature strength and reduces the creep rate in mechanically alloyed ferritic steels by about six orders of magnitude. In this work we present an experimental study using atom probe tomography, X-ray photoelectron spectroscopy, and positron annihilation spectroscopy combined with first principles modeling focusing on the distribution and behavior of yttria in pure iron prepared by mechanical alloying. Atom probe tomography and X-ray photoelectron spectroscopy measurements as well as positron annihilation spectroscopy conducted on powder particles directly after milling have revealed that a predominantly fraction of the yttria powder dissolves in the iron matrix and Y atoms occupy convenient positions, such as vacancies or dislocations. This is supported by ab initio calculations demonstrating that the formation energy for Y substitutional defects in bcc-Fe is significantly lower in the close neighborhood of vacancies.

Atomic-scale characterization of prior austenite grain boundaries in Fe–Mn-based maraging steel using site-specific atom probe tomography

The embrittlement and de-embrittlement behavior of an Fe–10Mn–1Pd (wt.%) maraging steel upon isothermal aging at 500°C is related to microstructural changes at prior austenite grain boundaries (PAGBs). Site-specific atom probe tomography measurements were performed to analyze the local chemistry of the PAGBs. Tensile tests and hardness measurements were conducted of the ternary alloy and of a binary non-hardenable Fe–10Mn alloy for comparison. Isothermal aging of the binary steel led to a decrease in strength along with a considerable increase in uniform elongation. The Pd-containing alloy, on the other hand, showed significant age-hardening, and an embrittlement and de-embrittlement transition was revealed. Ductile behavior was observed in the initial as-quenched and over-aged states, but there was zero tensile elongation in the intermediate under- and peak-aged regions, where intergranular fracture along the PAGBs occurred. In the brittle peak-aged state a large number of small nanometer-sized particles rich in Mn and Pd formed inside the grains and decorated the PAGBs. The precipitates grew in size on prolonged aging. Mn segregation to the PAGBs was revealed; the Mn concentration level at the boundaries varied with aging time and was highest in the peak-aged condition. Embrittlement and de-embrittlement mechanisms are discussed and compared to these observations.

Reverse α′ → γ transformation mechanisms of martensitic Fe–Mn and age-hardenable Fe–Mn–Pd alloys upon fast and slow continuous heating

The mechanisms governing the reverse martensite (α′) to austenite (γ) transformation (α′→γ) and the effect of prior precipitation on the austenite reversion are investigated for martensitic Fe–Mn alloys containing 5 and 10wt.% Mn and their age-hardenable variants with the addition of 1wt.% Pd, respectively. Dilatometric experiments employing heating rates between 0.5 and 200Kmin−1, atom-probe tomography measurements on continuously heated specimens and thermo-kinetic simulations were performed. On fast heating (200Kmin−1), the α′→γ transformation appeared in a single stage and can be regarded as a partitionless and interface-controlled reaction. In comparison to the binary alloys, the transformation temperatures of the Pd-containing steels are considerably increased, due to precipitates which act as obstacles to migrating austenite/martensite interfaces. For low heating rates of 0.5 and 2Kmin−1, splitting of the α′→γ transformation into two consecutive stages is observed for both the binary and the ternary alloys. With the assistance of thermo-kinetic simulations, a consistent description of this phenomenon is obtained. The first transformation stage is associated with the decomposition of the martensite matrix into Mn-rich and Mn-deficient regions, and the austenite formation is dominated by long-range diffusion. In the second stage, the austenite reversion mechanism changes and the Mn-depleted regions transform in a predominantly interface-controlled mode. This is corroborated by the results for the ternary alloys. The precipitates mainly impede the austenite formation in the second stage, which occurs over a considerably wider temperature range compared to the binary alloys.

Forced atomic mixing during severe plastic deformation: Chemical interactions and kinetically driven segregation

Shear mixing of the ternary alloy system Ag–Cu–Ni during ball-milling and high-pressure torsion was investigated to elucidate the effects of chemical interactions on phase formation. First, ball-milling of pure Ni with homogeneous Ag67Cu33 alloy powders at room temperature (RT) was studied for average Ni atomic concentrations of 4%, 9%, 15% and 25%. Additional samples with an average composition of Ag50Cu25Ni25 were ball-milled at ∼−15°C or subjected to high pressure torsion at ∼−125°C. X-ray diffraction and atom probe tomography measurements showed that Cu largely transferred from the Ag–Cu alloy phase to the Ni-rich phase at all temperatures, but that Ag and Ni did not significantly intermix. The Cu concentration in the steady state, moreover, was surprisingly higher in the Ni-rich phase than in the Ag-rich phase, and it was further enriched at the interphase boundary, even at −125°C. High-resolution transmission electron microscopy revealed that the sizes of the Ni/Cu precipitates and the grain size of the Ag-rich matrix were reduced to a few nanometers during RT or cryo-ball-milling, which is much finer than those observed after ball-milling of Cu–Ag or Ni–Ag binary powders. These findings illustrate that chemical effects can play an important role in phase formation during severe plastic deformation, but they also show that other kinetic factors can influence the final microstructure as well.

