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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Kink bands promote exceptional fracture resistance in a NbTaTiHf refractory medium-entropy alloy

Single-phase body-centered cubic (bcc) refractory medium- or high-entropy alloys can retain compressive strength at elevated temperatures but suffer from extremely low tensile ductility and fracture toughness. We examined the strength and fracture toughness of a bcc refractory alloy, NbTaTiHf, from 77 to 1473 kelvin. This alloy’s behavior differed from that of comparable systems by having fracture toughness over 253 MPa·m 1/2 , which we attribute to a dynamic competition between screw and edge dislocations in controlling the plasticity at a crack tip. Whereas the glide and intersection of screw and mixed dislocations promotes strain hardening controlling uniform deformation, the coordinated slip of <111> edge dislocations with {110} and {112} glide planes prolongs nonuniform strain through formation of kink bands. These bands suppress strain hardening by reorienting microscale bands of the crystal along directions of higher resolved shear stress and continually nucleate to accommodate localized strain and distribute damage away from a crack tip.

36 MATERIALS SCIENCE↗
DOE OSTI2024

A macro-micro approach for identifying crystal plasticity parameters for necking and failure in nickel-based alloy haynes 282

Here, this work develops a two-scales macro-micro approach to address the challenge in calibrating crystal plasticity microstructural models when samples undergo necking prior to fracture. The crystal plasticity models are crucial for predicting the materials’ plastic deformation and failure at the microstructure level, identifying the materials’ intrinsic properties as well as investigating the microstructure-properties relationships. However, after necking occurs, the experimentally measured stress-strain curves fail to reflect the materials ‘true’ stress-strain behavior and cannot be directly fitted into crystal plasticity models. The proposed macro-micro approach employs a top-down strategy to address this challenge, which has been studied with experimental tests on precipitation-strengthened Ni-based superalloy Haynes® 282®. In this approach, a macro rate-dependent anisotropic plasticity model with Voce-type hardening and Rice-Tracey damage law is first utilized to model the deformation and failure of the tensile bar, and calibrated by matching the stress-strain curves, necking strain, and reduction of area. Especially, to match the testing results under different applied strain rates, the rate-sensitivity parameter m and saturation stress in the elasticity model are modified to incorporate dependence on the local strain rate. Then, the ‘true’ stress-strain behaviors are extracted from the necking zone of the macro-model, which are used to calibrate a micro-model with explicit microstructures and governed by an extended crystal plasticity law. The consistency between the micro-model and macro-model are enforced during calibration. The calibration outcomes from the crystal plasticity model elucidate the materials intrinsic properties for slip, hardening, and failure, which is vital for further investigations into the microstructure-properties relationship and for accurate prediction of the material behavior under various test and service conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Validating the Use of Gaussian Process Regression for Adaptive Mapping of Residual Stress Fields

Probing the stress state using a high density of measurement points is time intensive and presents a limitation for what is experimentally feasible. Alternatively, individual strain fields used for determining stresses can be reconstructed from a subset of points using a Gaussian process regression (GPR). Results presented in this paper evidence that determining stresses from reconstructed strain fields is a viable approach for reducing the number of measurements needed to fully sample a component’s stress state. The approach was demonstrated by reconstructing the stress fields in wire-arc additively manufactured walls fabricated using either a mild steel or low-temperature transition feedstock. Effects of errors in individual GP reconstructed strain maps and how these errors propagate to the final stress maps were assessed. Implications of the initial sampling approach and how localized strains affect convergence are explored to give guidance on how best to implement a dynamic sampling experiment.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Strain rate effects on the mechanical behavior of porous titanium with different pore sizes

