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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 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Effect of Nd:YAG laser welding on microstructure and hardness of an Al-Li based alloy

Butt joints of 3.0 mm thick sheets of an Al-Li based alloy have been produced using Nd:YAG laser welding without filler metals. The hardness distribution and microstructure of the alloy and welded joints were investigated. The changes in the grain shapes, grain orientations, microtexture, and precipitates of the fusion zone were analyzed using optical microscope, electron back scattered diffraction (EBSD) and transmission electron microscopy (TEM). The results show that Nd:YAG laser welding leads to a change of the microhardness, grain shape, grain orientations, and a disappearance of the microtexture and precipitates. A narrow band of EQZ along the fusion boundary and a predominantly equiaxed dendritic structure are developed in the fusion zone. The formation of the predominately equiaxed dendritic grains is due to a heterogeneous nucleation mechanism aided by equilibrium A1{sub 3}Zr phases as well as the growth of pre-existing nuclei created by dendrite fragmentation, or by grain detachment resulted from Nd:YAG laser welding processes. In addition, Nd:YAG laser welding produces lower Vickers hardness than that of the base metal due to the decrease in the in quantity of {delta} Prime precipitates in the fusion zone. - Graphical Abstract: The grain shapes, grain orientations, microtexture, and precipitates of the solidified fusion zone were investigated and compared with the base metal using optical microscope, electron back scattered diffraction (EBSD) and transmission electron microscope (TEM). EBSD orientation map of laser welded joint in 5A90 alloys is presented in Fig. 3. It clearly shows that a narrow band EQZ along the fusion boundary and the predominantly equiaxed grains have been developed in the fusion zone of 5A90 alloys. Also, it is clear that the microstructure of the base metal is characterized by laminated grains with preferred orientations, whereas the fusion zone is predominately equiaxed grains in different colors having random orientations. Highlights: Black-Right-Pointing-Pointer The predominantly equiaxed dendritic structure is developed in the fusion zone. Black-Right-Pointing-Pointer The fusion zone with equiaxed grains shows random orientations and microtexture. Black-Right-Pointing-Pointer The loss in hardness in the fusion zone is due to the decrease in {delta} Prime precipitates. Black-Right-Pointing-Pointer The non-epitaxial growth occurs at fusion boundary. Black-Right-Pointing-Pointer The equilibrium A1{sub 3}Zr phases maybe the nuclei of new grains in the fusion zone.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Organic solvent vapor sensitive methylammonium lead trihalide film formation for efficient hybrid perovskite solar cells

In this study, the anisotropic electronic properties of the perovskite crystals originating from their non-cubic crystal structures can potentially give rise to the grain orientation correlated photovoltaic device performance. Here we report that an organic solvent vapor atmosphere introduced during the spin-coating and formation of perovskite films changes the orientation and size of perovskite grains. It was found that slightly larger but much more oriented methylammonium lead trihalide (CH 3 NH 3 PbI 3 ) grains could be obtained under 1,2-dichlorobenzene (DCB) and dimethyl sulfoxide (DMSO) vapor atmospheres. The devices with more oriented grains outperformed regular devices with more random grains by a 50 mV larger open circuit voltage as well as a slightly increased fill factor. The device efficiency enhancement can be attributed to the longer charge recombination lifetime resulting from the reduced trap density and oriented grains. This result is important in providing guidelines for comparing the results from various groups because organic solvent vapors are generally present in a sealed glovebox for perovskite solar cell fabrication.

14 SOLAR ENERGY↗
DOE OSTI2013

Development of crystallographic-orientation-dependent internal strains around a fatigue-crack tip during overloading and underloading

In-situ neutron diffraction was employed to directly measure the crystallographic-orientation-dependent (i.e. hkl) internal strains as a function of distance from the crack tip on the pre-cracked Hastelloy C-2000 compact-tension specimen. Both in-plane (IP) and through-thickness (TT) strain evolutions for various grain orientations were examined during tensile overloading and compressive underloading cycles. After overloading, underloading and their combination loadings were applied and unloaded, the significantly different (hkl) residual strain profiles were obtained in the vicinity of the crack tip. The load responses of the (200) grain orientation in both the IP and TT directions were more significant than those of any other orientations. It is suggested that the different orientation-dependent strain distributions around the crack tip are caused by the combined effects of elastic and plastic anisotropy of each (hkl) reflection upon loading and the subsequent development of residual stresses generated near the crack tip during unloading as a result of the plastic deformation. - Highlights: ► (hkl) strains are examined in situ using neutron diffraction. ► Distinct strain responses are developed around the crack tip under loading. ► The strain response of the (200) grain orientation is more significant. ► Possible mechanisms for the orientation-dependent strain responses are provided.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Electron microscopy of an aluminum layer grown on the vicinal surface of a gallium arsenide substrate

