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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 1,189 records · Page 66
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Icosahedral quasicrystalline (Ti₁.₆V₀.₄Ni)₁₀₀₋xScx alloys: Synthesis, structure and their application in Ni-MH batteries

Thanks to the revolutionary discovery of 5-fold symmetry contributed by Shechtman, quasicrystal is now recognized as another solid-state existing form. As the second largest class of quasicrystals, titanium-based icosahedral quasicrystals are very promising for hydrogen storage applications owing to their inherent abundant interstitial sites and favorable hydrogen-metal chemistry. In this context, (Ti₁.₆V₀.₄Ni)₁₀₀₋xScx (x=0.5–6) quaternary icosahedral quasicrystals have been successfully synthesized via arc-melting and subsequent melt-spinning techniques, and then their electrochemical performance toward hydrogen is explored. When the molar ratio of Sc addition is under 1%, a maximum discharge capacity of about 270 mA h g⁻¹ can be delivered. With further increasing Sc amount to 6%, good cycling stability as well as significantly retarded self-discharge rate (capacity retention 94% after 24 h relaxation) is observed. But meanwhile, the discharge capacities fall into 250-240 mA h g⁻¹, and the electrocatalytic activity improvement is highly demanded. - Graphical abstract: Quasicrystalline Ti–V–Ni–Sc hydrogen storage materials: Sc addition into Ti₁.₆V₀.₄Ni alloy forms the icosahedral phase (see picture). With optimal Sc dosage, the anodic cycling stability and self-discharge property are greatly enhanced. - Highlights: • Crystalline disallowed 5-fold symmetry is present in (Ti₁.₆V₀.₄Ni)₁₀₀₋xScx alloys. • Ti-based metastable quasicrystalline alloys can store hydrogen electrochemically. • A maximum discharge capacity of 270 mA h g⁻¹ can be delivered. • Advantageous cycle stability and self-discharge property benefit from Sc addition. • Ti and V dissolution is suppressed by an oxide layer resulting from Sc corrosion.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Observation of atomic scale compositional and displacive modulations in incommensurate melilite electrolytes

The paradigm that functional materials are adequately described as three-dimensional crystal structures is not universally tenable. Gallate melilites are efficient oxide ion conductors at intermediate temperatures (∼750 °C) with non-rational crystallographic modulations presumed to play a key role in significantly enhancing oxygen mobility. Lattice distortions associated with incommensuration are usually extrapolated from diffraction analysis of volumes greatly exceeding the scale of modulation. Therefore, opportunities for making direct nanometric measurements are exceptionally valuable for correlating structure with function. In [CaLn]{sub 2}[Ga]{sub 2}[Ga{sub 2}O{sub 7}]{sub 2} (Ln=Nd, La) melilites, atomic displacive and compositional modulation waves can be imaged by high angle annular dark field and bright field scanning transmission electron microscopy with contrast quantified through electron scattering simulation. Here, we present atomic scale observations of (3+2)-dimensional modulations in gallate melilites which expands our understanding of the ion conduction mechanism and provides guidance for enhancing the performance of solid oxide fuel cells through crystal chemical tailoring. - Highlights: • Characterise the (3+2)-dimensional melilite electrolytes using STEM technique. • Direct observation on displacive and compositional modulation in melilites. • Structural flexibility reduces when increasing interstitial oxygen. • Domain-like incommensurate modulation model is proposed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Complex high-temperature phase transitions in Li{sub 2}B{sub 12}H{sub 12} and Na{sub 2}B{sub 12}H{sub 12}