High strain-rate (up to 6600 s –1 ) and quasi-static compression tests are conducted on a powder-sintered porous titanium with different pore sizes (mean: 30 μm and 120 μm). In situ X-ray imaging is implemented to characterize the pores-scale deformation dynamics. The yield strength as a function of strain rate exhibits two stages of rate sensitivity, and the transition occurs at 1600 s –1 . X-ray images show that pore compaction and strain localizations occur preferentially at pores oriented perpendicular to the loading direction under quasi-static loading, but become more random under high strain-rate loading as a result of higher driving force and plastic deformation nucleation rate. Here, the more homogeneous nucleation of plastic deformation contributes to the increased rate sensitivity beyond 1600 s –1 . At the same strain rate, the yield strength of porous Ti as well as strain field homogeneity decreases significantly with increasing pore size. The small pore spacing in fine-pored Ti reduces the degree of stress concentrations around pores. Therefore, the higher stress concentrations in coarse-pored Ti lead to an earlier yield of matrix around pores and thus a lower bulk yield strength.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Proppant embedment in coal and shale: Impacts of stress hardening and sorption

During methane production in CBM reservoirs, the influence of proppant embedment and permeability damage cannot be neglected – especially where the wall-rock is soft. Effective stresses are elevated during methane recovery, increasing both normal loading stress and confinement and simultaneously overprinting sorption-induced volumetric strains. Experiments and analytic modeling are conducted to define key mechanisms controlling these competitive effects. We independently measure overall sample compaction (external LVDT) and local strain (strain gauge) in the matrix to deconvolve proppant embedment in a propped fracture for different conditions of confining stress. The results show symptomatic behaviors of elastic (shale) and elastoplastic (coal) responses of embedment. Different from shale, the evolution of embedment is convex upwards with increased stress where indented depth increases more rapidly as loading stress increases under constant confinement. In addition, a stress-hardening effect is found to play a pivotal role in determining the characteristics of indentation, which are examined in terms of evolution profiles, deformation regimes, embedment slopes, curvatures, yield points and irreversible indentations. Based on the experimental observations a semianalytical model predicts indentation and the evolution of propped permeability under recreated in-situ stress conditions. A simplified case study is conducted to further illustrate the evolution of aperture and permeability of a propped fracture in CBM reservoirs. The modeling results suggest that proppant embedment is significantly overestimated if the variable stress-hardening (VSH) effect is neglected, especially when effective stress is large. Moreover, a decrease in indentation depth possibly occurs during late stage methane production, resulting in a reversal/recovery in fracture closure. This is because desorption-induced shrinkage becomes the predominant effect, causing an increase in aperture and a reduction in the indented volume of proppant. The resulting recovery in permeability implies that the propped coal fracture has the potential to optimally facilitate methane production as a pathway, even at high closure stresses generated by methane drainage.

01 COAL, LIGNITE, AND PEAT↗
DOE OSTI2024

Core/Shell-Like Localized Emission at Atomically Thin Semiconductor–Au Interface

Localized emission in atomically thin semiconductors has sparked significant interest as single-photon sources. Despite comprehensive studies into the correlation between localized strain and exciton emission, the impacts of charge transfer on nanobubble emission remains elusive. Here, in this paper, we report the observation of core/shell-like localized emission from monolayer WSe 2 nanobubbles at room temperature through near-field studies. By altering the electronic junction between monolayer WSe 2 and the Au substrate, one can effectively adjust the semiconductor to metal junction from a Schottky to an Ohmic junction. Through concurrent analysis of topography, potential, tip-enhanced photoluminescence, and a piezo response force microscope, we attribute the core/shell-like emissions to strong piezoelectric potential aided by induced polarity at the WSe 2 –Au Schottky interface which results in spatial confinement of the excitons. Our findings present a new approach for manipulating charge confinement and engineering localized emission within atomically thin semiconductor nanobubbles. These insights hold implications for advancing the nano and quantum photonics with low-dimensional semiconductors.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
NASA NTRS1979

Measuring unsteady pressure on rotating compressor blades

The capability for accurate measurement of unsteady pressure on the surface of compressor and fan blades during engine operation was established. Tests were run on miniature semiconductor strain gage pressure transducers mounted in several arrangements. Both surface mountings and recessed flush mountings were tested. Test parameters included mounting arrangement, blade material, temperature, local strain in the blade, acceleration normal to the transducer diaphragm, centripetal acceleration, and pressure. Test results showed no failures of transducers or mountings and indicated an uncertainty of unsteady pressure measurement of approximately + or - 6%, plus 0.1 kPa for a typical application.