A thin Al layer grown by molecular-beam epitaxy on a misoriented GaAs (100) substrate is studied by transmission electron microscopy. Electron diffraction data and bright-field, dark-field, and high-resolution images show that, in the layer, there are Al grains of three types of crystallographic orientation: Al (100), Al (110), and Al (110)R. The specific structural features of the interfaces between the differently oriented grains and substrate are studied by digital processing of the high-resolution images. From quantitative analysis of the dark-field images, the relative content and sizes of the differently oriented grains are determined. It is found that atomic steps at the substrate surface cause an increase in the fraction and sizes of Al (110)R grains and a decrease in the fraction of Al (100) grains, compared to the corresponding fractions and sizes in the layer grown on a singular substrate surface.

36 MATERIALS SCIENCE↗
NASA NTRS2017

Measuring Shock Stage of ltokawa Regolith Grains by Electron Back-Scattered Diffraction and Synchrotron X-Ray Diffraction

We have been analyzing Itokawa samples in order to definitively establish the degree of shock experienced by the regolith of asteroid Itokawa, and to devise a bridge between shock determinations by standard light optical petrography, crystal structures as determined by electron and X-ray diffraction techniques. We are making measurements of olivine crystal structures and using these to elucidate critical regolith impact processes. We use electron back-scattered diffraction (EBSD) and synchrotron X-ray diffraction (SXRD). We are comparing the Itokawa samples to L and LL chondrite meteorites chosen to span the shock scale experienced by Itokawa, specifically Chainpur (LL3.4, Shock Stage 1), Semarkona (LL3.00, S2), Kilabo (LL6, S3), NWA100 (L6, S4) and Chelyabinsk (LL5, S4). In SXRD we measure the line broadening of olivine reflections as a measure of shock stage. In this presentation we concentrate on the EBSD work. We employed JSC's Supra 55 variable pressure FEG-SEM and Bruker EBSD system. We are not seeking actual strain values, but rather indirect strain-related measurements such as extent of intra-grain lattice rotation, and determining whether shock state "standards" (meteorite samples of accepted shock state, and appropriate small grain size) show strain measurements that may be statistically differentiated, using a sampling of particles (number and size range) typical of asteroid regoliths. Using our system we determined that a column pressure of 9 Pa and no C-coating on the sample was optimal. We varied camera exposure time and gain to optimize mapping performance, concluding that 320x240 pattern pixilation, frame averaging of 3, 15 kV, and low extractor voltage yielded an acceptable balance of hit rate (>90%), speed (11 fps) and map quality using an exposure time of 30 ms (gain 650). We found that there was no strong effect of step size on Grain Orientation Spread (GOS) and Grain Reference Orientation Deviation angle (GROD-a) distribution; there was some effect on grain average Kernel Average Misorientation (KAM) (reduced with smaller step size for the same grain), as expected. We monitored GOS, Maximum Orientation Spread (MOS) and GROD-a differences between whole olivine grains and sub-sampled areas, and found that there were significant differences between the whole grain dataset and subsets, as well as between subsets, likely due to sampling-related "noise". Also, in general (and logically) whole grains exhibit greater degrees of cumulative lattice rotation. Sampling size affects the apparent strain character of the grain, at least as measured by GOS, MOS and GROD-a. There were differences in the distribution frequencies of GOS and MOS between shock stages, and in plots of MOS and GOS vs. grain diameter. These results are generally consistent with those reported this year. However, it is unknown whether the differences between samples of different shock states exceeds the clustering of these values to the extent that shock stage determinations can still be made with confidence. We are investigating this by examination of meteorites with higher shock stage 4 to 5. Our research will improve our understanding of how small, primitive solar system bodies formed and evolved, and improve understanding of the processes that determine the history and future of habitability of environments on other solar system bodies. The results will directly enrich the ongoing asteroid and comet exploration missions by NASA and JAXA, and broaden our understanding of the origin and evolution of small bodies in the early solar system, and elucidate the nature of asteroid and comet regolith.

