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At least 1,333 records · Page 74
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

A novel reduction synthesis of the graphene/Mn{sub 3}O{sub 4} nanocomposite for supercapacitors

Graphene/Mn{sub 3}O{sub 4} nanocomposite is successfully synthesized from graphene oxide (GO)/MnO{sub 2} precursor using dilute hydrazine hydrate assisted hydrothermal reaction (DAH method). X-ray photoelectron spectroscopy (XPS) and Raman characterizations confirm the decrease of oxygen-containing functional groups of GO. The results indicate that GO have been reduced to graphene to a large degree to increase the electronic conductive channels. The morphology and the phase transformation of MnO{sub 2} can be ascribed to the “dissolution-recrystallization” mechanism. Such nanocomposite, as electrode material for supercapacitor, exhibits a high specific capacitance of 326.9 F g{sup −1}, almost 4 times that of GO/MnO{sub 2} precursor (81.3 F g{sup −1}). The good cycle stability of 94.6% capacitance retention after 1000 cycles, can be explained by the firm interfacial cohesion between graphene and Mn{sub 3}O{sub 4} nanoparticle. This soft chemical DAH process could be readily extended to the preparation of other classes of hybrids. - Graphical abstract: The phase transition can be explained by the “dissolution-recrystallization” model. The GO/MnO{sub 2} precursor is reduced into graphene/Mn{sub 3}O{sub 4} by the DAH process, and the electrochemical property is dramatically enhanced. - Highlights: • Graphene/Mn{sub 3}O{sub 4} nanocomposite is firstly synthesized successfully from GO/MnO{sub 2} precursor. • The phase transformation from MnO{sub 2} to Mn{sub 3}O{sub 4} has occurred. • The specific capacitance of the graphene/Mn{sub 3}O{sub 4} (326.9 F/g) is 4 times that of GO/MnO{sub 2} precursor.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Identifying the sources of ferromagnetism in sol-gel synthesized Zn{sub 1−x}Co{sub x}O (0≤x≤0.10) nanoparticles

We have carefully investigated the structural, optical and electronic properties and related them with changes in the magnetism of sol-gel synthesized Zn{sub 1−x}Co{sub x}O (0≤x≤0.10) nanoparticles. Samples with x≤0.05 were free of spurious phases. Samples with x≤0.03 were found to be with only high spin Co{sup 2+} ions into ZnO structure, whereas sample with x=0.05, exhibited the presence of high spin Co{sup 2+} and low spin Co{sup 3+}. We found that the intensity of the main EPR peak associated with Co{sup 2+} varies with the nominal Co content in a similar manner as the saturation magnetization and coercive field do. These results point out that the ferromagnetism in these samples should directly be correlated with the presence of divalent cobalt ions. Bound magnetic polaron (BMP) model and the charge transfer model are insufficient to explain the ferromagnetic properties of Zn{sub 1−x}Co{sub x}O nanoparticles. The room temperature ferromagnetism (RTFM) may be originated from a combination of several factors such as the interaction of high spin Co{sup 2+} ions, perturbation/alteration and/or changes in the electronic structure of ZnO close to the valence band edge and grain boundary effects. - Graphical abstract: The intensity of the main EPR peak associated with Co{sup 2+} varies with the nominal Co content in a similar manner as the saturation magnetization and coercive field do. These results point out that the ferromagnetism in these samples should directly be correlated with the presence of Co{sup 2+} ions. Display Omitted - Highlights: • Systematic and carefully study of physical-chemical properties of Zn{sub 1−x}Co{sub x}O nanoparticles. • Samples with x=0.01 and 0.03 were found to be with only high spin Co{sup 2+}. • Sample with x=0.05, exhibited the presence of high spin Co{sup 2+} and low spin Co{sup 3+}. • The BMP and charge transfer models seem not explain the ferromagnetic properties. • RTFM: high spin Co{sup 2+} ions, defects close to the valence band and grain boundary effects.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Photoluminescence and semiconductor properties of two novel lanthanide-mercury compounds with one-dimensional chain-like structures

