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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,117 records · Page 62
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Scanning electron microscopy, X-ray diffraction and thermal analysis study of the TiH{sub 2} foaming agent

The decomposition of commercially available TiH{sub 2} was investigated while performing different thermal treatments. TiH{sub 2} powder, which is widely used as a foaming agent, was heat treated at 450 Degree-Sign C for various times, from 15 min to 120 min. Scanning electron microscopy (SEM) images of the surfaces at different magnifications were obtained and interpreted. A Bragg-Brentano X-ray diffractometer was used to measure the X-ray diffraction (XRD) spectra on all five samples. A close examination of the diffraction spectra showed that for an as-received sample and samples undergoing the longest thermal treatment (1 and 2 h) these spectra can be explained as deriving from cubic TiH{sub 1.924}, while for the other two samples they can be explained as deriving from tetragonal TiH{sub 1.924}. A constant-unit-cell-volume phase transition between the cubic and tetragonal phases in TiH{sub 2-y}-type compounds had been described in the literature. The unit-cell parameters obtained from measured spectra confirm that within the measurement uncertainty the unit-cell volume is indeed constant in all five samples. Thermo-gravimetry (TG) and differential thermal analysis (DTA) measurements were performed on all the samples, showing that the intensity of the dehydrogenation depends on the previous treatment of the TiH{sub 2}. After the thermal analysis XRD of the samples was performed again and the material was found to exhibit a Ti-like unit cell, but slightly enlarged due to the unreleased hydrogen. - Highlights: Black-Right-Pointing-Pointer TiH{sub 2} samples were cubic or tetragonal TiH{sub 1.924} Black-Right-Pointing-Pointer Onset of the hydrogen release temperature increases with the pre-treatment time. Black-Right-Pointing-Pointer Thermal dehydrogenation for the as-prepared TiH{sub 2} is a three-step process. Black-Right-Pointing-Pointer After thermal analysis 2 residual hydrogen TiH{sub x} phases, close to {alpha}Ti, appeared.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Relationship between dislocations and residual stresses in cold-drawn pearlitic steel analyzed by energy-dispersive X-ray diffraction

We analyzed the dislocation distribution of cold-drawn pearlitic-steel wire by using the line-profile analysis based on the energy dispersive X-ray diffraction (EDXD). Although this line-profile analysis requires a high resolution in reciprocal space, the resolution for EDXD is generally poor due to the energy resolution of the detector. Our analysis demonstrated that the resolution in the reciprocal space can be maximized at small scattering angles. Using the line-profile analysis based on the EDXD, the microstructural parameters such as the crystallite size and the dislocation density of the ferrite phase in the pearlitic steel were successfully analyzed. In addition, the distribution of the residual stress of the ferrite phase of a pearlitic steel wire was also analyzed using the EDXD measurement. - Highlights: • Energy dispersive X-ray diffraction is applied to the line-profile analysis. • Distribution of dislocations in ferrite in the pearlitic steel wire is analyzed. • Relationship between dislocations and residual stress is discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Quantitative description of microstructure defects in hexagonal boron nitrides using X-ray diffraction analysis

A routine for simultaneous quantification of turbostratic disorder, amount of puckering and the dislocation and stacking fault density in hexagonal materials was proposed and tested on boron nitride powder samples that were synthesised using different methods. The routine allows the individual microstructure defects to be recognised according to their effect on the anisotropy of the X-ray diffraction line broadening. For quantification of the microstructure defects, the total line broadening is regarded as a linear combination of the contributions from the particular defects. The total line broadening is obtained from the line profile fitting. As testing material, graphitic boron nitride (h-BN) was employed in the form of hot-isostatically pressed h-BN, pyrolytic h-BN or a h-BN, which was chemically vapour deposited at a low temperature. The kind of the dominant microstructure defects determined from the broadening of the X-ray diffraction lines was verified by high resolution transmission electron microscopy. Their amount was attempted to be verified by alternative methods. - Highlights: • Reliable method for quantification of microstructure defects in BN was suggested. • The method is based on the analysis of anisotropic XRD line broadening. • This XRD line broadening is unique and characteristic of the respective defect. • Thus, the quantification of coexistent microstructure defects is possible. • The method was tested on hexagonal BN, which was produced by different techniques.