Zolensky, Michael↗
DOE OSTI2014

Magnetization reversal of in-plane uniaxial Co films and its dependence on epitaxial alignment

This work studies the influence of crystallographic alignment onto magnetization reversal in partially epitaxial Co films. A reproducible growth sequence was devised that allows for the continuous tuning of grain orientation disorder in Co films with uniaxial in-plane anisotropy by the controlled partial suppression of epitaxy. While all stable or meta-stable magnetization states occurring during a magnetic field cycle exhibit a uniform magnetization for fully epitaxial samples, non-uniform states appear for samples with sufficiently high grain orientation disorder. Simultaneously with the occurrence of stable domain states during the magnetization reversal, we observe a qualitative change of the applied field angle dependence of the coercive field. Upon increasing the grain orientation disorder, we observe a disappearance of transient domain wall propagation as the dominating reversal process, which is characterized by an increase of the coercive field for applied field angles away from the easy axis for well-ordered epitaxial samples. Upon reaching a certain disorder threshold level, we also find an anomalous magnetization reversal, which is characterized by a non-monotonic behavior of the remanent magnetization and coercive field as a function of the applied field angle in the vicinity of the nominal hard axis. This anomaly is a collective reversal mode that is caused by disorder-induced frustration and it can be qualitatively and even quantitatively explained by means of a two Stoner-Wohlfarth particle model. Its predictions are furthermore corroborated by Kerr microscopy and by Brillouin light scattering measurements.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Deformation mechanisms, defects, heat treatment, and thermal conductivity in large grain niobium

The physical and mechanical metallurgy underlying fabrication of large grain cavities for superconducting radio frequency accelerators is summarized, based on research of 1) grain orientations in ingots, 2) a metallurgical assessment of processing a large grain single cell cavity and a tube, 3) assessment of slip behavior of single crystal tensile samples extracted from a high purity ingot slice before and after annealing at 800 °C / 2 h, 4) development of crystal plasticity models based upon the single crystal experiments, and 5) assessment of how thermal conductivity is affected by strain, heat treatment, and exposure to hydrogen. Because of the large grains, the plastic anisotropy of deformation is exaggerated, and heterogeneous strains and localized defects are present to a much greater degree than expected in polycrystalline material, making it highly desirable to computationally anticipate potential forming problems before manufacturing cavities.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Nucleation and growth of damage in polycrystalline aluminum under dynamic tensile loading

Plate-impact experiments were conducted to study the features and mechanisms of void nucleation and growth in the polycrystalline of pure aluminum under dynamic loading. Soft-recovered samples have been analyzed by metallographic microscopy, electron back scattering diffraction (EBSD), and synchrotron radiation x-ray tomography technology. It was found that most of the void nucleation in grains neared the boundaries of “weak-orientation” grains and grew toward the grain boundaries with fractured small grains around the boundaries. This was mainly caused by the accumulation and interaction of slip systems in the “weak-orientation” grains. In addition, the micro voids were nearly octahedron because the octahedral slip systems were formed by 8 slip planes in the polycrystalline of pure aluminum. The EBSD results are consistent with the three-dimensional structure observed by synchrotron radiation x-ray.

36 MATERIALS SCIENCE↗
DOE OSTI2019

ODS Thin-wall Cladding Joining Technique Development

ABSTRACT The oxide dispersion strengthened (ODS) materials are typically fabricated by powder mechanical alloying to form uniform distribution of high concentration nanometer size oxide particles for exceptional high temperature mechanical strength and enhanced radiation tolerance. This report summarizes the recent development on the weld techniques of joining the ODS cladding to the endplug as a part of accident tolerant fuels (ATF) program for both advanced light water reactor (LWR) and advanced fuel cycle (AFC) with fast reactor fuel cladding. It is crucial for the weld of ODS material to maintain the uniform distribution and fine size of oxide particles in the matrix. Pressure Resistance Weld (PRW) is a solid state joining technique without melting therefore a preferred joining technique for ODS materials. Samples joined with PRW were pressure burst tested. Detailed microstructural characterization in this work includes non-destructive 3D x-ray ?m-resolution computer-tomography (?-CT), optical microscopy, SEM/EBSD for grain-orientation-mapping, TEM, STEM/ASTAR for high resolution grain orientation and phase mapping, and APT for high-resolution 3D composition analysis.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2012

Control of the azimuthal orientation of grains in polycrystalline Si films

Multi-directional ion beam irradiation is applied to control the crystal orientation of Si thin films during growth via plasma-enhanced chemical vapor deposition. The primary ion beam is generated by electron cyclotron resonance using an SiH{sub 4}/H{sub 2} mixture, and the auxiliary beam is obtained by redirecting a portion of the primary beam with a metal reflector. These two beams simultaneously hit the surface of the growing film from different directions. With this arrangement, we succeeded in controlling the orientation of the growth of deposited Si grains in virtually the same direction.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Mechanism of Texture and Precipitates Evolution in CGO Silicon Steel During High-Temperature Annealing Process

