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At least 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Experimental and Computational Investigation of High Entropy Alloys for Elevated-Temperature Applications

To create and design novel structural materials with enhanced creep-resistance, fundamental studies have been conducted on high-entropy alloys (HEAs), using (1) thermodynamic calculations, (2) mechanical tests, (3) neutron diffraction, (4) characterization techniques, and (5) crystal-plasticity finite-element modeling (CPFEM), to explore future candidates for next-generation power plants. All the constituent binary and ternary systems of the Al-Cr-Cu-Fe-Mn-Ni and Al-Co-Cr-Fe-Ni systems were thermodynamically modeled within the whole composition range. Comparisons between the calculated phase diagrams and literature data are in good agreement. Seven types of HEAs were fabricated from Al-Cr-Cu-Fe-Mn-Ni and Al-Co-Cr-Fe-Ni systems. The Al x CrCuFeMnNi HEAs have disordered [face-centered cubic (FCC) + body-centered cubic (BCC)] crystal structures, not FCC or BCC single structure. Excessive alloying of the Al element results in the change of both microstructural and mechanical properties in Al x CoCrFeNi HEAs. There are mainly three structural features in Al x CoCrFeNi: (1) the morphology, (2) the volume fractions of the constitute phases, and (3) existing temperatures of all six phases. After homogenization, the Al 0.3 CoCrFeNi material is a pure FCC solid solution. After aging at 700 °C for 500 hours, the optimal microstructure combinations, the FCC matrix, needle-like B2 phase within grains, and granular σ phase along grain boundary, is achieved for Al 0.3 CoCrFeNi. The cold-rolling process is utilized to reduce the grain size of Al 0.1 CoCrFeNi and Al 0.3 CoCrFeNi. The chemical elemental partitioning of FCC, BCC, B2, and σphases at different temperatures, before and after mechanical tests, in Al-Cr-Cu-Fe-Mn-Ni and Al-Co-Cr-Fe-Ni systems are quantitatively characterized by both synchrotron X-ray diffraction, neutron diffraction with levitation, scanning electron microscopy (SEM), advanced atom probe tomography (APT), and transmission electron microscopy (TEM). In-situ neutron diffraction experiments were conducted to study the strengthening effect of B2 phase on tensile properties of Al 0.3 CoCrFeNi HEAs directly. The results shows the creep behavior of Al 0.3 CoCrFeNi is superior to conventional alloys, and the heat treatment introduces secondary B2 phase into the FCC matrix, which increase the yielding strength, decrease the ductility, diminish the serrated flow during compression tests at high temperatures. In summary, the outcomes of the development of the HEAs with creep resistance include: (1) Suitable candidates, for the application to boilers and steam and gas turbines at temperatures above 760 °C and a stress of 35 MPa. (2) Fundamental understanding on the precipitate stability and deformation mechanisms of both single-phase and precipitate-strengthened alloys at room and elevated temperatures, and (3) The demonstration of an integrated approach, coupling modeling [thermodynamic calculations and crystal-plasticity finite-element modeling (CPFEM)] and focused experiments, to identify HEAs that outperform conventional alloys for high-temperature applications, which will be applicable for the discovery and development of other high-temperature materials in the power-generating industry.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Ice rule for a ferromagnetic nanosite network on the face-centered cubic lattice

The magnetic properties of an inverse opal-like cobalt-based structure having the symmetry of the face-centered cubic lattice are studied. The magnetization reversal of the structure in a magnetic field applied along the [11̄0] axis is described using a phenomenological model, which uses the ice rule for the local magnetization of nanostructure elements. This description predicts the absence of a long-range magnetic order in two <111> directions that are normal to the magnetic field. The magnetic structure is analyzed by smallangle neutron diffraction. Neutron diffraction patterns are measured in an external magnetic field varying from −1.2 to 1.2 T and applied along the crystallographic [11̄0] direction. A magnetic contribution to the neutron scattering intensity is extracted. It is shown that this contribution is close to zero over the entire applied magnetic field range for all scattering planes that are normal to the field, which agrees well with the proposed model.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Crystal structure and partial Ising-like magnetic ordering of orthorhombic D y 2 Ti O 5

The structure and magnetic properties of orthorhombic Dy 2 TiO 5 have been investigated using x-ray diffraction, neutron diffraction, and alternating current (ac)/direct current (dc) magnetic susceptibility measurements. In this paper, we report a continuous structural distortion below 100 K characterized by negative thermal expansion in the [0 1 0] direction. Neutron diffraction and magnetic susceptibility measurements revealed that two-dimensional (2D) magnetic ordering begins at 3.1 K, which is followed by a three-dimensional magnetic transition at 1.7 K. The magnetic structure has been solved through a representational analysis approach and can be indexed with the propagation vector k = [0 1/2 0]. The spin structure corresponds to a coplanar model of interwoven 2D “sheets” extending in the [0 1 0] direction. The local crystal field is different for each Dy 3+ ion (Dy1 and Dy2), one of which possesses strong uniaxial symmetry indicative of Ising-like magnetic ordering. In conclusion, consequently, two succeeding transitions under magnetic field are observed in the ac susceptibility, which are associated with flipping each Dy 3+ spin independently.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Superconducting and structural properties of {delta}-MoC{sub 0.681} cubic molybdenum carbide phase