Probing the local atomic structure of Sr-modified Al–Si alloys

Extended X-ray absorption fine structure (EXAFS) spectroscopy and atom probe tomography (APT) measurements were jointly used for the first time to probe the local structure around Sr atoms in Al–10%Sr master alloy and in Al–3%Si–0.04%Sr and Al–12.5%Si–0.04%Sr eutectic alloys in order to study the impact of trace levels of Sr on the morphological changes occurring in Al–Si binary eutectic alloy. EXAFS analysis shows the oxidation states of Sr is close to Sr0 in all the alloys studied. In the Al–10%Sr master alloy, Sr atoms tend to form the intermetallic compound Al4Sr. On the other hand, in Al–Si alloys, Sr atoms segregate towards Si-rich regions, preferentially bonding to Si atoms to form Al2Si2Sr-like clusters with a coordination environment consistent with bulk Al2Si2Sr. APT study reveals the presence of nanometer-sized Al-rich and Si-rich regions and support the EXAFS results. We speculate that addition of trace levels of Sr to Al–Si binary alloy system results in the initial formation of Al2Si2Sr-like clusters and the presence of such intermetallic clusters could play an important role such as poisoning of nucleation sites, thereby delaying the nucleation of eutectic phases and causing the morphological changes of the eutectic phases in the Al–Si alloys.

Improving aging and creep resistance in a dilute Al–Sc alloy by microalloying with Si, Zr and Er

Nanosized precipitates in an Al–0.06Scat.% alloy containing various microalloying additions were studied, with the goal of developing cost-effective aluminum alloys for high-temperature applications, using micro-hardness, electrical conductivity and atom-probe tomography measurements. Substituting 0.005at.% Er for the more expensive Sc maintains high ambient-temperature strength, and dramatically improves the high-temperature creep resistance, as anticipated from the increase in lattice parameter mismatch between the α-Al(fcc) matrix and the coherent L12-ordered Al3(Sc,Zr,Er) precipitates. A concentration of the slow-diffuser Zr as low as 0.02at.% is sufficient to provide coarsening resistance at 400°C (an homologous temperature of 0.72) for up to 66days by forming a Zr-enriched outer shell encapsulating the precipitates. Finally, adding 0.05at.% Si enhances ambient-temperature strength by increasing the number density of precipitates, while decreasing the homogenization and peak-aging heat-treatment times, which is caused by the Si atoms accelerating the Er and Sc diffusion kinetics. Si-containing alloys are also cost effective, owing to the existence of Si in commercial purity Al. But addition of Si reduces the precipitate coarsening resistance: the magnitude of this effect is, however, determined by the Si concentration.

Effect of nanoscale precipitation on strengthening of ferritic ODS Fe–Cr–Al alloy

The kinetics of Fe rich α and Cr rich α′ phase separation during aging of Fe–Cr–Al oxide dispersion strengthened alloys have been analysed with a combination of atom probe tomography and thermoelectric power measurements. The slight lattice parameter decrease measured by X-ray diffraction after thermal aging was associated with a lattice misfit between the emerging α and α′ domains. The elastic strain associated with this misfit and the different elastic moduli for the α and α′ phases would produce an increase in internal stresses that would retard the motion of dislocations and thus would be the main cause for the strengthening.

Atomistic insights into milling mechanisms in an Fe–Y2O3 model alloy

This experimental study combined with first principles modeling focuses on the distribution and behavior of yttria in pure iron powder particles prepared by mechanical alloying. A profound verification of the mechanism during milling is still missing in literature. Atom probe tomography and X-ray photoelectron spectroscopy measurements directly after mechanical alloying revealed yttria dissolved in the iron matrix, which later rearranged in clusters. These findings are corroborated by ab initio calculations demonstrating that the formation energy for Y substitutional defect in bcc-Fe is significantly lower in the close neighborhood of vacancies. X-ray diffraction measurements revealed that mechanical alloying for at least 12 hours caused a dramatic decrease in domain size and an extraordinary increase of defect density.