Englund, D. R.↗
DOE OSTI2023

Microstructure and strengthening of Al-6Ce-3Ni-0.7Fe (wt%) alloy manufactured by laser powder-bed fusion

In this study, an Al-6Ce-3Ni-0.7Fe (wt%) alloy was fabricated via laser powder-bed fusion and its microstructure, thermal stability, tensile properties, and creep properties are investigated for two rapid-solidification rates with different eutectic spacings. For both faster- and slower-cooled states, the as-fabricated alloy mostly shows elongated grains with very fine eutectic networks (~40 nm lamellar width) comprising a high-volume fraction (~18 %) of intermetallic phases. Faster-cooled samples, however, display a less continuous eutectic network with finer spacing (~100 nm), leading to higher Orowan strengthening and therefore superior microhardness and yield stress up to ~350 °C. Upon aging (300 - 450 °C), micron-size Al 9 (Ni,Fe) 2 needle-like precipitates form within grains, and the Al 11 Ce 3 eutectic network spheroidizes, while retaining a submicron width and spacing, resulting in 33–37 % drop in microhardness after 144 h aging at 400 °C. Limited tensile ductility (~6 %) and creep ductility (~1-2 %) are measured for both faster- and slower-cooled alloys due to an inhomogeneous microstructure, where cavitation preferentially initiates at melt-pool boundaries, precipitate-free zones, and/or denuded zones, eventually leading to local fracture. Denuded zones, induced by stress, form due to diffusional flow with stress-dependent orientations, and are identified as microstructurally weak regions leading to strain localization. The present alloy does not show a significant difference in 300 °C creep resistance between: (i) tensile and compressive loading, (ii) faster- and slower-cooled samples, and (iii) continuous and spheroidized eutectics. Creep resistance at 300 °C is comparable to that of a Ce-richer Al-10.5Ce-3.1Ni-1.2Mn (wt%) alloy, despite the formation of denuded zones and needle-like Al 9 (Ni,Fe) 2 precipitates.

36 MATERIALS SCIENCE↗
NASA NTRS1979

Measuring unsteady pressure on rotating compressor blades

Miniature semiconductor strain gage pressure transducers mounted in several arrangements were studied. Both surface mountings and recessed flush mountings were tested. Test parameters included mounting arrangement, blade material, temperature, local strain in the acceleration normal to the transducer diaphragm, centripetal acceleration, and pressure. Test results show no failures of transducers or mountings and indicate an uncertainty of unsteady pressure measurement of approximately + or - 6 percent + 0.1 kPa for a typical application. Two configurations were used on a rotating fan flutter program. Examples of transducer data and correction factors are presented.

Englund, D. R.↗
NASA NTRS2004

Synchrotron X-ray Microdiffraction Analysis of Proton Irradiated Polycrystalline Diamond Films

X-ray microdiffraction is a non-destructive technique that allows for depth-resolved, strain measurements with sub-micron spatial resolution. These capabilities make this technique promising for understanding the mechanical properties of MicroElectroMechanical Systems (MEMS). This investigation examined the local strain induced by irradiating a polycrystalline diamond thin film with a dose of 2x10(exp 17) H(+)per square centimeter protons. Preliminary results indicate that a measurable strain, on the order of 10(exp -3), was introduced into the film near the End of Range (EOR) region of the protons.