Differential scanning calorimetry measurements of Li{sub 2}B{sub 12}H{sub 12} and Na{sub 2}B{sub 12}H{sub 12} indicate hysteretic transformations to high-temperature phases at ≈615 K and 529 K, respectively, upon heating (1 K/min) from room temperature. X-ray and neutron powder diffraction measurements corroborate the phase-change behavior. For Li{sub 2}B{sub 12}H{sub 12}, the diffraction data are consistent with a previous study suggesting that the overall face-centered-cubic arrangement of icosahedral B{sub 12}H{sub 12}{sup 2−} anions is maintained upon transformation to the high-temperature polymorph, although the anions are now orientationally disordered and the Li{sup +} cations crystallographically disordered within an enlarged lattice. For Na{sub 2}B{sub 12}H{sub 12}, the diffraction data indicate the existence of three different high-temperature phases in addition to the known low-temperature monoclinic phase. The highest-temperature structure possesses Im3{sup -}m symmetry and exhibits a body-centered-cubic arrangement of orientationally disordered anions. The interstitial, disordered Na{sup +} cations appear to favor off-center positions within the distorted tetrahedral sites formed by the anions in this structure. An intermediate Pm3{sup -}n-symmetric phase at lower temperature is the result of a partial ordering of this higher-temperature structure. A third, minor, face-centered-cubic phase coexists with these high-temperature polymorphs. {sup 1}H NMR measurements of Li{sub 2}B{sub 12}H{sub 12} and Na{sub 2}B{sub 12}H{sub 12} reveal an approximately two-orders-of-magnitude increase in the reorientational jump rate of the anions in both cases upon transformation to their high-temperature structures. The enhanced anion mobilities were corroborated by neutron scattering fixed-window scans across the respective phase boundaries. The inherent cation disorder associated with these high-temperature polymorphs suggests their potential use as superionic conductors. - Graphical abstract: Na{sub 2}B{sub 12}H{sub 12} undergoes a phase transformation from an ordered monoclinic structure at low temperature to a partially disordered body-centered-cubic (bcc) structure at ≈529 K, and finally to a more-fully disordered bcc structure by ≈545 K. - Highlights: • Li{sub 2}B{sub 12}H{sub 12} undergoes a high-temperature hysteretic phase transformation at ≈615 K. • Na{sub 2}B{sub 12}H{sub 12} undergoes a high-temperature hysteretic phase transformation at ≈529 K. • Increased anion mobility is observed with NMR upon these H-T phase transformations. • Three high temperature phases are observed for Na{sub 2}B{sub 12}H{sub 12}, with increasing disorder.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Crystal structures and compressibility of novel iron borides Fe{sub 2}B{sub 7} and Fe{sub x}B{sub 50} synthesized at high pressure and high temperature

We present here a detailed description of the crystal structures of novel iron borides, Fe{sub 2}B{sub 7} and Fe{sub x}B{sub 50} with various iron content (x=1.01(1), 1.04(1), 1.32(1)), synthesized at high pressures and high temperatures. As revealed by high-pressure single-crystal X-ray diffraction, the structure of Fe{sub 2}B{sub 7} possesses short incompressible B–B bonds, which make it as stiff as diamond in one crystallographic direction. The volume compressibility of Fe{sub 2}B{sub 7} (the bulk modulus K{sub 0}= 259(1.8) GPa, K{sub 0}′= 4 (fixed)) is even lower than that of FeB{sub 4} and comparable with that of MnB{sub 4}, known for high bulk moduli among 3d metal borides. Fe{sub x}B{sub 50} adopts the structure of the tetragonal δ-B, in which Fe atoms occupy an interstitial position. Fe{sub x}B{sub 50} does not show considerable anisotropy in the elastic behavior. - Graphical abstract: Crystal structures of novel iron borides, Fe{sub 2}B{sub 7} and Fe{sub x}B{sub 50} (x=1.01(1), 1.04(1), 1.32(1)). - Highlights: • Novel iron borides, Fe{sub 2}B{sub 7} and Fe{sub x}B{sub 50}, were synthesized under HPHT conditions. • Fe{sub 2}B{sub 7} has a unique orthorhombic structure (space group Pbam). • Fe{sub 2}B{sub 7} possesses short incompressible B–B bonds that results in high bulk modulus. • Fe{sub x}B{sub 50} adopts the structure of the tetragonal δ-B composed of B{sub 12} icosahedra. • In Fe{sub x}B{sub 50} intraicosahedral bonds are stiffer than intericosahedral ones.

