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At least 1,333 records · Page 74
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

On Possible States of the Crystal Structure Preceding to a Phase Transition in Zn{sub 1 –} {sub x}V{sub x}Se (0.01 ≤ x ≤ 0.10) Crystals

The systematic new formations observed in the reciprocal lattice of the cubic structural modification of a II–VI compound are characterized using a detailed neutron diffraction study of bulk semiconducting ZnSe crystals with an increased vanadium content. Direct evidence that the additional sites k = (1/3 1/3 1/3) 2π/a (k is the wave vector and a is cubic unit cell parameter) observed by neutron scattering in the crystals, in the case when they belong to mutually penetrated rotated sublattices, contain a superstructure contribution formed by short-wave deformation, is obtained for the first time. This structure state is determined as a pretransition to the concentration fcc–hcp phase transformation, and the basis functions that allow one to analyze atomic displacements, the correlation between which create distortion-type superstructures, are indicated for the transition through one-arm channel, considering the transitions by the star of wave vector k{sub 5} of the fcc lattice.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Characterization of hot-pressed short ZrO{sub 2} fiber toughened ZrB{sub 2}-based ultra-high temperature ceramics

Two different ZrB{sub 2}-based ultra-high temperature ceramics were produced by hot pressing: ZrB{sub 2} + 20 vol.% SiC particle + 15 vol.% ZrO{sub 2} fiber and ZrB{sub 2} + 20 vol.% SiC whisker + 15 vol.% ZrO{sub 2} fiber. The microstructures were analyzed by using transmission electron microscopy and high-resolution transmission electron microscopy. It was shown that a clean interface without any impurities was identified in ZrB{sub 2}-based hybrid ceramics with SiC whiskers and ZrO{sub 2} fibers, which would significantly improve the toughening mechanism. The results of high-resolution transmission electron microscopy showed that stacking faults in SiC whiskers resulted from an insertion of a (111) layer, which would be one of the main reasons for material anisotropy. However, the interface between the SiC particle and ZrO{sub 2} fiber was found to be ambiguous in ZrB{sub 2}-based hybrid ceramics with SiC particles and ZrO{sub 2} fibers due to the slight reaction. The orientation relationship between t-ZrO{sub 2} and m-ZrO{sub 2} phases obeyed the classical correspondence: (100){sub m}//(100){sub t} and [001]{sub m}//〈001〉{sub t}, which further verified the feasibility of phase transformation toughening mechanism. - Highlights: • ZrB{sub 2}-based ceramics toughened by short ZrO{sub 2} fiber are characterized by TEM and HRTEM. • The orientation relationship of t- and m-ZrO{sub 2} are (100){sub m}//(100){sub t}, [001]{sub m}//〈001〉{sub t} • The clean interface without any impurities leads to improve the toughening mechanism.

36 MATERIALS SCIENCE↗
DOE OSTI2019

High-Temperature Heat Capacity and Thermal Extension of FeTa{sub 2}O{sub 6}

The heat capacity of synthesized iron tantalate FeTa{sub 2}O{sub 6} was measured in the temperature range of 323–1103 K by the ratio method using a thermal analyzer combining thermogravimetry and scanning calorimetry. The temperatures of phase transformations were determined. Structural changes and thermal expansion of oxide were studied in the range of 300–1173 K by high-temperature X-ray diffraction. The temperature dependences of the unit cell parameters were approximated by third order polynomials. The values of thermal expansion coefficients and anisotropy factors were calculated based on the obtained data.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Structure, phase transitions, and isotope effects in [(CH3)4N]2PuCl6

The single crystal X-ray diffraction structure of [(CH3)4N]2PuCl6 is presented for the first time, resolving long standing confusion and speculation regarding the structure of this compound in the literature. A temperature dependent study of this compound shows that the structure of [(CH3)4N]2PuCl6 undergoes no fewer than two phase transitions between 100 and 360 K. The phase of [(CH3)4N]2PuCl6 at room temperature is Fd-3c a = 26.012(3) Å. At 360 K, the structure is in space group Fm-3m with a = 13.088(1) Å. The plutonium octahedra and tetramethylammonium cations undergo a rotative displacement and the degree of rotation varies with temperature, giving rise to the phase transition from Fm-3m to Fd-3c as the crystal is cooled. Synthesis and structural studies of the deuterated salt [(CD3)4N]2PuCl6 suggest that there is an isotopic effect associated with this phase transition as revealed by a changing transition temperature in the deuterated versus protonated compound indicating that the donor-acceptor interactions between the tetramethylammonium cations and the hexachloroplutonate anions are driving the phase transformation.

