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.

This work reports the electrochemical and X-ray tomography characterization of crevice corrosion for an additively manufactured stainless steel. The cell design includes a three electrode configuration with an integrated crevice former for full electrochemical control. The crevice former design utilized a spring to induce force upon the crevice former and tape to defined the crevice. The cell was evaluated with a series of potentiostatic holds from 0.05 to 0.95 V _SCE to observe crevice corrosion initiation and propagation across a range of current densities. In this proof of concept demonstration, X-ray computed microtomography (XCMT) reconstructions were used to observe electrochemical potential dependent crevice corrosion propagation that was compared with coulometry and postmortem profilometry. Volume loss calculations based on XCMT reconstructions displayed good correlation with electrochemical measurements. We demonstrate this approach as a promising non-destructive tool for crevice corrosion characterization, limited primarily by the inherent limitations of X-ray tomography systems.

We report a dual-component anomalous Hall effect (AHE) in polycrystalline Fe3Ga4 thin films grown on STO (001) and Al2O3 substrates. Systematic magnetic and magnetotransport measurements reveal an AHE consisting of positive and negative contributions that coexist across a wide range of temperatures and magnetic phases. We find that both magnitudes are nearly equal in the low-temperature ferromagnetic (FM) phase, but that their relative ratio is reduced upon heating through the antiferromagnetic helical spin-spiral state where they compete with metamagnetism and topological Hall effects, maintaining finite values at least up to the high-temperature FM phase.

Environmentally assisted cracking can significantly affect the performance of high strength alloys and limit material selection to minimize the risk of subcritical crack growth in service. UNS N07718 is widely used in marine service applications and under a variety of conditions, such as: alternate immersion, different levels of cathodic protection, and freely corroding galvanic couples, because of its demonstrated corrosion and fracture resistance in these environments. In this work we developed a representative model of the material microstructure including the metal grains, the material texture, and the precipitates along the grain boundaries and within the grains. The microstructural model was subjected to the boundary conditions identified at the notch root of a fracture mechanics sample and the results are used as input for a simulation of hydrogen diffusion from the surface of the notch, assuming the material has been introduced to a hydrogen producing environment. The diffusion of hydrogen was modeled by Fick's law and included both hydrostatic stress and mobile dislocation velocity as driving forces. The influence of immobile dislocations was also modeled to account for the irreversible trapping. The results show that hydrostatic stress and immobile dislocation trapping can significantly alter the highest concentration of hydrogen and its location within the microstructure towards the fracture process zone. Mobile dislocation velocity has a small influence in determining the hydrogen distribution near the fracture process zone.

The automated indexing and orientation determination of backscattered Kikuchi patterns is an essential step in electron backscattered diffraction (EBSD) analysis. Here a new Radon-transform-based algorithm is developed within the software package PyEBSDIndex , which features a number of key improvements over what has been traditionally available. The Radon convolutions use derivatives of Gaussian kernels that more closely match EBSD band profiles, which is combined with sub-pixel localization of the peaks in the Radon transform. Additionally, the weighted quaternion estimator algorithm (QUEST) is leveraged to provide the final estimation of the crystal orientation. The combination of these techniques allows for high-accuracy indexing and precise orientation determination, with tests on simulated patterns showing mean orientation errors as low as 0.037° and a 95% confidence level of 0.073°. Additional testing of the effect of pattern noise shows that PyEBSDIndex performs similarly to the spherical harmonic transform indexing methods except in the most extreme levels of low pattern quality. A test case of indexing a dual-phase Ti-6Al-4V EBSD map finds that PyEBSDIndex differentiates phases equivalently to the commercial Hough indexing solution, with orientation noise 75% lower than the commercial solution. Finally, it is shown that PyEBSDIndex , by performing the image processing calculations on the GPU, is able to analyze patterns at unprecedented speeds, in some cases at over 45 000 patterns s −1 , thereby providing sufficient speed for newer, high-speed detectors. PyEBSD Index is open source and available at https://github.com/USNavalResearchLaboratory/PyEBSDIndex .