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

Hydrogenation studies on NdScSi and NdScGe

NdScSi and NdScGe were synthesized from the elements via arc-melting and subsequent annealing. Their ordered La{sub 2}Sb type structures, with space group I4/mmm, were refined from single crystal X-ray diffractometer data: a=428.94(6) and b=1570.5(3) pm, wR2=0.0395, 309 F{sup 2} values for NdScSi and a=431.2(1) and c=1581.3(5) pm, wR2=0.1220, 227 F{sup 2} values for NdScGe, with 11 variables per refinement. Hydrogen insertion was performed on both Nd-based intermetallics by solid/gas reaction. Hydrogen uptake keeps the pristine compound space group but yields an anisotropic expansion of the unit cell with a large increase of c (≈+7%) and a slight decrease of a (≈−1.7%) parameters. Hydrogen absorption at 350 °C and under 5 bar of H{sub 2} pressure shows that the hydride NdScSiH{sub 1.48(5)} is formed. An in-situ neutron diffraction study during the deuteration of NdScSi reveals for the first time in a CeScSi-type compound, the possibility to fill two interstitial sites with deuterium atoms, leading to the composition NdScSiD{sub 1.5} for the deuteride adopting then the La{sub 2}Fe{sub 2}Se{sub 2}O{sub 3}-type structure. From magnetization measurements, we evidence that hydrogenation strongly reduces the Curie temperature of NdScSi (T{sub C}=175 K) and NdScGe (T{sub C}=194 K) since NdScSiH{sub 1.5} and NdScGeH{sub x} undergo a magnetic transition at 4 K and around 2 K, respectively. - Highlights: • Crystal structures of NdScSi and NdScGe were refined from single crystal XRD data. • The hydrogenation of NdScSi leads to the formation of the NdScSiH{sub 1.5} hydride. • NdScSiD{sub 1.5} adopts the La{sub 2}Fe{sub 2}Se{sub 2}O{sub 3}-type structure. • Hydrogenation strongly reduces the ordering temperature of both intermetallics.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Directly obtained τ-phase MnAl, a high performance magnetic material for permanent magnets

The metastable tetragonal τ-phase has been directly obtained from casting Mn{sub 0.54}Al{sub 0.46} and (Mn{sub 0.55}Al{sub 0.45}){sub 100}C{sub 2} using the drop synthesis method. The as-casted samples were ball milled to decrease the particle size and relaxed at 500 °C for 1 h. The phase composition, crystallographic parameters, magnetic properties and microstructure were systematically studied. The results reveal that the τ-phase could be directly obtained from drop synthesis. The highest M{sub s} of 117 emu/g was achieved in the (Mn{sub 0.55}Al{sub 0.45}){sub 100}C{sub 2} where the τ-phase was stabilized by doping with carbon. Carbon doping increased the c/a ratio of the τ-phase as it occupies specific interstitial positions (½, ½, 0) in the structure. Furthermore, ball milling increases the coercivity (H{sub c}) at the expense of a decrease in magnetic saturation (M{sub s}). The increase in coercivity is explained by a decrease of grain size in conjunction with domain wall pinning due to defects introduced during the ball milling process. - Graphical abstract: The tetragonal τ-phase has been directly obtained from casting Mn{sub 0.54}Al{sub 0.46} and (Mn{sub 0.55}Al{sub 0.45}){sub 100}C{sub 2} using the drop synthesis method. The phase composition, crystallographic parameters, magnetic properties and microstructure were systematically studied. The highest M{sub s} of 117 emu/g was achieved for (Mn{sub 0.55}Al{sub 0.45}){sub 100}C{sub 2} ball milling increases the coercivity (H{sub c}) at the expense of a decrease in magnetic saturation (M{sub s}). - Highlights: • The ferromagnetic τ-phase has been directly obtained from casting. • The highest M{sub s} of 117 emu/g was achieved for (Mn{sub 0.55}Al{sub 0.45}){sub 100}C{sub 2}. • Ball milling increases the coercivity but decreases the magnetic saturation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Interplay between crystal and magnetic structures in YFe{sub 2}(H{sub α}D{sub 1−α}){sub 4.2} compounds studied by neutron diffraction