crystallography↗
DOE OSTI2017

Shock-Driven Hydrodynamic Instability Growth Near Phase Boundaries and Material Property Transitions: Final Report

Activities for this grant included: 1) Development of dynamic impact experiments to probe strength and phase transition influence on dynamic deformation, 2) development of modern strength and phase aware simulation capabilities, 3) and post-processing of experimental data with simulation and closed form analytical techniques. Two different dynamic experiments were developed to probe material strengths in solid metals (largely copper and iron in this effort). In the first experiment a flyer plate impacts a flat target with an opposite rippled surface that is partially supported by a weaker window material. Post mortem analysis of the target sample showed a strong and repeatable residual plastic deformation dependence on grain orientation. Yield strengths for strain rates near 10 5 s -1 and plastic strains near ~50% were estimated to be around 180 to 240 MPa, varying in this range with grain orientation. Unfortunately dynamic real-time measurements were difficult with this setup due to diagnostic laser scattering; hence, an additional experimental setup was developed to complement these results. In the second set of experiments a rippled surface was ablated by a controlled laser pulsed, which launched a rippled shock front to an opposite initially flat diagnostic surface that was monitored in real-time with spatially resolved velocimetry techniques, e.g., line VISAR in addition to Transient Imaging Displacement Interferometry (TIDI) displacement measurements. This setup limited the displacements at the diagnostic surface to a reasonable level for TIDI measurements (~ less than one micrometer). These experiments coupled with analytical and numerical solutions provided evidence that viscous and elastic deviatoric strength affect shock front perturbation evolution in clearly different ways. Particularly, normalized shock front perturbation amplitudes evolve with viscosity (η) and perturbation wavelength (λ) as η/λ, such that increasing viscosity (or decreasing the initial wavelength) delays the perturbation decay. Conversely our experimental data, analysis and simulations show that for materials with elastic yield strength Y the normalized shock perturbation amplitude evolves with Yλ/A 0 , which shows wavelength increases have the opposite effect as in viscous materials and perturbation decay is also dependent on initial amplitude A 0 (viscous materials are independent of this parameter). Materials where strength had clear strain rate dependence, e.g., such as a PTW material law, behaved similarly to materials with only an effective yield stress (elastic-perfectly plastic) in the shock front perturbation studies obeying a Y eff λA 0 relationship where Y eff was a constant (near ~400 MPa for Cu for strain rates around 10 6 s -1 ). Magnitude changes in strain rate would increase Y eff as would be expected from the PTW behavior, but small perturbations (typical of regions behind the shock front) near a mean had little effect. Additional work based on simulations showed that phase transformation kinetics can affect the behavior of the perturbed shock front as well as the evolution of the RM-like instability that develops due to the imprint of the perturbed shock front on the initially flat surface as the shock breaks out.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Luminescence switching in Ce{sup 3+} ion sensitized YPO{sub 4}:Tb{sup 3+} through Redox reaction

Highlights: • Simple polyol method for preparation of Ce{sup 3+} sensitized Tb{sup 3+} ion doped YPO{sub 4} nanophosphor. • Enhanced green color emission with incorporation of Ce{sup 3+} ion into the matrix of YPO{sub 4}:Tb{sup 3+} nanophosphor. • Highly crystalline tetragonal YPO{sub 4}:Tb{sup 3+} without any impurity phase. • Use of alternate oxidising and reducing agent for luminescence switching (ON and OFF). - Abstract: Ce{sup 3+} sensitized Tb{sup 3+} ion doped YPO{sub 4} nanophosphor is synthesized by simple polyol method using ethylene glycol as capping agent as well as reaction medium. Green emission from Tb{sup 3+} ion doped YPO{sub 4} nanophosphor has been enhanced with incorporation of Ce{sup 3+} ion into the matrix of YPO{sub 4}:Tb{sup 3+}. XRD of YPO{sub 4}:Tb{sup 3+} annealed at 900 °C reveals crystalline tetragonal structure without any impurity phase. Morphology of YPO{sub 4}:Tb{sup 3+} nanophosphor was also studied. Homogenous incorporation of rare earth, RE (RE = Tb{sup 3+}, Ce{sup 3+}) ion into the host matrix were given in EDAX spectra. TGA/DTA analysis shows phase transformation of the as-prepared sample. Also, it is found that the as-prepared sample were readily re-dispersible in water due to presence of −OH group. By using an alternate oxidising and reducing agent, behaviour of luminescence switching (ON and OFF) can be examined. Lifetime decay of Tb{sup 3+} ion doped YPO{sub 4} nanophosphor was also studied.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Two-dimensional X-ray diffraction and transmission electron microscopy study on the effect of magnetron sputtering atmosphere on GaN/SiC interface and gallium nitride thin film crystal structure