Newton, R. I.↗
NASA NTRS1979

Analysis of misalignment in the tension test

Many experimenters fail to appreciate the significance of test system alignment during a tensile test. Poor alignment can increase data scatter and significantly influence some of the test results. In this paper a comprehensive analytical study of the misalignment problem in the plastic strain range is presented and errors caused by misalignment estimated. The results show that the stress-strain curve is not significantly affected by misalignment. However, the strains at the outermost fibers of the specimen cross-section are strongly affected by misalignment. Therefore, the effect of misalignment is most important when properties sensitive to local strain concentrations are being investigated.

Wu, H. C.↗
DOE OSTI2021

Interstitial Nature of Mn 2+ Doping in 2D Perovskites

Halide perovskites doped with magnetic impurities (such as the transition metals Mn 2+ , Co 2+ , Ni 2+ ) are being explored for a wide range of applications beyond photovoltaics, such as spintronic devices, stable light-emitting diodes, single-photon emitters, and magneto-optical devices. However, despite several recent studies, there is no consensus on whether the doped magnetic ions will predominantly replace the octahedral B-site metal via substitution or reside at interstitial defect sites. Here, by performing correlated nanoscale X-ray microscopy, spatially and temporally resolved photoluminescence measurements, and magnetic force microscopy on the inorganic 2D perovskite Cs 2 PbI 2 Cl 2 , we show that doping Mn 2+ into the structure results in a lattice expansion. The observed lattice expansion contrasts with the predicted contraction expected to arise from the B-site metal substitution, thus implying that Mn 2+ does not replace the Pb 2+ sites. Photoluminescence and electron paramagnetic resonance measurements confirm the presence of Mn 2+ in the lattice, while correlated nano-XRD and X-ray fluorescence track the local strain and chemical composition. Density functional theory calculations predict that Mn 2+ atoms reside at the interstitial sites between two octahedra in the triangle formed by one Cl – and two I – atoms, which results in a locally expanded structure. These measurements show the fate of the transition metal dopants, the local structure, and optical emission when they are doped at dilute concentrations into a wide band gap semiconductor.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Edge-Confined Excitons in Monolayer Black Phosphorus

Quantum confinement of two-dimensional excitons in van der Waals materials via electrostatic trapping, lithographic patterning, Moiré potentials, and chemical implantation has enabled significant advances in tailoring light emission from nanostructures. While such approaches rely on complex preparation of materials, natural edges are a ubiquitous feature in layered materials and provide a different approach for investigating quantum-confined excitons. Here, we observe that certain edge sites of monolayer black phosphorus (BP) strongly localize the intrinsic quasi-one-dimensional excitons, yielding sharp spectral lines in photoluminescence, with nearly an order of magnitude line width reduction. Through structural characterization of BP edges using transmission electron microscopy and first-principles GW plus Bethe–Salpeter equation (GW-BSE) calculations of exemplary BP nanoribbons, we find that certain atomic reconstructions can strongly quantum-confine excitons resulting in distinct emission features, mediated by local strain and screening. Here we observe linearly polarized luminescence emission from edge reconstructions that preserve the mirror symmetry of the parent BP lattice, in agreement with calculations. Furthermore, we demonstrate efficient electrical switching of localized edge excitonic luminescence, whose sites act as excitonic transistors for emission. Localized emission from BP edges motivates exploration of nanoribbons and quantum dots as hosts for tunable narrowband light generation, with future potential to create atomic-like structures for quantum information processing applications as well as exploration of exotic phases that may reside in atomic edge structures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2005

Structural Anomaly Detection Using Fiber Optic Sensors and Inverse Finite Element Method

NASA Langley Research Center is investigating a variety of techniques for mitigating aircraft accidents due to structural component failure. One technique under consideration combines distributed fiber optic strain sensing with an inverse finite element method for detecting and characterizing structural anomalies anomalies that may provide early indication of airframe structure degradation. The technique identifies structural anomalies that result in observable changes in localized strain but do not impact the overall surface shape. Surface shape information is provided by an Inverse Finite Element Method that computes full-field displacements and internal loads using strain data from in-situ fiberoptic sensors. This paper describes a prototype of such a system and reports results from a series of laboratory tests conducted on a test coupon subjected to increasing levels of damage.

