Initially identified as a promising altermagnetic (AM) candidate, rutile RuO_2 has since become embroiled in controversy due to contradictory findings of modeling and measurements of the magnetic properties of bulk crystals and thin films. For example, despite observations of a bulk non-magnetic state using density functional theory, neutron scattering, and muon spin resonance measurements, patterned RuO_2 Hall bars and film heterostructures display magnetotransport signatures of magnetic ordering. Among the characteristics routinely cited as evidence for AM is the observation of exchange bias (EB) in an intimately contacted Fe-based ferromagnetic (FM) layer, which can arise due to interfacial coupling with a compensated antiferromagnet. Within this work, the origins of this EB coupling in Ru-capped RuO_2/Fe bilayers are investigated using polarized neutron diffraction, polarized neutron reflectometry, cross-sectional transmission electron microscopy, and super conducting quantum interference device measurements. These experiments reveal that the EB behavior is driven by the formation of an iron oxide interlayer containing Fe_3O_4 that undergoes a magnetic transition and pins interfacial moments within Fe at low temperature. These findings are confirmed by comparable measurements of Ni-based heterostructures, which do not display EB coupling, as well as magnetometry of additional Fe/Ru bilayers that display oxide-driven EB coupling despite the absence of the epitaxial RuO_2 layer. While these results do not directly refute the possibility of AM ordering in RuO_2 thin films, they reveal that EB, and related magnetotransport phenomena, cannot alone be considered evidence of this characteristic in the rutile structure due to interfacial chemical disorder.

A family of high entropy alloys rich in Al have been produced by arc-melting and their microstructures, predicted by thermodynamic modeling and observed experimentally, are reported along with the alloys’ density and Vickers microhardness. Alloy 1 ( Al_2.7 CrFeMnV) consisted of a polycrystalline single-phase BCC microstructure with relatively equiaxed grains. Alloy 2 ( Al_2.7 CrFeTiV) consisted of a polycrystalline BCC matrix containing G phase precipitates with complex dendritic shapes and a Laves phase with large aspect ratio lamellae. Alloy 3 ( Al_2.7 CrMnTiV) consisted of a BCC matrix containing lath-shaped AlTi L1_0 precipitates and regions with coherent cuboidal precipitates having an ordered BCC structure. This ordered BCC phase was not predicted by equilibrium thermodynamic calculations but a B2 phase was expected using a metastable prediction. The regions of Alloy 3 containing the ordered BCC cuboids showed evidence of enhanced Vickers microhardness. The alloys were characterized in the as-cast state by several techniques, including scanning and transmission electron microscopy, X-ray diffraction, atom-probe tomography, and electron backscatter diffraction, and these results were used to validate the thermodynamic predictions by Thermo-Calc.

Refractory multiprincipal element alloys (RMPEAs) are potential successors to incumbent high-temperature structural alloys, although efforts to improve oxidation resistance with large additions of passivating elements have led to embrittlement. RMPEAs containing group IV and V elements have a balance of properties including moderate ductility, low density, and the necessary formability. We find that oxidation of group IV-V RMPEAs induces hierarchical heterogeneities, ranging from nanoscale interstitial complexes to tertiary phases. This microstructural hierarchy considerably enhances hardness without indentation cracking, with values ranging between 12.1 and 22.6 GPa from the oxide-adjacent metal to the surface oxides, a 3.7 to 6.8× increase over the interstitial-free alloy. Our fundamental understanding of the oxygen influence on phase formation informs future alloy design to enhance oxidation resistance and obtain exceptional hardness while preserving plasticity.