The growth mechanisms of high quality GaN thin films on 6H-SiC by sputtering were investigated by X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM). The XRD θ-2θ scans show that high quality (0002) oriented GaN was deposited on 6H-SiC by reactive magnetron sputtering. Pole figures obtained by 2D-XRD clarify that GaN thin films are dominated by (0002) oriented wurtzite GaN and (111) oriented zinc-blende GaN. A thin amorphous silicon oxide layer on SiC surfaces observed by STEM plays a critical role in terms of the orientation information transfer from the substrate to the GaN epilayer. The addition of H{sub 2} into Ar and/or N{sub 2} during sputtering can reduce the thickness of the amorphous layer. Moreover, adding 5% H{sub 2} into Ar can facilitate a phase transformation from amorphous to crystalline in the silicon oxide layer and eliminate the unwanted (33{sup ¯}02) orientation in the GaN thin film. Fiber texture GaN thin films can be grown by adding 10% H{sub 2} into N{sub 2} due to the complex reaction between H{sub 2} and N{sub 2}.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

Hydrogen atom distribution and hydrogen induced site depopulation for the La{sub 2-x}Mg{sub x}Ni{sub 7}-H system

La{sub 2-x}Mg{sub x}Ni{sub 7} and its hydrides/deuterides were investigated by high resolution synchrotron powder X-ray and neutron diffraction. Upon deuteration the single phase sample of the intermetallic compound with the refined composition La{sub 1.63}Mg{sub 0.37}Ni{sub 7} (space group: P6{sub 3}/mmc) expands isotropically, in contrast to the Mg free phase. The hydrogen uptake, {approx}9 D/f.u., is higher than in La{sub 2}Ni{sub 7}D{sub 6.5}. The refined composition accounts for La{sub 1.63}Mg{sub 0.37}Ni{sub 7}D{sub 8.8} (beta-phase). Rietveld refinements using the neutron and synchrotron diffraction data suggest that deuterium atoms occupy 5 different interstitial sites within both AB{sub 2} and AB{sub 5} slabs, either in an ordered or a disordered way. All determined D sites have an occupancy >50% and the shortest D-D contact is 1.96(3) A. It is supposed that a competition between the tendency to form directional bonds and repulsive D-D (H-H) interactions is the most important factor that influences the distribution of deuterium atoms in this structure. A hitherto unknown second, alpha-phase with composition La{sub 1.63}Mg{sub 0.37}Ni{sub 7}D{sub 0.56}, crystallizing with the same hexagonal symmetry as La{sub 1.63}Mg{sub 0.37}Ni{sub 7}D{sub 8.8}, has been discovered. The unit cell parameters for this D-poor phase differ slightly from those of the intermetallic. Alpha-phase displays only one D site (4f, space group: P6{sub 3}/mmc) occupied >50%, which is not populated in the D-rich beta-phase. This hydrogen/deuterium induced site depopulation can be explained by repulsive D-D (H-H) interactions that are likely to influence non-occupancy of certain interstices in metal lattice when absorbing hydrogen. - Graphical abstract: The detailed D atoms arrangement in La{sub 1.63}Mg{sub 0.37}Ni{sub 7}D{sub 8.8} differs significantly from the previously reported La{sub 1.5}Mg{sub 0.5}Ni{sub 7}D{sub 8.9(9.1)}. The present model consists of only five deuterium sites as opposed to nine proposed for La{sub 1.5}Mg{sub 0.5}Ni{sub 7}D{sub 8.9(9.1)}. The reported four remaining deuterium atom positions in La{sub 1.5}Mg{sub 0.5}Ni{sub 7}D{sub 8.9(9.1)} were not found in the investigated La{sub 1.63}Mg{sub 0.37}Ni{sub 7}D{sub 8.8}. The five Ni atoms have deuterium among their nearest neighbors, which surround them in a way similar to configurations observed in some complex transition metal hydrides and already reported for metallic hydrides. In the presented deuterium-rich phase, deformed tetrahedron, rigid trigonal pyramids as well as disordered and deformed saddle-like configuration are observed. Highlights: Black-Right-Pointing-Pointer Alpha- and beta-phase for La{sub 2-x}Mg{sub x}Ni{sub 7}-H system have been characterized. Black-Right-Pointing-Pointer Five different interstitial sites are occupied by deuterium/hydrogen atoms in the beta-phase. Black-Right-Pointing-Pointer One D/H site has been determined in the alpha-phase. Black-Right-Pointing-Pointer Deuterium/hydrogen induced site depopulation during phase transformation is observed. Black-Right-Pointing-Pointer Ni atoms tend to have tetrahedral-like D/H atom coordination.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Modeling of melting and erosion in high velocity microparticle impacts