Grain-oriented silicon steel is an important soft magnetic material for electrical applications. A conventional grain-oriented silicon steel with final thicknesses of 0.27, 0.23 and 0.20 mm was prepared separately by a two-stage cold rolling process. Texture evolution, precipitates distribution and Goss textures formed during secondary recrystallization were analyzed by electron backscattered diffraction and field emission scanning electron microscopy. The results show that plate thickness influences the distribution density of secondary phase particles and the content of favorable texture components such as {111} <112>, {111} <110> and {112} <110> texture. Decreasing plate thickness increases the areal density of secondary phase particles, the temperature of secondary recrystallization, which also increases the content of favorable texture component, and the strength of Goss texture in secondary recrystallization. Goss grains have higher particle density than the grains with other orientations, which facilitates their abnormal growth by swallowing surrounding grains with lower particle density.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Modeling the effect of neighboring grains on twin growth in HCP polycrystals

In this paper, we study the dependence of neighboring grain orientation on the local stress state around a deformation twin in a hexagonal close packed (HCP) crystal and its effects on the resistance against twin thickening. We use a recently developed, full-field elasto-visco-plastic formulation based on fast Fourier transforms that accounts for the twinning shear transformation imposed by the twin lamella. The study is applied to Mg, Zr and Ti, since these HCP metals tend to deform by activation of different types of slip modes. The analysis shows that the local stress along the twin boundary are strongly controlled by the relative orientation of the easiest deformation modes in the neighboring grain with respect to the twin lamella in the parent grain. A geometric expression that captures this parent-neighbor relationship is proposed and incorporated into a larger scale, mean-field visco-plastic self-consistent model to simulate the role of neighboring grain orientation on twin thickening. We demonstrate that the approach improves the prediction of twin area fraction distribution when compared with experimental observations.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Texture Analysis of Inconel 718 with Different Modes During Single-Track Laser Surface Re-Melting

An in-depth understanding of the texture formation in melt pools allows for the modification of the surface layer microstructure and corresponding material properties, providing an opportunity to integrate laser surface re-melting into metal additive manufacturing. This study investigates crystallographic texture formation at different cooling rates in single melting tracks on the Inconel 718 (IN718) plate produced by laser surface re-melting. The cooling rate varies from 2.31 × 10 5 °C/s to 9.56 × 10 5 °C/s with the increase in scanning rates from 400 mm/s to 1200 mm/s, measured by recently developed real-time temperature monitoring of melt pools. Columnar grains are dominant, with distinct crystallographic textures forming in the melt pools. At a slower scanning speed, the keyhole mode shows three different textures forming at different depths (crystallographically layered structure), while, at a faster scanning speed, the conduction mode shows only random grain orientation. There are no pores/voids detected, and the columnar grain morphology and columnar grain width (8.6 μm to 12.4 μm) follow the analysis framework in terms of thermal gradient and solidification rate analysis. This implies that laser surface re-melting provides the potential to modify the surface structure from a random grain orientation to a crystallographically layered structure.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Biaxial texture development in aluminum nitride layers during off-axis sputter deposition

Polycrystalline aluminum nitride (AlN) layers were deposited by pulsed-dc reactive magnetron sputtering from a variable deposition angle {alpha} = 0 Degree-Sign -84 Degree-Sign in 5 mTorr pure N{sub 2} at room temperature. X-ray diffraction pole figure analyses show that layers deposited from a normal angle ({alpha} = 0 Degree-Sign ) exhibit fiber texture, with a random in-plane grain orientation and the c-axis tilted by 42 Degree-Sign {+-} 2 Degree-Sign off the substrate normal, yielding wurtzite AlN grains with the {l_brace}1012{r_brace} plane approximately parallel ({+-}2 Degree-Sign ) to the substrate surface. However, as {alpha} is increased to 45 Degree-Sign , two preferred in-plane grain orientations emerge, with populations I and II having the c-axis tilted toward and away from the deposition flux, by 53 Degree-Sign {+-} 2 Degree-Sign and 47 Degree-Sign {+-} 1 Degree-Sign off the substrate normal, respectively. Increasing {alpha} further to 65 Degree-Sign and 84 Degree-Sign , results in the development of a single population II with a 43 Degree-Sign {+-} 1 Degree-Sign tilt. This developing biaxial texture is attributed to a competitive growth mode under conditions where the adatom mobility is sufficient to cause intergrain mass transport, but insufficient for the thermodynamically favored low energy {l_brace}0001{r_brace} planes to align parallel to the layer surface. Consequently, AlN nuclei are initially randomly oriented and form a kinetically determined crystal habit exposing {l_brace}0001{r_brace} and {l_brace}1120{r_brace} facets. The expected direction of its highest growth rate is 49 Degree-Sign {+-} 5 Degree-Sign tilted relative to the c-axis, in good agreement with the 42 Degree-Sign -53 Degree-Sign measured tilt. The in-plane preferred orientation for {alpha} > 0 Degree-Sign is well explained by the orientation dependence in the cross section of the asymmetric pyramidal nuclei to capture directional deposition flux. The observed tilt is ideal for shear mode electromechanical coupling, which is maximized at 48 Degree-Sign .