A family of high entropy alloys rich in Al have been produced by arc-melting and their microstructures, predicted by thermodynamic modeling and observed experimentally, are reported along with the alloys' density and Vickers microhardness. Alloy 1 (Al2.7CrFeMnV) 2 consisted of a polycrystalline single-phase BCC microstructure with relatively equiaxed grains. Alloy 2 (Al2.7CrFeTiV) consisted of a polycrystalline BCC matrix containing G phase precipitates with complex dendritic shapes and a Laves phase with large aspect ratio lamellae. Alloy 3 (Al2.7CrMnTiV) consisted of a BCC matrix containing lath-shaped AlTi L1(0) precipitates and regions with coherent cuboidal precipitates having an ordered BCC structure. This ordered BCC phase was not predicted by equilibrium thermodynamic calculations but a B2 phase was expected using a metastable prediction. The regions of Alloy 3 containing the ordered BCC cuboids showed evidence of enhanced Vickers microhardness. The alloys were characterized in the as-cast state by several techniques, including scanning and transmission electron microscopy, X-ray diffraction, atom-probe tomography, and electron backscatter diffraction, and these results were used to validate the thermodynamic predictions by Thermo-Calc.

A promising high-strength Fe – 17.7Mn – 4.7Cr – 0.48C – 10Ni – 5Al – 4Cu wt.% Austenitic steel was solutionized, then aged for 3 or 10 h at 580°C producing a pronounced precipitation hardening response primarily due to the formation nanoscale NiAl precipitates. Density functional theory (DFT), molecular dynamics (MD), and discrete dislocation dynamics (DDD) calculations were combined with microstructural data from atom-probe tomography (APT), informing theoretical strengthening models to predict yield strength at different stages of precipitation as a function of NiAl size, volume fraction, and composition. These yield strength predictions were compared with experimental microhardness measurements of the various ageing conditions, including the peak microhardness of 490 HV, which corresponds to an estimated alloy yield strength of 1200 MPa. Comparing MD calculations with theoretical models showed that anti-phase boundary (APB) formation was the predominant barrier to dislocation motion posed by the NiAl precipitates. Using single dislocation particle strengthening models with NiAl APB energies calculated from DFT, good agreement was observed between DDD calculations and the 10 h peak-aged experimental measurements, while agreement with the 3 h experimental measurements required reducing the NiAl APB energy. These results demonstrate the utility of the undertaken approach integrating simulations and experiments across multiple length-scales, particularly the presented coarse-grained DDD simulation method towards modeling materials strengthened by very fine precipitates. The results further suggest that the observed NiAl precipitates may adopt an alternate crystal structure early in their formation.

Refractory high-entropy alloys (RHEAs), comprised of elements from Groups 4 (Ti, Zr, Hf), 5 (V, Nb, Ta), and 6 (Cr, Mo, W), are a new class of BCC alloys that offer particular promise for ultra-high-temperature applications.The first RHEAs were developed in 2010 by Senkov and colleagues at the U.S. Air Force Research Laboratory [1] and their ability to retain high strength up to 1600 °C [2] stimulated considerable research over the next decade [3].While these nascent alloys offer potentially superior hightemperature performance over existing Ni-based superalloys and conventional refractory alloys, they are often limited by poor oxidation resistance.Atom-probe tomography (APT) is uniquely capable of measuring atomic clustering and chemical short range order, offering insights into the stability of RHEAs in high-temperature environments.APT analysis of RHEAs is complicated, however, by their propensity for "multi-hits" [4], where two or more ions strike the atom-probe detector from a single laser or voltage pulse.These multi-hits introduce uncertainty and noise in the data, and can lead to inaccurate and biased compositional measurements.Here we report on the APT analysis of the phases formed within an equimolar NbTiZr RHEA during high-temperature exposure to oxygen.Fig. 1 shows ion correlation histograms [5] for the double ion hit events during APT analysis of this alloy.While a majority (74%) of ions are detected in single events, analysis of the multi-hit events can reveal important aspects of the APT data.In particular, the correlation histogram indicates that almost all of the background noise in the mass spectrum originates from delayed evaporation of Nb-Nb and Ti-Nb pairs.The ion correlation histograms also demonstrate the preponderance of NbO and ZrO molecular species detected by APT.Improving clarity of APT data from oxygen-rich RHEAs is an important step in understanding their behavior in the extreme environments of their target applications [6].