High velocity collisions can cause melting, a situation involving elasticity, plasticity, and phase transformations in tandem, which is very challenging for mechanical modeling. Here, this paper advances the use of smoothed particle hydrodynamics (SPH) simulations for this complex situation, investigating stainless steel microspheres impacting a tin substrate. After extensive validation against a range of mechanical and geometrical measurements of experimental impacts, the model provides a detailed, quantitative view of melt volume production over a broad range of impact velocities. It also enables direct decomposition of the impact energy partitioning, including analysis of the energy absorbed by plasticity in the impactor, the system kinetic energy, and the adiabatic heating of the substrate in addition to the melting itself. Since these quantities are not available experimentally, SPH modeling substantially enhances the quantitative understanding of such impacts. Finally, the model speaks to the phase characteristics of the ejecta formed during impact, confirming the experimental inference that the ejected material likely contains a significant fraction of solid material.

Constitutive modeling↗
DOE OSTI2013

Non-destructive evaluation of degradation in EB-PVD thermal barrier coatings by infrared reflectance spectroscopy

At room temperature and atmospheric conditions infrared reflectance spectroscopy and X-ray diffraction were employed for the detection of the phase transformation and residual stress within thermal barrier coatings (TBC). The TBC's samples initially consisted of the porous ceramic topcoat deposited by electron beam plasma vapor deposition, a bond coat and a superalloy substrate. Reflectance spectroscopy scans were performed from 7497 cm{sup -1} to 68 cm{sup -1} to analysis the fingerprint region as well as the chemical bonding region. These regions should indicate if a detectable change within the TBC response is a result of thermal degradation of the microstructure and the changes in yttrium dispersion throughout the yttrium stabilized zirconium. The thermal degradation was induced by thermal cycling the samples to 1100 Degree-Sign C and then cooling them in an atmospheric environment. X-ray diffraction was also used to detect the phase composition within the TBC samples and see if either would clearly identify failure prior to actual spallation. The eventual measurability and quantify-ability of the phase changes within the TBC's may be used as an effective non-destructive evaluation (NDE) technique that would allow personnel in the field to know when servicing of the turbine blade was necessary.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Manipulate the Electronic and Magnetic States in NiCo 2 O 4 Films through Electric-Field-Induced Protonation at Elevated Temperature

Ionic-liquid-gating- (ILG-) induced proton evolution has emerged as a novel strategy to realize electron doping and manipulate the electronic and magnetic ground states in complex oxides. While the study of a wide range of systems (e.g., SrCoO 2.5 , VO 2 , WO 3 , etc.) has demonstrated important opportunities to incorporate protons through ILG, protonation remains a big challenge for many others. Furthermore, the mechanism of proton intercalation from the ionic liquid/solid interface to whole film has not yet been revealed. Here, with a model system of inverse spinel NiCo 2 O 4 , an increase in system temperature during ILG forms a single but effective method to efficiently achieve protonation. Moreover, the ILG induces a novel phase transformation in NiCo 2 O 4 from ferrimagnetic metallic into antiferromagnetic insulating with protonation at elevated temperatures. Finally, this study shows that environmental temperature is an efficient tuning knob to manipulate ILG-induced ionic evolution.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Martensite crystallography and chemistry in dual phase and fully martensitic steels

Highlights: • Lath martensite islands and bands were indexed in variations of dual-phase steels. • Detail insight was obtained by combined EBSD, APT, TEM, nano-indentation analysis. • The islands predominantly consist of one main packet with all six variants. • The bands are composed of connected domains, each largely made of one main packet. • Tempering at ~450 °C causes carbon partition to lath boundaries and dislocation cores. - Abstract: Lath martensite is important in industry because it is the key strengthening component in many advanced high strength steels. The study of crystallography and chemistry of lath martensite is extensive in the literature, however, mostly based on fully martensitic steels. In this work, lath martensite in dual phase steels is investigated with a focus on the substructure identification of the martensite islands and microstructural bands using electron backscattered diffraction, and on the influence of the accompanied tempering process during industrial coating process on the distribution of alloying elements using atom probe tomography. Unlike findings for the fully martensitic steels, no martensite islands with all 24 Kurdjumov-Sachs variants have been observed. Almost all martensite islands contain only one main packet with all six variants and minor variants from the remaining three packets of the same prior austenite grain. Similarly, the martensite bands are typically composed of connected domains originating from prior austenite grains, each containing one main packets (mostly with all variants) and few separate variants. The effect of tempering at ~450 °C (due to the industrial zinc coating process) has also been investigated. The results show a strong carbon partitioning to lath boundaries and Cottrell atmospheres at dislocation core regions due to the thermal process of coating. In contrast, auto-tempering contributes to the carbon redistribution only in a limited manner. The substitutional elements are all homogenously distributed. The phase transformation process has two effects on the material: mechanically, the earlier-formed laths are larger and softer and therefore more ductile (as revealed by nanoindentation); chemically, due to the higher dislocation density inside the later-formed laths, which are generally smaller, carbon Cottrell atmospheres are predominantly observed.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Beam current effect as a potential challenge in SEM-EBSD in situ tensile testing