36 MATERIALS SCIENCE↗
DOE OSTI2015

On the measurement of austenite in supermartensitic stainless steel by X-ray diffraction

Sections of a 13Cr supermartensitic stainless steel were investigated to determine the optimum sample preparation for measurement of the austenite content by X-ray diffraction. The surface of several samples was mechanically ground or polished using media of grit sizes in the range 1–120 μm. The strained surface layer was afterwards removed stepwise by electropolishing, and the austenite content measured at each step. It was found that any level of mechanical grinding or polishing results in a reduction of the measured austenite fraction relative to the true bulk value, and that coarser grinding media impart greater damage and greater reduction in the measured austenite content. The results thus highlight the importance of the electropolishing step in preparation of such samples, but suggest that the American Society for Testing and Materials standard E975-03 substantially overestimates the amount of material which needs to be removed to recover the true “bulk” content. - Highlights: • Quantitative Rietveld analysis of austenite/martensite ratio in supermartensitic stainless steels • Critical evaluation of sample preparation for residual austenite measurements by X-ray diffraction • Highlighting of the importance of electropolishing as a final preparation step.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Dislocation density evolution of AA 7020-T6 investigated by in-situ synchrotron diffraction under tensile load

The dislocation density evolution along the loading axis of a textured AA 7020-T6 aluminum alloy during uniaxial tension was investigated by in-situ synchrotron diffraction. The highly parallel synchrotron beam at the High Energy Materials Science beamline P07 in PETRA III, DESY, offers excellent conditions to separate different influences for line broadening from which micro-strains are obtained using the modified Williamson–Hall method which is also for defect density investigations. During tensile loading the dislocation density evolution was documented from the as-received material (initial micro-strain state) to the relaxation of the strains during elastic deformation. After yield, the increasing rate of dislocation density growth was relatively fast till half-way between yield and UTS. After that, the rate started to decrease and the dislocation density fluctuated as the elongation increased due to the generation and annihilation of dislocations. When dislocation generation is dominant, the correlation between the flow stress and dislocation density satisfies the Taylor equation. Besides, a method to correct the thickness effect on peak broadening is developed in the present study. - Highlights: • In-situ synchrotron diffraction was applied to characterize peak broadening. • Dislocation evolution along the loading axis during uniaxial tension was investigated. • A method to correct the sample thickness effect on peak broadening was developed. • Dislocation density and flow stress satisfy the Taylor equation at a certain range. • The texture before load and after sample fracture was analyzed.

36 MATERIALS SCIENCE↗
DOE OSTI2016

A transmission Kikuchi diffraction study of cementite in a quenched and tempered steel

This is the first transmission Kikuchi diffraction (TKD) study to report the indexing of nano-sized cementite as distinct structures and its orientation relationship with the body-centered cubic matrix in a quenched and tempered steel. Crystallographic analysis via TKD and selected area diffraction returned the well-known Bagaryatskii and Isaichev orientation relationships. However, the indexing of nano-sized cementite via TKD was sensitive to the thickness of the electron transparent region such that TEM remains the most precise method to characterise such precipitates. - Highlights: • Nano-sized cementite in a QT steel has been investigated by TKD and TEM. • Cementite has been indexed as distinct structures via TKD. • Crystallographic analysis returned the Bagaryatskii and Isaichev ORs. • Success of TKD is sensitive to the thickness of the electron transparent region. • TEM remains the most precise technique to characterise nano-sized precipitates.

36 MATERIALS SCIENCE↗
DOE OSTI2016

High quality transmission Kikuchi diffraction analysis of deformed alloys - Case study

Modern scanning electron microscopes (SEM) equipped with thermally assisted field emission guns (Schottky FEG) are capable of imaging with a resolution in the range of several nanometers or better. Simultaneously, the high electron beam current can be used, which enables fast chemical and crystallographic analysis with a higher resolution than is normally offered by SEM with a tungsten cathode. The current resolution that limits the EDS and EBSD analysis is related to materials' physics, particularly to the electron-specimen interaction volume. The application of thin, electron-transparent specimens, instead of bulk samples, improves the resolution and allows for the detailed analysis of very fine microstructural features. Beside the typical imaging mode, it is possible to use a standard EBSD camera in such a configuration that only transmitted and scattered electrons are detected. This modern approach was successfully applied to various materials giving rise to significant resolution improvement, especially for the light element magnesium based alloys. This paper presents an insight into the application of the transmission Kikuchi diffraction (TKD) technique applied to the most troublesome, heavily-deformed materials. In particular, the values of the highest possible acquisition rates for high resolution and high quality mapping were estimated within typical imaging conditions of stainless steel and magnesium-yttrium alloy. - Highlights: •Monte Carlo simulations were used to simulate EBSD camera intensity for various measuring conditions. •Transmission Kikuchi diffraction parameters were evaluated for highly deformed, light and heavy elements based alloys. •High quality maps with 20 nm spatial resolution were acquired for Mg and Fe based alloys. •High speed TKD measurements were performed at acquisition rates comparable to the reflection EBSD.