We report a detailed magnetic structure investigation of YFe{sub 2}(H{sub α}D{sub 1−α}){sub 4.2} (α=0, 0.64, 1) compounds presenting a strong (H,D) isotope effect by neutron diffraction and Mössbauer spectroscopy analysis. They crystallize in the same monoclinic structure (Pc space group) with 8 inequivalent Fe sites having different H(D) environment. At low temperature, the compounds are ferromagnetic (FM) and show an easy magnetization axis perpendicular to the b axis and only slightly tilted away from the c axis. Upon heating, they display a first order transition from a ferromagnetic towards an antiferromagnetic (AFM) structure at T{sub M0} which is sensitive to the H/D isotope nature. The AFM cell is described by doubling the crystal cell along the monoclinic b axis. It presents an unusual coexistence of non magnetic Fe layer sandwiched by two thicker ferromagnetic Fe layers which are antiparallel to each other. This FM-AFM transition is driven by the loss of ordered moment on one Fe site (Fe7) through an itinerant electron metamagnetic (IEM) behaviour. The key role of the Fe7 position is assigned to both its hydrogen rich atomic environment and its geometric position. Above T{sub M0} a field induced metamagnetic transition is observed from the AFM towards the FM structure accompanied by a cell volume increase. Both thermal and magnetic field dependence of the magnetic structure are found strongly related to the anisotropic cell distortion induced by (H,D) order in interstitial sites. - Graphical abstract: Representation of the FM-AFM magnetic structures of YFe{sub 2}D{sub 4.2} deuteride. - Highlights: • YFe{sub 2}(H,D){sub 4.2} compounds undergoes a isotope sensitive FM-AFM transition at T{sub M0}. • The FM structure is formed of Fe moments perpendicular to the monoclinic b axis. • AFM structure is formed by antiparallel Fe layers separated by non-magnetic Fe layer. • One Fe site among eight loses its moment at T{sub M0} due to larger Fe–H bonding. • Magnetic properties are driven by the monoclinic distortion induced by D order.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Oxygen-storage behavior and local structure in Ti-substituted YMnO{sub 3}

Hexagonal manganates RMnO{sub 3} (R=Y, Ho, Dy) have been recently shown to exhibit oxygen-storage capacities promising for three-way catalysts, air-separation, and related technologies. Here, we demonstrate that Ti substitution for Mn can be used to chemically tune the oxygen-breathing properties of these materials towards practical applications. Specifically, Y(Mn{sub 1−x}Ti{sub x})O{sub 3} solid solutions exhibit facile oxygen absorption/desorption via reversible Ti{sup 3+}↔Ti{sup 4+} and Mn{sup 3+}↔Mn{sup 4+} reactions already in ambient air at ≈400 °C and ≈250 °C, respectively. On cooling, the oxidation of both cations is accompanied by oxygen uptake yielding a formula YMn{sup 3+}{sub 1−x-y}Mn{sup 4+}{sub y}Ti{sup 4+}{sub x}O{sub 3+δ}. The presence of Ti promotes the oxidation of Mn{sup 3+} to Mn{sup 4+}, which is almost negligible for YMnO{sub 3} in air, thereby increasing the uptake of oxygen beyond that required for a given Ti{sup 4+} concentration. The reversibility of the redox reactions is limited by sluggish kinetics; however, the oxidation process continues, if slowly, even at room temperature. The extra oxygen atoms are accommodated by the large interstices within a triangular lattice formed by the [MnO{sub 5}] trigonal bipyramids. According to bond distances from Rietveld refinements using the neutron diffraction data, the YMnO{sub 3} structure features under-bonded Mn and even more severely under-bonded oxygen atoms that form the trigonal bases of the [MnO{sub 5}] bipyramids. The tensile bond strain around the 5-fold coordinated Mn site and the strong preference of Ti{sup 4+}(and Mn{sup 4+}) for higher coordination numbers likely provide driving forces for the oxidation reaction. Reverse Monte Carlo refinements of the local atomic displacements using neutron total scattering revealed how the excess oxygen atoms are accommodated in the structure by correlated local displacements of the host atoms. Large displacements of the under-bonded host oxygen atoms play a key part in this lattice-relaxation process, facilitating reversible exchange of significant amounts of oxygen with atmosphere. - Graphical abstract: Concurrent redox reactions involving Ti and Mn yield facile absorption/desorption of excess oxygen. - Highlights: • Concurrent redox reactions involving Ti and Mn yield oxygen absorption/desorption. • Excess oxygen is accommodated as interstitials via correlated atomic shifts. • Oxygen breathing is facilitated by the under-bonding of host Mn and O atoms.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Identify OH groups in TiOF{sub 2} and their impact on the lithium intercalation properties