This study reports on the as-cast microstructures of AlCrFeMnV, AlCrFeTiV, and AlCrMnTiV equimolar multi-principal element alloys. Thermodynamic modeling predicts that these alloys solidify as BCC solid solutions, with additional phases forming at lower temperatures. We present the measured densities, the elastic modulus values measured by nanoindentation, and the Vickers microhardness of the alloys. These properties are correlated to the underlying microstructures, as measured by x-ray diffraction, scanning electron microscopy, and atom-probe tomography. The densities of the alloys range from 5.53 to 6.42 ×103 kgm−3. The elastic modulus of AlCrFeMnV, AlCrFeTiV, and AlCrMnTiV are 211, 194, and 175 GPa, respectively, and their Vickers microhardness values are 3.9, 6.1, and 5.7 GPa, respectively. After arc-melting, AlCrFeMnV and AlCrFeTiV are BCC solid solutions while AlCrMnTiV has two phases: a BCC phase and a minor hexagonal C14 Laves phase that is enriched in V and Cr.

Solid solution strengthening remains the basis for many industrial alloys, yet chemical short-range order (CSRO) can also play a significant role in the strengthening of alloys with appreciable alloying additions, such as in Austenitic stainless steels. In this work, we study the evolution of CSRO under various annealing temperatures and its role on the mechanical strength of a ternary Fe–12Ni–18Cr (at.%) alloy using molecular dynamics (MD) and Monte Carlo atomistic simulations and experimental measurements from mechanical microindentation and atom-probe tomography (APT). A general model that incorporates the role of CSRO into a solute–solution strengthening model is proposed, based on a family of analytical models due to Varvenne et al. (2016) which takes into account dislocation-solute misfit interaction, and a more recent extension due to Nag and Curtin (2020) which incorporates the role of solute–solute interactions during dislocation slip. Predictions of the modified model are validated against MD shearing simulations using two commonly used embedded atom model (EAM) interatomic potentials for Austenitic stainless steel alloys, and mechanical microindentation experiments on cast alloys of the same composition. Our results suggest that, while significant CSRO is predicted in the simulations, there is little experimental evidence in cast and annealed alloys analyzed by APT.

Volatile memristor devices are used in neuromorphic computing systems because they mimic the spike action of neurons in the brain [1].A physical artificial neural network can be constructed using threshold (i.e., rapid) switching phase change materials such as TiO 2 or NbO 2 to meet the non-linear and "short term memory" requirements for a reservoir computing architecture.In the case of NbO 2 , an induced current applied to the device causes localized Joule heating to enable a reversible insulator-to-metal phase transition between the rutile (R-NbO 2 ; E g = 1 eV) and tetragonal (T-NbO 2 ) crystal structures [2].This phase transition has been shown to be consistent with a Peierls transition model, in which conducting chains of Nb dimers are formed through a small reorganization of NbO 6 octahedra.Practical NbO 2 memristor devices can be constructed using atomic layer deposition (ALD) growth, followed by annealing to generate a final, transitionable, thin film [3].Since Nb-oxide has multiple metastable phases, several Nb 2 O 5 and NbO 2 phase transitions were observed during annealing to form a contiguous NbO 2 thin film [4].Introducing hydrogen plasma exposure in the ALD chamber between successive deposition layers can lead to intermediate NbO x compositions with unique memristor capability.Here we used correlated transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS), and atom-probe tomography (APT) on a set of ALD-deposited Nb-oxide thin films annealed to 800 °C and 1000 °C (for 20 min each), along with an unheated control sample.Cross-sections of each sample were prepared by FIB liftout methods with a ThermoFisher Helios G3 DualBeam FIB-SEM.The distribution of Nb-oxide phases within the thin films were measured by electron energy-loss spectroscopy (EELS) with an aberration Nion UltraSTEM 200-X, using a Gatan Enfinium EELS spectrometer modified with a Quantun Detectors MerlinEELS direct electron detector.EELS spectrum images were acquired at the Nb N-edge and plasmon (∼20 eV), O K-edge (∼532 eV), Nb L-edge (∼2370 eV).Fine-structure at these EELS edges provides sensitive information about Nb cation coordination and NbO 6 octahedra edge-vs.-cornersharing.Reference EELS spectra exist for the stable rutile R-NbO 2 and monoclinic H-Nb 2 O 5 phases [5], but none of the other metastable phases identified by Twigg et al. [4] in similar ALD-deposited thin films (e.g., T-NbO 2 , T-Nb 2 O 5 , and B-Nb 2 O 5 ).Distinct phases in spectrum images were identified using PCA and cluster analysis with HyperSpy 1.7. 3 [6] Characterization of the unheated sample reveals that three compositions of NbO x were deposited in a multilayer film by plasma-assisted ALD (Figure 1).The first layer (30 nm) at the bottom of the stack is crystalline and shows O-K and Nb-N spectra that match R-NbO 2 .Above this layer is 30 nm of amorphous film, where the O-K spectra of top 15 nm is consistent with the predominantly corner-sharing NbO 6 octahedra of Nb 2 O 5 phases.The O-K spectrum of the middle 15 nm layer does not match either of the two reference spectra, and the broad peak at 540 eV indicates that complex O coordination is present.The fully annealed sample retains the multilayer structure of the initial ALD thin film.Octahedra edge-vs.-cornersharing does not change after annealing, as O K-edge spectra for each layer does not change from the initial thin film.However, the peak distance between Nb L 2 and L 3 peaks does increase by 0.5 eV from crystallization of Nb 2 O 5 .These data indicate that alternate NbO x compositions can be stabilized for memristor devices by H plasma-assisted ALD growth.However, vesicle growth along interlayer boundaries observed by TEM may cause difficulties in memristor switching due to blocking the formation of contiguous phase transformation channels through the device [7].