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.

The automated indexing and orientation determination of backscattered Kikuchi patterns is an essential step in electron backscattered diffraction (EBSD) analysis. Here a new Radon-transform-based algorithm is developed within the software package PyEBSDIndex , which features a number of key improvements over what has been traditionally available. The Radon convolutions use derivatives of Gaussian kernels that more closely match EBSD band profiles, which is combined with sub-pixel localization of the peaks in the Radon transform. Additionally, the weighted quaternion estimator algorithm (QUEST) is leveraged to provide the final estimation of the crystal orientation. The combination of these techniques allows for high-accuracy indexing and precise orientation determination, with tests on simulated patterns showing mean orientation errors as low as 0.037° and a 95% confidence level of 0.073°. Additional testing of the effect of pattern noise shows that PyEBSDIndex performs similarly to the spherical harmonic transform indexing methods except in the most extreme levels of low pattern quality. A test case of indexing a dual-phase Ti-6Al-4V EBSD map finds that PyEBSDIndex differentiates phases equivalently to the commercial Hough indexing solution, with orientation noise 75% lower than the commercial solution. Finally, it is shown that PyEBSDIndex , by performing the image processing calculations on the GPU, is able to analyze patterns at unprecedented speeds, in some cases at over 45 000 patterns s −1 , thereby providing sufficient speed for newer, high-speed detectors. PyEBSD Index is open source and available at https://github.com/USNavalResearchLaboratory/PyEBSDIndex.

Non-destructive 3D characterization techniques are particularly important for materials synthesized by advanced manufacturing techniques that display disparity from their namesake identifiers (e.g. UNS S31600 or SS316). This is a particular concern for non-equilibrium materials, such as additively manufactured (AM) alloys, that often contain porosity, surface roughness, phases, microstructures and internal stress not exhibited in wrought or cast counterparts. This work examines chemical, electrochemical and microstructural evolution of AM and wrought SS316 under a crevice corrosion configuration. These results include chronoamperometric signatures, potentiometric sensors to track pH and X-ray computed micro-tomography 3D reconstructions during crevice corrosion. In-situ measurements are combined with initial and post-mortem SEM and EDS to better understand two AM316 materials that exhibit ennobled and more active behavior relative to wrought SS316. Local composition and phase heterogeneity at different length scales is shown to be a primary driver for contrasting crevice corrosion performance.

With the emergence of altermagnetism, a theoretical spin state has been revealed, which could address the long-standing scaling and frequency challenges of modern spintronics in high-performance computing. The experimental realization of high-speed and low-power spintronics has brought about an especially high renewed interest in the preparation and characterization of antiferromagnetic materials. Among the candidates for future altermagnetic applications is RuO2, which has been modeled to boast a large spin-orbit coupling-related band splitting, a high Neel temperature, and a robust inherent sheet conductivity. Investigation of this material for future altermagnetic applications necessarily involves the ready preparation of single-crystal films with controlled orientations, which to date is almost exclusively accomplished through the use of oxide molecular beam epitaxy or pulsed metal organic chemical vapor deposition. Within this report, the crystalline qualities of RuO2 films prepared through high-temperature direct current reactive sputtering are investigated. It is shown that utilization of TiO2 substrates with different orientations [(001), (101), and (110)] provides a means to template the orientation of RuO2 and that the crystalline quality of the deposited film depends strongly on the lattice match between the two materials. Through X-ray diffraction, atomic force microscopy, and electron back scatter diffraction measurements, it is shown that high-quality epitaxial films can be grown on all three substrates, while the (101)-oriented TiO2 presents the best template for the strain-relaxed and low-roughness single-crystal RuO2 unit cell. This fast and accessible process is enabling for the experimental investigation of the altermagnetic properties of this material, which is a requirement ahead of its application in future spintronics devices.

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].