The superconducting and lattice properties of {delta}-MoC{sub 0.681} were studied by electromagnetic measurements, synchrotron X-ray diffraction, neutron diffraction, and electron diffraction. The superconducting properties (T{sub c}=12 K) of {delta}-MoC{sub 0.681} were well characterized by a weak coupling model. The carbon vacancies present in the host cubic structure were found to be robust, although the material was synthesized from stoichiometric carbon and Mo powder under a high-pressure of 6 GPa. A thermodynamically-stable structure with ordered vacancies did not account for the robust features of {delta}-MoC{sub 0.681} since the vacancies are unlikely to be ordered in long range in the host structure. A model based on inherent phonon instability theoretically predicted for a stoichiometric MoC phase might be responsible for the robust features of {delta}-MoC{sub 0.681}. - Graphical Abstract: The cubic molybdenum carbide shows an excellent superconductivity with robust carbon vacancies. Inherent phonon instability theoretically predicted for a stoichiometric MoC phase might be responsible for the vacancies rather than a thermodynamically-stable structure with vacancies ordering. Highlights: Black-Right-Pointing-Pointer The 12 K superconductivity is well characterized by a weakly coupling model. Black-Right-Pointing-Pointer Carbon vacancies are robust and disordered in the cubic host structure. Black-Right-Pointing-Pointer Inherent phonon instability might be responsible for the robust carbon vacancies in {delta}-MoC{sub 0.681}.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Structure and component dynamics in binary mixtures of poly(2-(dimethylamino)ethyl methacrylate) with water and tetrahydrofuran: A diffraction, calorimetric, and dielectric spectroscopy study

We have combined X-ray diffraction, neutron diffraction with polarization analysis, small angle neutron scattering, differential scanning calorimetry, and broad band dielectric spectroscopy to investigate the structure and dynamics of binary mixtures of poly (2-(dimethylamino)ethyl methacrylate) with either water or tetrahydrofuran (THF) at different concentrations. Aqueous mixtures are characterized by a highly heterogeneous structure where water clusters coexist with an underlying nano-segregation of main chains and side groups of the polymeric matrix. THF molecules are homogeneously distributed among the polymeric nano-domains for concentrations of one THF molecule/monomer or lower. A more heterogeneous situation is found for higher THF amounts, but without evidences for solvent clusters. In THF-mixtures, we observe a remarkable reduction of the glass-transition temperature which is enhanced with increasing amount of solvent but seems to reach saturation at high THF concentrations. Adding THF markedly reduces the activation energy of the polymer β-relaxation. The presence of THF molecules seemingly hinders a slow component of this process which is active in the dry state. The aqueous mixtures present a strikingly broad glass-transition feature, revealing a highly heterogeneous behavior in agreement with the structural study. Regarding the solvent dynamics, deep in the glassy state all data can be described by an Arrhenius temperature dependence with a rather similar activation energy. However, the values of the characteristic times are about three orders of magnitude smaller for THF than for water. Water dynamics display a crossover toward increasingly higher apparent activation energies in the region of the onset of the glass transition, supporting its interpretation as a consequence of the freezing of the structural relaxation of the surrounding matrix. The absence of such a crossover (at least in the wide dynamic window here accessed) in THF is attributed to the lack of cooperativity effects in the relaxation of these molecules within the polymeric matrix.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2010

Temperature and composition dependent structural investigation of the defect perovskite series Sr{sub 1-x}Ti{sub 1-2x}Nb{sub 2x}O{sub 3}, 0{<=}x{<=}0.2

The crystal structure of the defect perovskite series Sr{sub 1-x}Ti{sub 1-2x}Nb{sub 2x}O{sub 3} has been investigated over a range of temperatures using high-resolution synchrotron X-ray diffraction, neutron diffraction and electron diffraction. Three distinct regions were observed: 0<x{<=}0.125 was a solid solution of Sr{sub 1-x}Ti{sub 1-2x}Nb{sub 2x}O{sub 3} with minor SrTiO{sub 3} intergrowth, 0.125<x{<=}0.2 was a pure Sr{sub 1-x}Ti{sub 1-2x}Nb{sub 2x}O{sub 3} solid solution adopting the cubic perovskite type structure (Pm3-bar m) and for x>0.2 Sr{sub 0.8}Ti{sub 0.6}Nb{sub 0.4}O{sub 3} and Sr{sub 3}TiNb{sub 4}O{sub 15} formed a two phase region. The cubic structure for Sr{sub 0.8}Ti{sub 0.6}Nb{sub 0.4}O{sub 3} was stable over the temperature range 90-1248 K and the thermal expansion co-efficient was determined to be 8.72(9)x10{sup -6} K{sup -1}. Electron diffraction studies revealed diffuse scattering due to local scale Ti/Nb displacements and slightly enhanced octahedral rotations that did not lead to long range order. The octahedral rotations were observed to 'lock-in' at temperatures below {approx}75 K resulting in a tetragonal structure (I4/mcm) with anti-phase octahedral tilting about the c-axis. - Graphical abstract: Change in cell dimensions for the series Sr{sub 1-x}Ti{sub 1-2x}Nb{sub 2x}O{sub 3}, 0{<=}x{<=}0.2.