36 MATERIALS SCIENCE↗
DOE OSTI2018

On the improvement of measurement accuracy of retained austenite in steel with X-ray diffraction

Highlights: • Improved procedure to determine austenite in steels with crystallographic texture • Extension to X-ray area detectors • Electropolishing is essential to get correct results. - Abstract: An improved measurement scheme for the determination of the retained austenite fraction with X-ray diffraction is extended to area detectors. The scheme reduces the error due to crystallographic texture in phase analysis of steel by averaging the results of diffraction patterns at different sample positions. The method is based on the standard ASTM E975, but it can be easily extended to other crystal structures, multi-phase materials or X-ray wavelengths. Phase transformation, induced by mechanical sample preparation, may lead to a significant reduction of the retained austenite fraction. Electropolishing before the measurement is beneficial to remove the layer affected by sample preparation.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Resolving the geometrically necessary dislocation content in severely deformed aluminum by transmission Kikuchi diffraction

Highlights: • Grain fragmentation and steady-state stages occur during HPT processing. • A gradual transformation of LAGBs to HAGBs is characterized in both regimes. • During plastic deformation, dislocations gather collectively as geometrical walls. • The presence of GNDs is more pronounced in the vicinity of already existing HAGBs. • The density of GNDs is greater in the fragmentation stage. - Abstract: In this paper, severe plastic deformation (SPD) is applied to commercially pure aluminum. Monotonic high pressure torsion (HPT) processing is employed at room temperature, and the microstructure of samples deformed up to an equivalent strain of 50 is investigated by electron backscatter diffraction (EBSD). The distribution pattern and the density of geometrically necessary dislocations (GNDs) are evaluated by examination of transmission Kikuchi diffraction (TKD) maps. Three different methodologies are utilized for assessment of the GND density. It was observed that two distinct stages of grain fragmentation and steady-state occur during processing. During the first stage, a severe grain refinement was observed as the average grain size decreased from ~85 μm to ~1 μm at an equivalent strain of 10. Quantification of the density of dislocations in both deformation regimes showed that, independent of the choice of model, the GND density is greater in the fragmentation stage than in the steady-state stage. This observation was linked with the prevalence of the continuous dynamic recrystallization (CDRX) phenomenon in each stage. Furthermore, a significant presence of GNDs in the steady-state stage was characterized. Formation of microstructure, grain refinement and saturation of grain size are discussed in the light of statistics of GNDs.

36 MATERIALS SCIENCE↗
DOE OSTI2020

An algorithm for resolving intragranular orientation fields using coupled far-field and near-field high energy $\mathrm{X}$-ray diffraction microscopy

Here, we present a novel algorithm for reconstructing spatial intragranular lattice orientation fields using both far-field and near-field high energy X-ray diffraction microscopy (HEDM) measurements. An established far-field indexing algorithm is modified to include lattice orientation distribution information (grain orientation envelopes) in addition to average grain orientations. The near-field data reduction algorithm utilizes this enriched far-field orientation data as a seed for reconstructing intragranular spatial maps of lattice orientation from the diffraction images. The primary benefit of the new algorithm is a significant decrease in the number of trial calculations that must be performed in the reconstruction process compared to existing methodologies while maintaining scalability. The resulting gains in efficiency facilitate the use of relatively modest computational resources and improve throughput at the point of measurement. We provide two example applications: volumetric orientation field reconstructions for a Ti-Al alloy both before and after the application of 3% uniaxial strain. The results showcase the efficiency of the new method and the ability to resolve subtle changes in microstructure, which are associated with incipient plastic deformation.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Cooling dynamics of two titanium alloys during laser powder bed fusion probed with in situ X-ray imaging and diffraction