Highlights: • This paper reports two novel lanthanide-mercury compounds with 1D chain. • Diffuse reflectance spectroscopy reveals that they have wide optical band gaps. • One shows red photoluminescence which is assigned to the {sup 5}D{sub 0} → {sup 7}F{sub J} (Eu{sup 3+}). • The other shows green photoluminescence which is assigned to the {sup 5}D{sub 4} → {sup 7}F{sub J} (Tb{sup 3+}). • Energy transfer mechanism is explained by the energy level diagram. - Abstract: Two novel lanthanide-mercury compounds [Ln(IA){sub 3}(H{sub 2}O){sub 2}]{sub n}(Hg{sub 3}Cl{sub 9}){sub n}·4nH{sub 2}O (Ln = Eu (1) and Tb (2); IA = isonicotinic acid) have been synthesized under solvothermal conditions and structurally characterized by single-crystal X-ray diffraction. Both complexes exhibit one-dimensional (1D) chain-like structure with a novel isolated (Hg{sub 3}Cl{sub 9}){sup 3-} anion. Solid-state diffuse reflectance spectroscopy reveals the presence of wide optical band gaps of 2.81 eV and 3.47 eV, suggesting that they are potentially wide band gap organic semiconductor materials. Solid-state photoluminescence measurements uncover that they show red or green light emission bands, respectively. The emission bands can be assigned to the characteristic emission of the 4f electrons intrashell transitions of {sup 5}D{sub 0} → {sup 7}F{sub J} (Eu{sup 3+}, J = 2 and 4) and {sup 5}D{sub 4} → {sup 7}F{sub J} (Tb{sup 3+}, J = 6, 5, 4 and 3), respectively. Energy transfer mechanism is explained by the energy level diagram of Eu{sup 3+} ion, Tb{sup 3+} ion and isonicotinic acid ligand. The title compounds have remarkable CIE chromaticity coordinates of (0.6438, 0.3559) and (0.3172, 0.4773). This suggests that they may be potential candidates with red or green light emitting materials for white light emitting diodes (LEDs).

36 MATERIALS SCIENCE↗
DOE OSTI2018

The dynamics of integration, viral suppression and cell-cell transmission in the development of occult Hepatitis B virus infection

Out of several phases of HBV infection, the least understood phase is occult hepatitis B virus infection. The paucity of data due to non-availability of biological tissues and the prerequisite of ultra-sensitive assays for the detection of occult hepatitis B virus infection prompted us to utilize mathematical modeling in determining mechanisms that lead to occult hepatitis B virus infection and characteristics of HBV infection during occult hepatitis B virus infection. In this paper, we proposed two mathematical models (M1 and M2), considering two different phenomenon for episomal maintenance and accumulation of covalently closed circular DNA (cccDNA) in infected hepatocytes: (i) M1 – recirculation of the relaxed circular DNA/double-stranded linear DNA from cytoplasm to the nucleus, and (ii) M2 – reinfection of infected hepatocytes with virions. We further incorporated the dynamics of integrated Hepatitis B virus DNA (iHBV) to investigate its role in the development of occult hepatitis B virus infection. The analysis showed that the main mechanism for the spread of infection during occult hepatitis B virus infection is cell-to-cell transmission and not cell-free virus transmission. A significant viral suppression (of at least 99% from its peak production values) was essential but not sufficient in the development of occult hepatitis B virus infection under M1; however under M2, the viral suppression was neither sufficient nor essential as the inhibition of the production of HBsAg without viral suppression can also explain the development of occult hepatitis B virus infection. Our analysis also revealed that occult hepatitis B virus infection seropositive cases are more likely to progress into liver cirrhosis compared to occult hepatitis B virus infection seronegative cases. The iHBV was found to be mostly silent (by either being absent or non-productive for HBsAg) during occult hepatitis B virus infection. The viral suppression is neither essential nor sufficient to explain the development of occult hepatitis B virus infection on its own. Not only the viral suppression but the inhibition -of the production and the export of HBsAg from cccDNA and iHBV also plays an important role in the development of occult hepatitis B virus infection. Finally, this is the first study, which incorporates the dynamics of iHBV and shows that HBV primarily spreads via cell-cell transmission during occult hepatitis B virus infection.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2020