36 MATERIALS SCIENCE↗
NASA NTRS2009

Mechanistic Study of Delamination Fracture in Al-Li Alloy C458 (2099)

Delamination fracture has limited the use of lightweight Al-Li alloys. In the present study, electron backscattered diffraction (EBSD) methods were used to characterize crack paths in Al-Li alloy C458 (2099). Secondary delamination cracks in fracture toughness samples showed a pronounced tendency for fracture between grain variants of the same deformation texture component. These results were analyzed by EBSD mapping methods and simulated with finite element analyses. Simulation procedures include a description of material anisotropy, local grain orientations, and fracture utilizing crystal plasticity and cohesive zone elements. Taylor factors computed for each grain orientation subjected to normal and shear stresses indicated that grain pairs with the largest Taylor factor differences were adjacent to boundaries that failed by delamination. Examination of matching delamination fracture surface pairs revealed pronounced slip bands in only one of the grains bordering the delamination. These results, along with EBSD studies, plasticity simulations, and Auger electron spectroscopy observations support a hypothesis that delamination fracture occurs due to poor slip accommodation along boundaries between grains with greatly differing plastic response.

Tayon, W. A.↗
DOE OSTI2015

Microstructure evolution of 7050 Al alloy during age-forming

The microstructure evolution of the 7050 Al alloy treated by age-forming was studied using a designed device which can simulate the age-forming process. The grain shape, grain boundary misorientation and grain orientation evolution of 7050 Al alloy during age-forming have been quantitatively characterized by electron backscattering diffraction technique. The results show that age-forming produced abundant low-angle boundaries and elongated grains, which attributed to stress induced dislocation movement and grain boundary migration during the age-forming process. On the other side, the stress along rolling direction caused some unstable orientation grains to rotate towards the Brass and S orientations during the age-forming process. Hence, the intensity of the rolling texture orientation in age-formed samples is enhanced. But this effect decays gradually with increasing aging time, since stress decreases and precipitation hardening occurs during the age-forming process. - Highlights: • Quantitative analysis of grain evolution of 7050 Al alloys during age-forming • Stress induces some grain rotation of 7050 Al alloys during age-forming. • Creep leads to elongate grain of 7050 Al alloys during age-forming. • Obtains a trend on texture evolution during age-forming applied stress.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Preheated Microstructure Vis-à-vis Process Parameters and Characterization of Orange Peel in Incremental Forming of AA1050 Sheets

The present investigation systematically analyzes and compares the influence of microstructure and crystallographic texture of AA1050 sheet to that of the most critical process parameters, namely tool dimension, and incremental step depth, in single-point incremental forming (SPIF) process. The sheet samples were preheated at different temperatures to reform the original, i.e., distorted and misoriented grain distribution. The results from experimental and statistical analyses report higher sensitivity of global spring back, surface topography and forming limits strains to variations in microstructure and texture than tool diameter and incremental step depth. The preheating of the sheets resulted in the formation of preferential texture by grain reformation, which leads to the increase in the fraction of homogeneous orientated grains. Such grains increase the maximum forming limit strains and reduce the global spring back in SPIF. Fractography analysis of the failed components revealed that improvement in formability due to grain homogenization (developed texture) may be attributed to larger average void diameter. Empirical models were also developed, and optimized levels of parameters were identified to maximize the efficiency of the SPIF process. Further, qualitative and quantitative characterization of orange peel roughness was carried out by segregating the external formed surface into surfaces subjected to orange peel roughness and tool-induced texture.

36 MATERIALS SCIENCE↗