Journal Article Insight Into Precipitation Synergy of Nano β-NiAl + Cu + Carbide in Austenitic Steel by Atom-Probe Tomography Get access Colin A Stewart, Colin A Stewart US Naval Research Laboratory, Washington, DC, USA Corresponding Author: colin.stewart@nrl.navy.mil Search for other works by this author on: Oxford Academic Google Scholar Richard W Fonda, Richard W Fonda US Naval Research Laboratory, Washington, DC, USA Search for other works by this author on: Oxford Academic Google Scholar Keith E Knipling, Keith E Knipling US Naval Research Laboratory, Washington, DC, USA Search for other works by this author on: Oxford Academic Google Scholar Patrick G Callahan, Patrick G Callahan US Naval Research Laboratory, Washington, DC, USA Search for other works by this author on: Oxford Academic Google Scholar Paul K Lambert Paul K Lambert US Naval Surface Warfare Center Carderock Division, Bethesda, MD, USA Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 28, Issue S1, 1 August 2022, Pages 312–313, https://doi.org/10.1017/S1431927622002021 Published: 01 August 2022

The joining process for oxide dispersion strengthened (ODS) alloys remains a key challenge facing the nuclear community. The microstructure and mechanical properties were characterized in the base material and friction stir welded ODS MA956 irradiated with 5 MeV Fe2+ ions from 400 to 500 degrees C up to 25 dpa. Nanoindentation was performed to assess changes in hardness and yield stress, and the dispersed barrier hardening (DBH) model was applied to described results. A combination of scanning transmission electron microscopy (STEM) and atom probe tomography (APT) were used to assess evolution of the microstructure including dispersoids, network dislocations and dislocation loops, nanoclusters, and solid solution concentrations. Overall, softening was observed as a result of increased dose, which was exacerbated at 500 degrees C. The formation and coarsening of new dispersoids was noted while nanoclusters tended to dissolve in the base material, and were not observed in the stir zone. Solute nanocluster evolution was identified as a primary driver of the changes in mechanical properties. Published by Elsevier B.V.

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Journal Article Analysis of Multiple-ion Events in Atom Probe Tomography Studies of MoNbTi and HfNbTaTiZr Refractory High Entropy Alloys Get access Patrick G Callahan, Patrick G Callahan Materials Science and Technology Division, US Naval Research Laboratory, Washington, DC, USA Corresponding author: patrick.callahan@nrl.navy.mil Search for other works by this author on: Oxford Academic Google Scholar Keith E Knipling, Keith E Knipling Materials Science and Technology Division, US Naval Research Laboratory, Washington, DC, USA Search for other works by this author on: Oxford Academic Google Scholar D Beaudry D Beaudry Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, MD, USA Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 28, Issue S1, 1 August 2022, Pages 702–703, https://doi.org/10.1017/S1431927622003270 Published: 01 August 2022