Quach, Cuong C.↗
DOE OSTI2025

Unveiling nano-scale crystal deformation using coherent X-ray dynamical diffraction

Visualization of internal deformation fields in crystalline materials helps bridge the gap between theoretical models and practical applications. Applying Bragg coherent diffraction imaging under X-ray dynamical diffraction conditions provides a promising approach to the longstanding challenge of investigating the deformation fields in micron-sized crystals. Here, we present an automatic differentiation-based reconstruction method that integrates dynamical scattering theory to accurately reconstruct deformation fields in large crystals. Using this forward model, our simulated and experimental results demonstrate that three-dimensional local strain information inside a large crystal can be accurately reconstructed under coherent X-ray dynamical diffraction conditions with Bragg coherent X-ray diffraction imaging. These findings open an avenue for extending the investigation of local deformation fields to microscale crystals while maintaining nanoscale resolution, leveraging the enhanced coherence and brightness of advanced X-ray sources.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1995

Sweeping and straining effects in sound generation by high Reynolds number isotropic turbulence

The sound radiated by isotropic turbulence is computed using inertial range scaling expressions for the relevant two time and two point correlations. The result depends on whether the decay of Eulerian time correlations is dominated by large scale sweeping or by local straining: the straining hypothesis leads to an expression for total acoustic power, whereas the sweeping hypothesis leads to a more recent result.

Zhou, YE↗
DOE OSTI2022

Conversion of Classical Light Emission from a Nanoparticle‐Strained WSe 2 Monolayer into Quantum Light Emission via Electron Beam Irradiation

Abstract Solid‐state single photon emitters (SPEs) within atomically thin transition metal dichalcogenides (TMDs) have recently attracted interest as scalable quantum light sources for quantum photonic technologies. Among TMDs, WSe 2 monolayers (MLs) are promising for the deterministic fabrication and engineering of SPEs using local strain fields. The ability to reliably produce isolatable SPEs in WSe 2 is currently impeded by the presence of numerous spectrally overlapping states that occur at strained locations. Here nanoparticle (NP) arrays with precisely defined positions and sizes are employed to deterministically create strain fields in WSe 2 MLs, thus enabling the systematic investigation and control of SPE formation. Using this platform, electron beam irradiation at NP‐strained locations transforms spectrally overlapped sub‐bandgap emission states into isolatable, anti‐bunched quantum emitters. The dependence of the emission spectra of WSe 2 MLs as a function of strain magnitude and exposure time to electron beam irradiation is quantified and provides insight into the mechanism for SPE production. Excitons selectively funnel through strongly coupled sub‐bandgap states introduced by electron beam irradiation, which suppresses spectrally overlapping emission pathways and leads to measurable anti‐bunched behavior. The findings provide a strategy to generate isolatable SPEs in 2D materials with a well‐defined energy range.

2D material↗
NASA NTRS1987

Effect of copper-rich regions on tensile properties of VPPA weldments of 2219-T87 aluminum

This study examines the relations between tensile properties and microstructural features of variable polarity plasma arc (VPPA) weldments of 2219-T87 aluminum. Crack initiation and weld failure of transverse tensile specimens of single and multipass weldments were studied. The specimens fractured on the rising portion of the stress-strain curve prior to necking, signifying that an increase in strength would accompany an increase in ductility. Of particular interest is a shallow, typically 0.001-0.003-in. (0.03-0.08-mm) deep, copper-rich region located in the crown and root corners of the weld. This region is a primary source of crack initiation and growth, due to its brittle nature and highly strained location. The brittle regions were removed by electropolishing and machining to determine their effect on weld tensile properties. The removal increased the ductility of the weld specimens, and in the case of single pass welds, actually increased the load carrying capacity. Local strain measurements and metallographic and chemical analyses are presented.

Hartman, J. A.↗