Vertical effective stress and temperature as controls of quartz cementation in sandstones: Evidence from North Sea Fulmar and Gulf of Mexico Wilcox sandstones

Here we present quantitative petrographic data, high spatial resolution oxygen isotope analyses of quartz cement, basin modelling and a kinetic model for quartz precipitation for two Paleocene-Eocene Wilcox Group sandstones from Texas and two Jurassic Fulmar Formation sandstones from the Central North Sea. At each location, one sandstone has been buried to ca. 145 °C and one to ca. 185 °C. A key difference between the Wilcox and Fulmar burial histories is that the Wilcox sandstones are currently at higher vertical effective stresses and, from basin modelling studies, have been subjected to generally higher vertical effective stresses through their burial history. The amounts of quartz cement in the Wilcox sandstones are between 12 and 18%, and between 2 and 6% in the Fulmar sandstones. High-spatial-resolution oxygen isotope data obtained from the quartz cements suggest temperature ranges for quartz precipitation from 60 to 80 °C to values approaching maximum burial temperature. Factors such as grain coatings or the timing of petroleum emplacement cannot explain the differences in the amounts of quartz cement. Petrographic data show that most of the silica for quartz cement can be derived from intergranular pressure dissolution. Although the sample set is small, we interpret the results to suggest that the differences in quartz cementation in Fulmar and Wilcox sandstones can be explained better by differences in their vertical effective stress history than their temperature history; in this case, the supply of silica rather than the precipitation of quartz becomes an important control on the rate and extent of cementation.

58 GEOSCIENCES↗
DOE OSTI2012

Microstructural evolution in adiabatic shear bands of copper at high strain rates: Electron backscatter diffraction characterization

The microstructural evolution of adiabatic shear bands in annealed copper with different large strains at high strain rates has been investigated by electron backscatter diffraction. The results show that mechanical twinning can occur with minimal contribution to shear localization under dynamic loading. Elongated ultrafine grains with widths of 100-300 nm are observed during the evolution of the adiabatic shear bands. A rotational dynamic recrystallization mechanism is proposed to explain the formation of the elongated ultrafine grains. - Highlights: Black-Right-Pointing-Pointer The microstructural evolution of ASB is studied by electron backscatter diffraction. Black-Right-Pointing-Pointer Twinning can occur in ASB while the contribution to shear localization is slight. Black-Right-Pointing-Pointer Elongated ultrafine grains are observed during the evolution process of ASB. Black-Right-Pointing-Pointer A possible mechanism is proposed to explain the microstructure evolution of ASB.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Microstructural analysis of laser weld fusion zone in Haynes 282 superalloy

Analytical electron microscopy and spectroscopy analyses of the fusion zone (FZ) microstructure in autogenous laser beam welded Haynes 282 (HY 282) superalloy were performed. The micro-segregation patterns observed in the FZ indicate that Co, Cr and Al exhibited a nearly uniform distribution between the dendrite core and interdendritic regions while Ti and Mo were rejected into the interdendritic liquid during the weld solidification. Transmission electron diffraction analysis and energy dispersive X-ray microanalysis revealed the second phase particles formed along the FZ interdendritic region to be Ti-Mo rich MC-type carbide particles. Weld FZ solidification cracking, which is sometimes associated with the formation of {gamma}-{gamma}' eutectic in {gamma}' precipitation strengthened nickel-base superalloys, was not observed in the HY 282 superalloy. Modified primary solidification path due to carbon addition in the newly developed superalloy is used to explain preclusion of weld FZ solidification cracking in the material. - Highlights: Black-Right-Pointing-Pointer A newly developed superalloy was welded by CO{sub 2} laser beam joining technique. Black-Right-Pointing-Pointer Electron microscopy characterization of the weld microstructure was performed. Black-Right-Pointing-Pointer Identified interdendritic microconstituents consist of MC-type carbides. Black-Right-Pointing-Pointer Modification of primary solidification path is used to explain cracking resistance.