Stochastic mesoscale inhomogeneity of material properties and material symmetries are investigated in a 3D-printed material. The analysis involves a spatially-dependent characterization of the microstructure in 316 L stainless steel, obtained through electron backscatter diffraction imaging. These data are subsequently fed into a Voigt–Reuss–Hill homogenization approximation to produce maps of elasticity tensor coefficients along the path of experimental probing. Information-theoretic stochastic models corresponding to this stiffness random field are then introduced. The case of orthotropic fields is first defined as a high-fidelity model, the realizations of which are consistent with the elasticity maps. To investigate the role of material symmetries, an isotropic approximation is next introduced through ad-hoc projections (using various metrics). Both stochastic representations are identified using the dataset. In particular, the correlation length along the characterization path is identified using a maximum likelihood estimator. Uncertainty propagation is finally performed on a complex geometry, using a Monte Carlo analysis. It is shown that mechanical predictions in the linear elastic regime are mostly sensitive to material symmetry but weakly depend on the spatial correlation length in the considered propagation scenario.

The goal of this work is to enable more rapid composition determination and mapping in materials by energy dispersive X-ray spectroscopy (EDS) using non-local means filtering of the collected spectra.Non-local means filtering is an image processing technique that is effective at denoising images by adjusting pixel intensities through a non-local smoothing kernel [1].This technique was recently modified to operate on multi-dimensional datasets for post-processing of electron backscattered diffraction patterns (EBSPs).It has proven to be an effective tool for denoising EBSPs by averaging an EBSP with patterns within a chosen distance where the weighting factor for the average is dependent on a similarity metric of the information between the two patterns [2].This method has been shown to significantly reduce the noise in the signal, but also to preserve sharp interfaces within the map.This work focuses on adapting this method to denoising hyperspectral EDS data, the so-called non-local spectra averaging (NLSPEC) in the examination of nickel aluminum bronze (NAB).Nickel aluminum bronzes are alloys that are Cu based with 6-13 wt % Al, up to 7 wt % Fe, up to 7 wt % Ni, and up to 1.5 wt % Mn [3].They are regularly used in marine applications that require a combination of strength, fatigue life, and corrosion resistance.The microstructure can be quite complex and consists of a Cu rich α matrix with some retained high temperature β phase, and several precipitate phases, κ I -κ IV which, depending on type, are rich in Al, Fe, or Ni [4,5].Some NAB castings are also quite large and have widely varying cooling rates depending on location in the casting.NAB samples were cooled at several controlled rates in a dilatometer in order to study the microstructure that is developed upon slow cooling.EDS spectra from 2D regions of the samples were collected with an EDAX Octane EDS detector attached to a Tescan Mira scanning electron microscope.The EDS spectra from each pixel in the 2D regions were denoised using NLSPEC method described above.Figure 1 shows a secondary electron image from a NAB sample that was cooled at approximately 0.15 °C/s, maps of the counts for the Cu Kα peak at 8.04 keV summed at +/-50 eV surrounding it as collected and processed by NLSPEC, and also shows the original and processed spectrum from the location indicated by the arrow in the Cu Kα maps.The example spectra are from the Cu-rich α phase.NLSPEC provides a method for quickly reducing the noise in EDS data, both the spectra and in maps generated from the spectra, while not introducing blurring of the interfaces, and erasing small features such as the κ II precipitates in the map.We believe it is a promising method for enabling faster data collection times while maintaining quality [6].

Advanced experimental and numerical approaches are being developed to capture the localization of plasticity at the nanometer scale as a function of the multiscale and heterogeneous microstructure present in metallic materials. These innovative approaches promise new avenues to understand microstructural effects on mechanical properties, accelerate alloy design, and enable more accurate mechanical property prediction. This article provides an overview of emerging approaches with a focus on the localization of plasticity by crystallographic slip. New insights into the mechanisms and mechanics of strain localization are addressed. The consequences of the localization of plasticity by deformation slip for mechanical properties of metallic materials are also detailed.

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.