A detailed investigation on the chemical composition of the cubic form of titanium oxyfluoride-based compound reveals the presence of OH groups substituting the oxide/ fluoride sublattice. The substitution of oxide by hydroxyl groups induces the presence of titanium vacancies (□) which were characterized by {sup 1}H and {sup 19}F solid-state NMR. {sup 1}H NMR shows that OH groups are present as bridging Ti-OH-Ti or terminal OH groups, i.e. sitting close to a titanium vacancy Ti-OH-□. The electrochemical properties vs. Li{sup +}/Li indicates that the presence of OH groups prevents the intercalation of lithium in the upper voltage region (1.2–3 V vs. Li{sup +}/Li). Indeed, a partial dehydroxylation of the framework enables to improve the reversibility of the lithium insertion/de-insertion processes. Since the presence of OH groups in this type of compounds is usual and depends on the synthesis method employed, this work enables to rationalize the different electrochemical behaviors reported in the literature and further highlights the importance of a good knowledge of the chemical composition with regard to the physico-chemical properties. - Graphical abstract: The substitution of oxide by hydroxyl groups inducing the formation of titanium vacancies (□), i.e., Ti{sub 1−x}□{sub x}O{sub 1-4x}(OH){sub 4x+y}F{sub 2−y}, was characterized by solid-state {sup 1}H and {sup 19}F NMR. - Highlights: • Evidences of the presence of OH groups and titanium vacancies in titanium oxyfluoride. • {sup 1}H NMR showed the presence of Ti-OH-Ti and Ti-OH-□ species. • The presence of OH groups limits the insertion of lithium within the interstitial sites.

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

Structural unit charge density and molecular cation templating effects on orientational geometric isomerism and interlayer spacing in 2-D uranyl sulfates

The new uranyl aqua sulfate compound [({CH_3}{sub 3}N{sub 2}C{sub 3}H{sub 3}{CH_2COOH})(H{sub 3}O)] [(UO{sub 2}){sub 2}(SO{sub 4}){sub 3}(H{sub 2}O)]·1.5H{sub 2}O was synthesized and the crystal structure determined. The two-dimensional uranyl-aqua sulfate compound exhibits a previously reported anionic structural unit topology, but features an interstitial complex incorporating hydronium in the form of a disorder Zundel cation and the cation moiety from the ionic liquid 1-carboxymethyl 3-methylimidazolium bistriflimide [HBetMIm][Tf{sub 2}N], which forms hydrogen bonding dimer pairs. An analysis of similar uranyl sulfate complexes from the literature is conducted to explore the relative importance and role of structure directing factors in alkylammonium charge-balanced two-dimensional uranyl sulfates. A correlation is observed between structural unit charge density and interlayer spacing, with a concomitant direction of structural and orientational geometric isomerism of the anionic structural unit. We discuss the role of synthetic conditions and molecular cation properties in directing structural unit formation through charge density matching and hydrogen bonding.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Electron doping of NdNiO 3 thin films using dual chamber CaH 2 annealing