36 MATERIALS SCIENCE↗
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 OSTI2014

EBSD analysis of (10–12) twinning activity in Mg–3Al–1Zn alloy during compression

The (10–12) twinning activity of Mg–3Al–1Zn magnesium alloy during uniaxial compression at room temperature has been investigated by electron backscatter diffraction. The results indicated that the twinning activity was closely related with two angles: one was the angle between the c-axis and the compression direction and the other was the angle between the a-axis and the titling direction in the basal plane for a given relation between the c-axis and the compression direction. These two parameters can be used to explain which twinning variant will operate under the given strain path. For the grains containing a single (10–12) twinning variant, the (10–12) twinning variant occurred in a wide range of Schmid factor values (0 < Schmid factor < 0.5) and the Schmid factor rank of 1 or 2 was the most commonly observed. By contrast, for the grains containing two (10–12) twinning variants, the (10–12) twinning activity exhibited a stronger orientation dependence and the combinations of Schmid factor ranks 1–3 and 1–2 were the most commonly observed. - Highlights: • Twinning activity of AZ31 magnesium alloy was investigated by EBSD. • (10–12) twinning shows a strong orientation dependence. • Two angles can be used to explain which twin variant will operate.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Grain size effect on electrical resistivity of bulk nanograined Bi{sub 2}Te{sub 3} material

The bulk nanograined Bi{sub 2}Te{sub 3} material with various mean grain sizes changing from ~ 97 nm to ~ 51 nm was prepared by microwave assisted solvothermal method and hot pseudo-isostatic pressure. It was found that the specific electrical resistivity of the material increases as mean grain size decreases. Such kind of the grain effect on the resistivity can be attributed to enhanced electron scattering at the grain boundaries. The Mayadas–Shatzkes model was applied to explain experimental results. In this model the grain boundaries are regarded as potential barriers which have to be overcome by the electrons. The reflectivity R of the grain boundaries for the material under study was estimated to be equal to ~ 0.7. - Highlights: • The bulk nanograined Bi{sub 2}Te{sub 3} material with various mean grain sizes was prepared. • It was found that the electrical resistivity of the material increases as grain size decreases. • The Mayadas–Shatzkes model was applied to explain experimental results. • The reflectivity R of the grain boundaries was estimated to be equal to ~ 0.7.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Structural characterization of “carbide-free” bainite in a Fe–0.2C–1.5Si–2.5Mn steel

Low-alloyed, low C containing carbide-free bainitic steels are attractive candidates for applications in the automotive industry due to their well-balanced combination of high strength and ductility achieved in an economic way. In this work, their complex microstructure consisting of a mixture of bainitic ferrite, austenite with different morphologies and stabilities, martensite, M/A constituent and a few carbides has been investigated with metallographic and high-resolution techniques. After specific isothermal heat treatments in a dilatometer, a combination of LePera and Nital etching was applied to distinguish between bainite and martensite. Site-specific atom probe tips were prepared by means of scanning electron microscopy, electron backscatter diffraction and focused ion beam, revealing that “carbide-free” bainite consists of C depleted bainitic ferrite, C enriched retained austenite and occasional ε-carbides. Furthermore, it was found that the M/A constituent is highly dislocated and mainly martensitic. Its C content is increased compared to the nominal composition, but below the values obtained for retained austenite, explaining the lower transformation resistance. - Highlights: • Detailed top-down characterization of low C “carbide-free” bainitic steel • APT of all constituents in “carbide-free” bainite • Identification of ε-carbide based on its C content determined by APT • M/A constituent is mainly martensitic with austenitic areas at the boundaries • Lower C content of M/A constituent explains its lower stability.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Growth mechanism and magnetic and electrochemical properties of Na{sub 0.44}MnO{sub 2} nanorods as cathode material for Na-ion batteries