Metal parts produced by laser powder bed fusion (LPBF) additive manufacturing exhibit characteristic microstructures comparable to those observed in laser welding. The primary cause of this characteristic microstructure is rapid, localized heating and cooling cycles, which result in extreme thermal gradients where material solidification is followed by fast cooling in the solid state. The final microstructure and mechanical performance are also influenced by pore formation caused by melt pool fluid dynamics. Here, we use high speed, in situ X-ray diffraction to probe the kinetics of cooling and solid-solid phase transitions after laser melting in two aerospace titanium alloys: Ti-6Al-4V, an α + β alloy; and Ti-5Al-5V-5Mo-3Cr, a near-β alloy. We complement these diffraction studies with in situ X-ray imaging to probe melt pool dynamics and pore formation. From these two complementary probes, we quantify pore formation during melting and the subsequent microstructural evolution as the material rapidly cools after solidification. These results are critical for understanding defect formation and residual stress development in different titanium alloys under LPBF conditions and can help inform process models to predict final part performance.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Structural stability of the Li-ion conductor Li7La3Zr2O12 investigated by high-pressure in-situ X-ray diffraction and Raman spectroscopy

Highlights: • Lithium ion conductor of Garnet-related-type structure Li{sub 7}La{sub 3}Zr{sub 2}O{sub 12}. • Conversion(M.E.) • High-pressure behavior of Li{sub 7}La{sub 3}Zr{sub 2}O{sub 12} is investigated up to 70 GPa. • High pressure in-situ X-ray diffraction measurements and Raman spectroscopy. • Tetragonal Li{sub 7}La{sub 3}Zr{sub 2}O{sub 12} transforms to high-pressure cubic one under high pressure. - Abstract: The high-pressure behavior of Li{sub 7}La{sub 3}Zr{sub 2}O{sub 12} (LLZO) with a garnet-related structure has been investigated for the first time by high pressure in situ X-ray diffraction measurements and Raman spectroscopy using a diamond anvil cell. It was found that the tetragonal LLZO transforms to the high-pressure cubic phase (HP-c-LLZO) at 71.8 GPa in a N{sub 2} medium and at 25.8 GPa in the absence of pressure mediums. Uniaxial compression in the absence of pressure mediums induced the phase transition at much lower pressures than in the hydrostatic case when N{sub 2} was used as the pressure medium. High pressure in situ Raman spectroscopy suggested that the Li coordination in HP-c-LLZO was different from that reported in cubic LLZO, which had been stabilized by doping Al at ambient pressure and room temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Hydrogen embrittlement behaviors of additive manufactured maraging steel investigated by in situ high-energy X-ray diffraction

The microstructures and mechanical properties of additively manufactured (AM) maraging steel were investigated before and after hydrogen charging. The hydrogen-charged specimen suffers severe hydrogen embrittlement and its strength and plasticity decline obviously. The mechanism of ductile-to-brittle transition of AM maraging steel after hydrogen charging was studied by in situ high energy X-ray diffraction and electron backscatter diffraction techniques. Martensitic transformation occurs in the hydrogen-charged specimen under low stress. The premature phase transformation along with hydrogen-induced microcracks resulting in the brittle fracture of the hydrogen-charged specimen.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Understanding the deformation behavior of the γ rich transformative Fe 38.5 Mn 20 Co 20 Cr 15 Si 5 Cu 1.5 complex concentrated alloy using in situ synchrotron diffraction

In situ tensile testing coupled with synchrotron x-ray diffraction was used to study the deformation behavior of metastability-engineered Fe 38.5 Mn 20 Co 20 Cr 15 Si 5 Cu 1.5 complex concentrated alloy. Monitoring the evolution of phase fraction and strain hardening response allowed the determination of true critical stress for the onset of the transformation-induced plasticity (TRIP) to be ∼375 MPa, preceded by slip starting at ∼255 MPa. In situ EBSD was used to validate the critical stress for transformation at the microstructure level and observe slip traces to confirm prior slip activity before the transformation. Further, a modeling framework based on stacking fault energy (SFE) was developed to predict the critical stress for transformation. Modeling suggested the SFE of the alloy to fall nearly 15 mJ/m 2 , which agrees well with SFE values calculated using synchrotron peak shifting (12 mJ/m 2 ) and thermodynamic calculation (11 mJ/m 2 ). As a result of γ-fcc to ε-hcp phase transformation, new {0002} ε planes emerged parallel to unaligned {111} γ planes with the tensile loading following S-N orientation relationship. Such selective emergence of new diffraction rings corresponding to ε phase is understood based on the reorientation of γ crystals with reference to tensile axis. In conclusion, this approach can be extended to effectively design alloys based on critical stress required for activating different deformation mechanisms to further push the limits of the strength-ductility envelope.