Hydrogen donor doping has been exploited as a strategy to manipulate the electronic structure and electrical properties of functional oxide systems. Especially, the development of synthetic methods to achieve electron doping of perovskite rare-earth nickelate thin films utilizing interstitial hydrogen is highly desirable considering the rich electronic phase diagram hosting several functional properties. Here, in this work, we present the hydrogenation of NdNiO 3 (NNO) thin films using CaH 2 annealing and the resulting giant modulation of electrical resistivity in hydrogenated NNO (H–NNO) thin films. Magnetron sputtering was employed to deposit epitaxial ~60 nm-thin NNO films on single crystal LaAlO 3 (LAO) substrates. The formation of the pristine perovskite NNO phase was realized after annealing the films at 500 °C for 24 h. CaH 2 annealing of NNO thin films for time durations ranging from 1 to 6 h was performed in a vacuumed ampule with two interconnected chambers at 280 °C. The two-chamber design enables a simple and clean approach for hydrogen doping without physical contact between the powder and sample of interest. X-ray diffraction and Raman spectroscopy revealed the formation of NNO in pristine samples, and the subsequent hydrogen incorporation upon CaH 2 annealing without forming any impurity phases. The conversion of the oxidation state of Ni towards +2 upon CaH 2 annealing was probed using X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. Consequently, a substantial increase in the room-temperature resistivity was observed upon CaH 2 annealing indicating the formation of a strongly correlated electronic configuration of Ni in H–NNO due to electron doping. Overall, the findings of this study highlight the versatility of CaH 2 annealing as an electron doping method to tune the electrical properties of correlated oxides at low temperatures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Reaction path model of the formation of abiotic immiscible hydrocarbon fluids in subducted carbonated serpentinites, Lanzo Massif (Western Italian Alps)

Fluids generated from subducted slabs participate in the cycling of deep carbon in the crust and upper mantle. In these fluids, aqueous carbon species vary in oxidation state between +IV and -IV depending on whether the fluids are oxidizing or reducing, respectively. Most studies of subduction-zone fluids have focused on oxidized carbon species. However, recent studies of natural samples have demonstrated the occurrence of deep, reducing fluids, generated in both subducted oceanic upper mantle and crustal rocks. CH 4 -H 2 -rich fluid inclusions in subducted carbonated serpentinites have demonstrated the existence of abiotic, immiscible, hydrocarbon fluids at upper mantle conditions. To investigate the formation of such immiscible hydrocarbon fluids during the evolution of subducted carbonated serpentinites, we used equilibrium constants from the Deep Earth Water model to carry out predictive chemical mass transfer modeling to simulate the alteration reactions. A novel feature of the models was the inclusion of an immiscible hydrocarbon fluid containing six components (CH 4,f , C 2 H 6,f , C 3 H 8,f , isoC 4 H 10,f , CO 2,f , H 2,f ). This feature enabled prediction of the formation of a separate immiscible fluid in equilibrium with aqueous species and minerals. We developed a predictive reaction path model of invasive H 2,f reacting with carbonated serpentinites and interstitial aqueous fluids for comparison with the natural samples from the Lanzo Massif, western Italian Alps. Over a range of temperatures and pressures, immiscible hydrocarbon fluids formed in association with altered mineral assemblages. Reaction progress caused the transformation of carbonated serpentinites and the formation of clinopyroxene, brucite, graphite, and hydrocarbon fluids, along with changes of pH, logfO 2 , and aqueous species. CH 4,f was the most abundant hydrocarbon species in all the models. The overall results at 2.0 GPa and 400 to 450 °C were consistent with the natural samples from the Lanzo Massif. Interestingly, large amounts of H 2 O formed due to oxidation of H 2 . More hydrocarbons and H 2 O formed in models with lower fluid/rock mass ratios or with more reactant H 2 . Models at different pressure and temperature conditions showed similar results with some variation in the relative stabilities of aragonite, graphite and olivine solid solution, and associated differences in mineral sequences, hydrocarbon fluids, values of aqueous species, and the final log fO 2 and pH. As a result, our models strongly support the laboratory and field evidence that reduction of carbonated serpentinites by infiltrating H 2 fluids can cause the formation of immiscible, abiotic hydrocarbon fluids in subduction zones.