Nanorods of Na{sub 0.44}MnO{sub 2} are a promising cathode material for Na-ion batteries due to their large surface area and single crystalline structure. We report the growth mechanism of Na{sub 0.44}MnO{sub 2} nanorods via solid state synthesis and their physical properties. The structure and the morphology of the Na{sub 0.44}MnO{sub 2} nanorods are investigated by X-ray diffraction (XRD), scanning and tunneling electron microscopy (SEM and TEM), and energy-dispersive X-ray (EDX) techniques. The growth mechanism of the rods is investigated and the effects of vapor pressure and partial melting of Na-rich regions are discussed. The magnetic measurements show an antiferromagnetic phase transition at 25 K and the μ{sub eff} is determined as 3.41 and 3.24 μ{sub B} from the χ–T curve and theoretical calculation, respectively. The electronic configuration and spin state of Mn{sup 3+} and Mn{sup 4+} are discussed in detail. The electrochemical properties of the cell fabricated using the nanorods are investigated and the peaks in the voltammogram are attributed to the diffusion of Na ions from different sites. Na intercalation process is explained by one and two Margules and van Laar models. - Highlights: • We synthesized Na{sub 0.44}MnO{sub 2} nanorods via a simple solid state reaction technique. • Our studies show that excess Na plays a crucial role in the nanorod formation. • Magnetization measurements show that Mn{sup 3+} ions are in LS and HS states. • The electrochemical properties of the cell fabricated using the nanorods are investigated. • Na intercalation process is explained by one and two Margules and van Laar models.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Formation and evolution of platelet-like Ti-rich precipitates in the V–4Cr–4Ti alloy

The goal of the present investigation is to explain the obviously different appearances of Ti-rich precipitates in vanadium alloy and in steels. To achieve the goal, the formation and evolution of the precipitates in the as-cast and the heat treated V–4Cr–4Ti samples were investigated using optical and electron microscopies. The precipitates were found to be rare in the as-cast samples, and a high density of the precipitates occur in the samples subjected to isothermal holding at 600–1300 °C. The precipitates preferentially distribute within the grains rather than at the grain boundaries. All of the precipitates are platelet-like, with NaCl structure, in three-dimensional space. The further observation using high-resolution electron microscopy (HREM) reveals that a high density of twins occurs in the growth front of the precipitates, whereas the middle of the precipitates is twin-free. Meanwhile, enrichment of titanium atoms was observed in the middle of the precipitates. These results indicate that the precipitates form by a displacive transformation, followed by a diffusional process to enrich titanium further and eliminate the twined structure. - Highlights: • Precipitates in the V–4Cr–4Ti alloy are Ti-(CN) with NaCl structure. • The Ti-rich precipitates are platelet-like in three dimensional space. • Twins occur in both ends of the precipitates and the midst of these is twin-free. • Twinning and its elimination is explained by displacive followed by diffusion.

36 MATERIALS SCIENCE↗
DOE OSTI2016

A comparison of EBSD based strain indicators for the study of Fe-3Si steel subjected to cyclic loading

The current work aims at proposing an EBSD-based indicator for fatigue damage of a Fe-3Si steel. At the same time direct observation of dislocation structures is provided by electron channelling contrast imaging (ECCI). The investigation consisted in processing the EBSD data from patterns collected on specimen subjected to low cycle fatigue. It revealed two different regimes depending on the applied total strain variation which is explained by the identification of the dislocations structures and their evolution. At low strain variation, strain accommodation occurs by planar glide of dislocations uniformly distributed throughout the grains. No misorientation evolution is observed. At higher strain variation, the vein-channel structure is observed within the grain and the wall-channel structure in the vicinity of grain boundaries. The misorientation between these two dislocation structures is evaluated at about 0.7° which is detected by the EBSD analyses and explains the increase of the different misorientation based criteria. The EBSD study enables also the prediction of crack initiation mode. Finally, this study points out the limits of the EBSD technique as no misorientation evolution is detected at small strain variation. Indeed, the lattice distortion is too weak to be detected by conventional EBSD. - Highlights: • Microstructure investigation of the fatigue behaviour of an iron-silicon steel • Use of cECCI to investigate the fatigue dislocations structures • Characterisation of local plastic accommodation through EBSD misorientation criteria.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Precipitation behaviour of γ′ phase in Alloy 693