Atom probe tomography characterizations of high nickel, low copper surveillance RPV welds irradiated to high fluences

The Ringhals Units 3 and 4 reactors in Sweden are pressurized water reactors (PWRs) designed and supplied by Westinghouse Electric Company, with commercial operation in 1981 and 1983, respectively. The reactor pressure vessels (RPVs) for both reactors were fabricated with ring forgings of SA 508 class 2 steel. Surveillance blocks for both units were fabricated using the same weld wire heat, welding procedures, and base metals used for the RPVs. The primary interest in these weld metals is because they have very high nickel contents, with 1.58 and 1.66wt.% for Unit 3 and Unit 4, respectively. The nickel content in Unit 4 is the highest reported nickel content for any Westinghouse PWR. Although both welds contain less than 0.10wt.% copper, the weld metals have exhibited high irradiation-induced Charpy 41-J transition temperature shifts in surveillance testing. The Charpy impact 41-J shifts and corresponding fluences are 192°C at 5.0×1023n/m2 (>1MeV) for Unit 3 and 162°C at 6.0×1023n/m2 (>1MeV) for Unit 4. These relatively low-copper, high-nickel, radiation-sensitive welds relate to the issue of so-called late-blooming nickel–manganese–silicon phases. Atom probe tomography measurements have revealed ∼2nm-diameter irradiation-induced precipitates containing manganese, nickel, and silicon, with phosphorus evident in some of the precipitates. However, only a relatively few number of copper atoms are contained within the precipitates. The larger increase in the transition temperature shift in the higher copper weld metal from the Ringhals R3 Unit is associated with copper-enriched regions within the manganese–nickel–silicon-enriched precipitates rather than changes in their size or number density.

Positron Annihilation Study of Neutron-Irradiated Nuclear Reactor Pressure Vessel Steels and their Model Alloy: Effect of Purity on the Post-Irradiation Annealing Behavior

Post-irradiation annealing (PIA) behavior of irradiation-induced microstructural changes and hardening of two kind of A533B (first generation (1stGENS: 0.16 wt.% Cu) and second generation (2ndGENS: 0.04 wt.% Cu)) steels after neutron-irradiation of 3.9 × 1019 n cm–2 at 290 °C has been studied by positron annihilation spectroscopy, atom probe tomography and Vickers microhardness measurements. In the 1stGENS, clear two recovery stages are observed: (i) as-irradiated state to 450 °C and (ii) 450 to 600 °C. The first stage is due to annealing out of the most of irradiation-induced vacancy-related defects (VRDs), and the second stage corresponds to dissolving irradiation-induced Cu-rich solute nano-clusters (CRSCs). The experimental hardening is almost twice of the hardening due to the CRSCs estimated by Russell-Brown model below 350 °C, but almost the same as the estimation from 400 to 550 °C. In the 2ndGENS, the VRDs and non-Cu-rich solute nano-clusters (NCRSCs) recover at 450 °C. No CRSC has been formed even in all the annealing process. The experimental hardening is almost twice of the hardening estimated due to the NCRSCs by Russell-Brown model below 400 °C.

Phase-partitioning and site-substitution patterns of molybdenum in a model Ni-Al-Mo superalloy: An atom-probe tomographic and first-principles study

Atom-probe tomography (APT) and first-principles calculations are employed to investigate the partitioning of Mo in the γ(f.c.c.)-and γ′(L12)-phases in a model Ni-6.5Al-9.9Mo at. % superalloy. Mo is experimentally observed to partition preferentially to the γ(f.c.c.)-matrix, which is consistent with the smaller value of the γ(f.c.c.)-matrix substitutional formation-energy, with a driving force of 0.707 eV for partitioning as determined by first-principles calculations. APT measurements of the γ′(L12)-precipitate-phase composition and Al-, Mo-centered partial radial distribution functions indicate that Mo occupies the Al sublattice sites of the Ni3Al(L12) phase. The preferential site-substitution of Mo at Al sublattice sites is confirmed by first-principles calculations.

Effect of quantum well cap layer thickness on the microstructure and performance of InGaN/GaN solar cells

A two-step GaN barrier growth methodology was developed for InxGa1−xN/GaN multiple quantum well solar cells in which a lower temperature GaN cap layer was grown on top of the quantum wells (QWs) and then followed by a higher temperature GaN barrier layer. The performance of the solar cells improved markedly by increasing the low temperature GaN cap layer thickness from 1.5 to 3.0 nm. High-angle annular dark field scanning transmission electron microscopy and atom probe tomography measurements showed that increasing the GaN cap layer thickness improved the uniformity and increased the average indium content of the QWs.

Influence of recrystallization on phase separation kinetics of oxide dispersion strengthened Fe–Cr–Al alloy

The effect of different starting microstructures on the kinetics of Fe-rich (α) and Cr-rich (α′) phase separation during aging of Fe–Cr–Al oxide dispersion strengthened (ODS) alloys has been analyzed with a combination of atom probe tomography and thermoelectric power measurements. The results revealed that the high recrystallization temperature necessary to produce a coarse grained microstructure in Fe-base ODS alloys affects the randomness of Cr-atom distributions and defect density , which consequently affect the phase separation kinetics at low annealing temperatures.