58 GEOSCIENCES↗
DOE OSTI2013

Structure analysis of aluminium silicon manganese nitride precipitates formed in grain-oriented electrical steels

We report a detailed structural and chemical characterisation of aluminium silicon manganese nitrides that act as grain growth inhibitors in industrially processed grain-oriented (GO) electrical steels. The compounds are characterised using energy dispersive X-ray spectrometry (EDX) and energy filtered transmission electron microscopy (EFTEM), while their crystal structures are analysed using X-ray diffraction (XRD) and TEM in electron diffraction (ED), dark-field, high-resolution and automated crystallographic orientation mapping (ACOM) modes. The chemical bonding character is determined using electron energy loss spectroscopy (EELS). Despite the wide variation in composition, all the precipitates exhibit a hexagonal close-packed (h.c.p.) crystal structure and lattice parameters of aluminium nitride. The EDX measurement of ∼ 900 stoichiometrically different precipitates indicates intermediate structures between pure aluminium nitride and pure silicon manganese nitride, with a constant Si/Mn atomic ratio of ∼ 4. It is demonstrated that aluminium and silicon are interchangeably precipitated with the same local arrangement, while both Mn{sup 2+} and Mn{sup 3+} are incorporated in the h.c.p. silicon nitride interstitial sites. The oxidation of the silicon manganese nitrides most likely originates from the incorporation of oxygen during the decarburisation annealing process, thus creating extended planar defects such as stacking faults and inversion domain boundaries. The chemical composition of the inhibitors may be written as (AlN){sub x}(SiMn{sub 0.25}N{sub y}O{sub z}){sub 1−x} with x ranging from 0 to 1. - Highlights: • We study the structure of (Al,Si,Mn)N inhibitors in grain oriented electrical steels. • Inhibitors have the hexagonal close-packed symmetry with lattice parameters of AlN. • Inhibitors are intermediate structures between pure AlN and (Si,Mn)N with Si/Mn ∼ 4. • Al and Si share the same local arrangement; Mn is incorporated in both Mn{sup 2+} and Mn{sup 3+}. • Oxygen incorporation is invoked to account for the thermal stability of (Al,Si,Mn)N.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Atom probe study of grain boundary segregation in technically pure molybdenum

Molybdenum, a metal with excellent physical, chemical and high-temperature properties, is an interesting material for applications in lighting-technology, high performance electronics, high temperature furnace construction and coating technology. However, its applicability as a structural material is limited because of the poor oxidation resistance at high temperatures and a brittle-to-ductile transition around room temperature, which is influenced by the grain size and the content of interstitial impurities at the grain boundaries. Due to the progress of the powder metallurgical production during the last decades, the amount of impurities in the current quality of molybdenum has become so small that surface sensitive techniques are not applicable anymore. Therefore, the atom probe, which allows the detection of small amounts of impurities as well as their location, seems to be a more suitable technique. However, a site-specific specimen preparation procedure for grain boundaries in refractory metals with a dual focused ion beam/scanning electron microscope is still required. The present investigation describes the development and successful application of such a site-specific preparation technique for grain boundaries in molybdenum, which is significantly improved by a combination with transmission electron microscopy. This complimentary technique helps to improve the visibility of grain boundaries during the last preparation steps and to evidence the presence of grain and subgrain boundaries without segregants in atom probe specimens. Furthermore, in industrially processed and recrystallized molybdenum sheets grain boundary segregation of oxygen, nitrogen and potassium is successfully detected close to segregated regions which are believed to be former sinter pores. - Highlights: • First study of grain boundary segregation in molybdenum by atom probe • Site-specific preparation technique by FIB and TEM successfully developed • Grain boundary segregation of oxygen, nitrogen and potassium found • Segregation in former sinter-pores detected • Presence of grain boundaries without segregation evidenced.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Elemental segregation and subsequent precipitation during solidification of continuous cast Nb–V–Ti high-strength low-alloy steels