Highlights: • In situ SEM-EBSD tensile tests were conducted using 304L steels with 8 and 10.4% Ni. • KAM and GROD evolution, twinning, and phase instability were investigated. • Phase transformation in 8% Ni-steel was sensitive to the electron beam current. In situ mechanical tensile tests coupled with electron backscatter diffraction (EBSD) analysis were conducted using specimens of two 304L steels with ~8 and 10.4% Ni content, respectively. Twinning was observed in steel with 10.4% Ni content, whereas steel with 8% Ni content demonstrated martensitic transformation. Testing showed that the kinetics of martensitic transformation in steel with 8% Ni content was influenced by the electron beam current used during EBSD scans. Continuous scanning using 32 nA nominal beam current led to suppression of the phase instability in the near-surface layer, but acceptable comparable-to-the-bulk results were obtained using 8 nA beam current. The exact mechanism of the beam current role is unclear, but it appears to be neutral regarding Kernel Average Misorientation behavior and deformation-induced twinning.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Microstructural Modeling of the α + β Phase in Ti-6Al-4V: A Diffusion-Based Approach

Complex heat treatment operations and advanced manufacturing processes such as laser or electron-beam welding will see the metallic workpiece experience a considerable range of temperatures and heating/cooling rates. These intrinsic conditions will have a significant bearing upon the microstructure of the material, and in turn upon the thermo-mechanical properties. In this work, a diffusion-based approach to model the growth and shrinkage of precipitates in the alpha + beta field of the common titanium alloy Ti-6Al-4V is established. Further, the numerical model is extended using a JMA-type approach to explore the dependency of the beta-transus temperature on extremely high heating rates, whereby dissolution alone is too slow to accurately describe the alpha to beta-phase transformation. Experimental heat treatments at rates of 5, 50, and 500 °C/s were performed, and metallographic analysis of the samples was used to validate the numerical modeling framework predictions for lamellar shrinkage, while data from the literature has been used to evaluate the numerical modeling framework for the growth of equiaxed microstructures. The agreement between measurements and numerical predictions was found to be good.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Size-dependent transition of deformation mechanism, and nonlinear elasticity in Ni{sub 3}Al nanowires

A size-dependent transition of deformation mechanism is revealed in Ni{sub 3}Al nanowire under atomistic uniaxial tension. Deformation twinning is replaced by phase transformation when the diameter of Ni{sub 3}Al nanowire reduces to a critical value near 4 nm. Enhanced size-dependent nonlinear elasticity is observed in the nanowires, in comparison to their bulk counterpart which is benchmarked by combined density functional and atomistic study. This study provide fundamental understanding on the size-dependent deformation mechanisms of nanostructured alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2019

The Oxidation Mechanism of Pure Magnesium Powder Particles: A Mathematical Approach

In this study, the mechanism of nonisothermal oxidation of magnesium powder was determined using a tension analysis, which was performed by mathematical modeling and the finite element method. The results revealed that phase transformation of magnesium hydroxide to magnesium oxide in the oxide layer plays an important role in this process so that formation of magnesium hydroxide in the early stages of oxidation (< 450 °C) leads to the formation of compressive stresses through the oxide layer, and as a result, the beginning of measurable oxidation is delayed. The results obtained by the mathematical model and finite element method were validated by the experimental results.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Low-temperature variation of magnetic order in a nonmagnetic n-Ge:As semiconductor in the vicinity of the metal-insulator phase transition

The phenomenon of the low-temperature transition from antiferro- to ferromagnetic ordering of impurity spins in a nonmagnetic compensated n-Ge:As semiconductor near the metal-insulator phase transformation has been experimentally observed. The effect is manifested by rather sharp changes in the spin density and g-factor in the electron spin resonance spectra. As the relative content of a compensating impurity (gallium) is reduced below 0.7, the transition temperature begins to decrease and, at a degree of compensation below 0.3, drops below the studied temperature range (i.e., below 2 K).

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