This paper reports the precipitation behaviour of γ′ phase in the newly developed Ni-base superalloy Alloy 693. Precipitation of γ′ particles is studied at different temperatures ranging from 800 °C to 950 °C under isothermal conditions for various time periods. Monomodal, bimodal and trimodal size distributions of γ′ precipitates have been observed depending on the temperature of annealing and the cooling rates. These studies have been carried out using scanning and transmission electron microscopy. Results are compared with existing literature and explained on the basis of classical nucleation and growth theory. - Highlights: •The alloy exhibits multi-modal distribution of γ′ precipitates formed in aged Alloy 693. •The γ′ phase has been found contain up to 4 at% of Nb. •Results have been explained on the basis of classical nucleation and growth models.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Background of SAM atom-fraction profiles

Atom-fraction profiles acquired by SAM (scanning Auger microprobe) have important applications, e.g. in the context of alloy surface engineering by infusion of carbon or nitrogen through the alloy surface. However, such profiles often exhibit an artifact in form of a background with a level that anti-correlates with the local atom fraction. This article presents a theory explaining this phenomenon as a consequence of the way in which random noise in the spectrum propagates into the discretized differentiated spectrum that is used for quantification. The resulting model of “energy channel statistics” leads to a useful semi-quantitative background reduction procedure, which is validated by applying it to simulated data. Subsequently, the procedure is applied to an example of experimental SAM data. The analysis leads to conclusions regarding optimum experimental acquisition conditions. The proposed method of background reduction is based on general principles and should be useful for a broad variety of applications. - Highlights: • Atom-fraction–depth profiles of carbon measured by scanning Auger microprobe • Strong background, varies with local carbon concentration. • Needs correction e.g. for quantitative comparison with simulations • Quantitative theory explains background. • Provides background removal strategy and practical advice for acquisition.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Electron-irradiation-induced Cr segregation in Fe-Cr model alloy pre-implanted with hydrogen ions

Highlights: • Streaks appeared in the diffraction patterns of hydrogen-implanted Fe-Cr alloy after electron irradiation in an HVEM. • A kind of needle-like Cr precipitates give rise to the streaks. • The streaks are explained reasonably by shape effect of precipitates. - Abstract: Reduced activation ferritic/martensitic (RAFM) steels are widely accepted as the candidate structural materials for fusion reactors. To gain knowledge about Cr behaviour of these steels in such environments, Fe-Cr binary alloys have to be investigated. In this study, Fe-10 at.% Cr ferritic model alloy pre-implanted with hydrogen ions was electron-irradiated in a high voltage electron microscope (HVEM) at the temperature ranging from 300 °C to 600 °C. Streaks were observed in the diffraction patterns in the model alloy after electron irradiation at 450–550 °C and explained reasonably by shape effect. The analysis of transmission electron microscopy (TEM) reveals that the observed streaks are induced by a kind of needle-like precipitates with the directions along <100>. The dependence of the length and density of the precipitates on the temperature were studied. In addition, contrast experiments were conducted with pure Fe and Fe-10 at.% Cr alloy with and without hydrogen ion implantation, respectively. After electron irradiation, no streak was observed in the hydrogen-implanted pure Fe and Fe-10 at.% Cr alloy without the hydrogen implantation. Therefore, it is assumed that the needle-like precipitates could be the complex of Cr and H, which segregate and precipitate along <100> direction.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Twining and dynamic recrystallization in austenitic Alloy 718 during friction welding

Highlights: • DDRX was the primary grain refinement mechanism in friction welding of Alloy 718. • Two models were developed to explain the twin formation mechanism. • Intragranular twins were caused by grain boundary growth incidents. • Intergranular twin boundaries were ascribed to boundary growth stagnation. - Abstract: Quantitative boundary misorientation analysis verified that discontinuous dynamic recrystallization (DDRX) was the primary grain refinement mechanism in friction welding of Alloy 718. Two models were developed to explain the formation mechanism of intragranular twins and intergranular twins, respectively. Intragranular twins were caused by grain boundary growth incidents. After twin initiation, the twin boundary migrated toward the plastic-deformed grain interior to reduce the stored energy. Additionally, intergranular twin boundaries were ascribed to boundary growth stagnation. When boundary growth stagnation occurred, twin boundaries could form and altered the orientation of the boundary migration front, so that boundary migration could progress, leaving a single twin boundary behind.

36 MATERIALS SCIENCE↗