Complex concentrated alloy↗
DOE OSTI2019

High-energy synchrotron radiation X-ray diffraction measurements during in situ aging of a NiTi-15 at. % Hf high temperature shape memory alloy

NiTiHf high temperature shape memory alloys (HTSMAs) have begun to show considerable progress in their ability to be manufactured and are now on the cusp of becoming viable alloys for use in the aerospace industry, mainly as actuation devices. By aging the alloys at intermediate temperatures between 450 and up to 700 °C, Ni-rich NiTiHf HTSMAs form the H-phase, a Ni-rich nanoscale precipitate which can be used to strengthen the alloy and improve the shape memory response; however, precipitate phase evolution and growth has not yet been studied in-situ nor have the effects of previous thermo-mechanical processing on their formation. In this study, H-phase precipitate formation is observed experimentally in situ during aging of a Ni-rich NiTiHf alloy using high energy synchrotron radiation X-ray diffraction (SR-XRD) measurements. The effects of prior thermo-mechanical treatments, i.e. hot and cold rolling, on its formation rate and coarsening rate are compared. It is shown that growth of the H-phase can be observed in situ using SR-XRD and the H-phase formation rate seems to exhibit linear growth when aging at 550 °C, while increasing the temperature to 650 °C shows a logarithmic trend that plateaus after approximately 1 h of aging. Other properties such as incubation time, austenite lattice strains, texture evolution, and overall comparison between hot and cold rolling are discussed. New potential peak age conditions are suggested based on the diffraction data and evaluated using three-point bending and TEM imaging.

36 MATERIALS SCIENCE↗
DOE OSTI2019

EXPLORING THE STRUCTURE OF GLASS-FORMING LIQUIDS USING HIGH ENERGY X-RAY DIFFRACTION, CONTAINERLESS METHODOLOGY AND STATE-OF-THE-ART MOLECULAR DYNAMICS SIMULATION

High energy X-ray diffraction can be combined with containerless techniques to provide information on the atomic arrangements in glass-forming liquids in stable and metastable regimes. The high incident energies provide bulk diffraction data to high values of scattering vector which enables significantly more robust analysis of the local and medium-range order that influences important physical properties such as viscosity and crystal nucleation. These combined techniques have been applied to a range of oxide liquids. In this contribution we illustrate addition of further dimensions to phase space by controlling the partial pressure of oxygen that permits the study liquids containing iron. The advantages of rapid data acquisition are also demonstrated in a study of tellurite glass-forming systems where a transition from ergodic to non-ergodic regimes in the deeply supercooled liquid is shown. Finally we demonstrate how descriptions of the liquid structure can be developed by combining HEXRD with molecular dynamics simulations.

containerless techniques↗
DOE OSTI2017

In-situ neutron diffraction study on the tension-compression fatigue behavior of a twinning induced plasticity steel

Grain orientation dependent behavior during tension-compression type of fatigue loading in a TWIP steel was studied using in-situ neutron diffraction. Orientation zones with dominant behavior of (1) twinning-de-twinning, (2) twinning-re-twinning followed by twinning-de-twinning, (3) twinning followed by dislocation slip and (4) dislocation slip were identified. Jumps of the orientation density were evidenced in neutron diffraction peaks which explains the macroscopic asymmetric behavior. The asymmetric behavior in early stage of fatigue loading is mainly due to small volume fraction of twins in comparison with that at later stage. As a result, easy activation of the de-twin makes the macroscopically unloading behavior nonlinear.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Evaluating the Taylor hardening model in polycrystalline Ti using high energy X-ray diffraction microscopy

High energy X-ray diffraction microscopy (HEDM) was employed to index nearly 1000 grains in a polycrystalline Ti specimen and characterize their deformation during an incremental tensile test. For each grain, the positions of its associated diffraction peaks were used for analyzing its evolving crystal orientation and stress tensor. The azimuthal breadth of each peak at different load steps was measured, allowing measurement of initial yielding and providing an estimate of the dislocation density evolution in each grain. Furthermore, the Taylor hardening model was evaluated grain by grain using the above data, indicating that it can represent hardening in most grains. The yield stress and the strain hardening coefficient for all grains were statistically analyzed with respect to grain orientation, grain size, slip systems, and the surrounding neighborhood to examine how the yield stress and hardening are correlated.

36 MATERIALS SCIENCE↗