An integrated computational materials engineering (ICME) approach was used to design a fully Austenitic, Mn-stabilized steel strengthened by three nano-scale precipitate phases: β-NiAl, FCC-Cu, and M23C6 carbides. Thermo-Calc was used to predict the effects of Ni, Al, and Cu additions on phase stability in a Fe–17.7Mn–4.7Cr–0.48C (wt.%) base Austenitic composition. These predictions were verified experimentally by (1) synthesizing alloys of this Austenitic base alloy modified with Cu, Ni+Al, or Cu+Ni+Al, (2) measuring the mechanical properties by microindentation, and (3) characterizing the nano-scale microstructures by atom probe tomography (APT). After ageing at 580°C, selected based on Thermo-Calc modeling, only minor hardening was observed in samples modified with either Cu or Ni+Al. However, the alloy with combined additions of Cu together with Ni+Al exhibited significant hardening (490 HV, corresponding to an estimated σy ≈ 1200 MPa), which was attributed to the formation of FCC-Cu and β-NiAl nano-precipitates.

This paper provides a systematic examination of microstructure and corrosion behavior of additively manufactured 316L stainless steel after post-processing by surface finishing, isothermal heat treating, or hot isostatic pressing. The effects of isothermal heat treatments from 500 °C to 1300 °C and hot isostatic press processing from 1000 °C to 1200 °C were correlated to the evolution of microstructure, pore morphology and volume fraction, microhardness measurements, and corrosion behavior to reveal the different post-processing temperature regimes and their corresponding characteristics. In particular, this study found that the AM 316L microstructures and properties are stable up to nearly 800 °C. Higher temperatures eliminate the fine solidification structure, causing a drop in microhardness and corrosion resistance. Corrosion was primarily driven by the porosity content of the exposed surface and near-surface regions, which is unaffected by isothermal or HIP post-processing, and the corrosion was not significantly affected by recrystallization, which begins around 1050 °C and is nearly complete by 1200 °C. The amount of porosity was not markedly affected by isothermal post-processing, but could be significantly reduced by HIP processing.

Refractory multiprincipal element alloys (MPEAs) are promising materials to meet the demands of aggressive structural applications, yet require fundamentally different avenues for accommodating plastic deformation in the body-centered cubic (bcc) variants of these alloys. We show a desirable combination of homogeneous plastic deformability and strength in the bcc MPEA MoNbTi, enabled by the rugged atomic environment through which dislocations must navigate. Our observations of dislocation motion and atomistic calculations unveil the unexpected dominance of nonscrew character dislocations and numerous slip planes for dislocation glide. This behavior lends credence to theories that explain the exceptional high temperature strength of similar alloys. Our results advance a defect-aware perspective to alloy design strategies for materials capable of performance across the temperature spectrum.

The Sb concentration profile in an nBn photodetector containing an InAs/InAsSb type-II superlattice is collected and analyzed using atom probe tomography. A 3D reconstruction comprises the full composition of 31 periods. The Sb concentration profile is evaluated for the entire 31 period stack, as well as each individual period using segregation models from Muraki and Wood. Trends in the asymmetric Sb profile show a consistent non-negligible Sb concentration in the InAs layers and a lower Sb concentration in the InAsSb with respect to the target concentration.

Friction stir welding (FSW) has proven to be a viable technique for joining a wide variety of alloys. However, thick section welding of alpha and near-alpha Ti alloys has proven particularly challenging. Previous research at the Naval Research Laboratory using Ni markers in CP Ti friction stir welds indicated that elemental additions of Ni to the joint line can provide substantial benefits for improved weldability of these alloys. The current study surveys the effects of Ni and other elemental additions to CP Ti friction stir welds to determine their influence on the resultant weld microstructure, weld surface finish, and welding machine forces. These results reveal that Ni provides the most benefits for the concentrations examined, but other elements may also provide benefits at lower concentrations. The addition of these elements may improve the weldability and weld quality for FSW of CP Ti, enabling thick section welding of this and similar alloys.

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