In this study, elemental segregation during solidification and subsequent precipitation behaviour in a continuous cast Nb–V–Ti high-strength low-alloy steel was investigated by optical microscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy and thermodynamic modelling. It is known that for steels with low carbon contents the pearlite that forms on slow cooling does so where the interdendritic liquid was present prior to final solidification. The alloying elements of Nb, Ti, Mn and V segregate into the interdendritic liquid during solidification, while Al preferentially segregates into the solidifying solid phase. The composition analysis on the slab samples verified the predicted element segregation behaviour, with a smaller difference in the concentrations of Mn and V in the pearlite and dendritic ferrite regions being observed compared to the Nb levels. Small (30–100 nm) spherical or irregular shaped Nb-rich precipitates (Nb(C, N) and (Nb, V)(C, N)) were mainly found in the pearlite regions, while angular Al-rich (60–300 nm) precipitates were found in the dendritic ferrite regions, in the form of AlN and complex AlN–V(C, N) precipitates. Small isolated ferrite regions surrounded by pearlite were observed in the microstructure and has two origins: one type is dendritic ferrite that appears as an isolated island due to a sectioning effect when observing the two-dimensional microstructure; the other is a ferrite idiomorph that forms in the interdendritic region due to the low carbon content of the steel. Accordingly, in these isolated ferrite islands two different precipitation behaviours are found; predominantly Al-rich particles in the dendritic regions or predominantly Nb-rich precipitates in the interdendritic ferrite idiomorphs. No Al-rich precipitates were observed in the interdendritic regions (pearlite or isolated ferrite idiomorphs) despite the Thermo-Calc predictions indicating a higher volume fraction of AlN in these regions compared to the dendritic regions. This is believed to be due to back diffusion of nitrogen after solidification reducing the available nitrogen, which reacts with the high Nb levels present. - Highlights: • Nb-rich precipitates (Nb(C, N) and (Nb, V)(C, N)) were mainly found in pearlite. • Angular Al-rich precipitates (AlN and AlN–V(C, N)) were found in dendritic ferrite. • Precipitation behaviour in isolated ferrite islands within pearlite was revealed. • Segregation of substitutional elements during solidification is retained. • Back diffusion of interstitial elements (C, N) occurred.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Microstructural characterization of weld joints of 9Cr reduced activation ferritic martensitic steel fabricated by different joining methods

This paper presents a detailed electron microscopy study on the microstructure of various regions of weldment fabricated by three welding methods namely tungsten inert gas welding, electron beam welding and laser beam welding in an indigenously developed 9Cr reduced activation ferritic/martensitic steel. Electron back scatter diffraction studies showed a random micro-texture in all the three welds. Microstructural changes during thermal exposures were studied and corroborated with hardness and optimized conditions for the post weld heat treatment have been identified for this steel. Hollomon–Jaffe parameter has been used to estimate the extent of tempering. The activation energy for the tempering process has been evaluated and found to be corresponding to interstitial diffusion of carbon in ferrite matrix. The type and microchemistry of secondary phases in different regions of the weldment have been identified by analytical transmission electron microscopy. - Highlights: • Comparison of microstructural parameters in TIG, electron beam and laser welds of RAFM steel • EBSD studies to illustrate the absence of preferred orientation and identification of prior austenite grain size using phase identification map • Optimization of PWHT conditions for indigenous RAFM steel • Study of kinetics of tempering and estimation of apparent activation energy of the process.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Microstructure evolution and mechanical behavior of a high strength dual-phase steel under monotonic loading

Transmission electron microscopy (TEM) microstructures of a high-strength dual-phase steel DP800 have been examined after moderate plastic deformations in simple shear and uniaxial tension. Special attention has been paid to the effect of the intergranular hard phase (martensite) on the microstructure evolution in the near-grain boundary regions. Quantitative parameters of dislocation patterning have been determined and compared with the similar characteristics of previously examined single-phase steels. The dislocation patterning in the interiors of the ferrite grains in DP800 steel is found to be similar to that already observed in the single-phase IF (Interstitial Free) steel whereas the martensite-affected zones present a delay in patterning and display very high gradients of continuous (gradual) disorientations associated with local internal stresses. The above stresses are shown to control the work-hardening of dual-phase materials at moderate strains for monotonic loading and are assumed to influence their microstructure evolution and mechanical behavior under strain-path changes. - Highlights: • The microstructure evolution has been studied by TEM in a DP800 steel. • It is influenced by both martensite and dislocations in the initial state. • The DP800 steel presents a high work-hardening rate due to internal stresses.

36 MATERIALS SCIENCE↗