36 MATERIALS SCIENCE↗
DOE OSTI2017

The Structure of Liquid and Amorphous Hafnia

Understanding the atomic structure of amorphous solids is important in predicting and tuning their macroscopic behavior. Here, we use a combination of high-energy X-ray diffraction, neutron diffraction, and molecular dynamics simulations to benchmark the atomic interactions in the high temperature stable liquid and low-density amorphous solid states of hafnia. The diffraction results reveal an average Hf–O coordination number of ~7 exists in both the liquid and amorphous nanoparticle forms studied. The measured pair distribution functions are compared to those generated from several simulation models in the literature. We have also performed ab initio and classical molecular dynamics simulations that show density has a strong effect on the polyhedral connectivity. The liquid shows a broad distribution of Hf–Hf interactions, while the formation of low-density amorphous nanoclusters can reproduce the sharp split peak in the Hf–Hf partial pair distribution function observed in experiment. The agglomeration of amorphous nanoparticles condensed from the gas phase is associated with the formation of both edge-sharing and corner-sharing HfO6,7 polyhedra resembling that observed in the monoclinic phase.

36 MATERIALS SCIENCE↗
DOE OSTI2014

A search for temperature induced time-dependent structural transitions in 10mol%Sc 2 O 3 –1mol%CeO 2 –ZrO 2 and 8mol%Y 2 O 3 –ZrO 2 electrolyte ceramics

Both Sc 2 O 3 –CeO 2 -stabilized-ZrO 2 (SCSZ) and Y 2 O 3 -stabilized-ZrO 2 (YSZ) show similar Young's modulus damping at 175–400 °C and 200–400 °C, respectively, by impulse excitation acoustic technique. The phase transition in SCSZ is considered responsible for the damping; however for YSZ, such a phase transition has never been reported. To clarify the relation of damping and structural transition in these two materials, the time-dependent high temperature stabilities are studied by in situ X-ray diffraction, neutron diffraction and Raman scattering during long-term annealing of SCSZ at 350 °C and YSZ at 275 °C. The cubic-to-rhombohedral transition in SCSZ is detected. However, the existence of t' phase in YSZ raw powders is confirmed without significant changes of the cubic structure during isothermal annealing. It is concluded that the phase transition contributes to the Young's modulus damping in SCSZ, while the structural transition is excluded from the reason for damping in YSZ.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Crystal structures of orthorhombic, hexagonal, and cubic compounds of the Sm{sub (x)}Yb{sub (2−x)}TiO{sub 5} series

A series of single phase compounds with nominal stoichiometry Sm{sub (x)}Yb{sub (2−x)}TiO{sub 5} (x=2, 1.4, 1, 0.6, and 0) have been successfully fabricated to generate a range of crystal structures covering the most common polymorphs previously discovered in the Ln{sub 2}TiO{sub 5} series (Ln=lanthanides and yttrium). Four of the five samples have not been previously fabricated in bulk, single phase form so their crystal structures are refined and detailed using powder synchrotron and single crystal x-ray diffraction, neutron diffraction and transmission electron microscopy. Based on the phase information from diffraction data, there are four crystal structure types in this series; orthorhombic Pnma, hexagonal P6{sub 3}/mmc, cubic (pyrochlore-like) Fd-3m and cubic (fluorite-like) Fm-3m. The cubic materials show modulated structures with variation between long and short range ordering and the variety of diffraction techniques were used to describe these complex crystal structure types. - Graphical abstract: A high resolution image of the compound Sm{sub 0.6}Yb{sub 1.4}TiO{sub 5} showing contrast from lattice fringes and the corresponding fast Fourier transform (FFT) of the HREM image with pyrochlore related diffraction spots marked “P” and fluorite marked “F”. The crystal is oriented down the [1 1 0] zone axis based on the Fd-3m structure. The ideal crystal structure (no vacancies) of the cubic, pyrochlore-like (Sm{sub 0.6}Yb{sub 1.4}TiO{sub 5}). - Highlights: • First fabrication of bulk single-phase material with stoichiometry Sm{sub 2}TiO{sub 5}. • Systematic study of crystal structure types within Ln{sub 2}TiO{sub 5} series (Ln=lanthanides). • A novel technique using IFFT of HREM images to study cubic structures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Tunable structural and magnetic properties of NiAs-type Mn x Sb (1.00≤x≤1.30) compounds

Mn-based alloys can exhibit a variety of magnetic properties related to their tunable exchange interactions. Here, structural and magnetic properties of hexagonal NiAs-type Mn x Sb (1.00≤x≤1.30) compounds are studied with a combination of X-ray diffraction, neutron diffraction, and magnetic measurements. It is found that the magnetization, Curie temperature, and magnetocrystalline anisotropy of Mn x Sb compounds can be tuned by controlling Mn concentration and doping at interstitial (2d) sites. With increasing Mn concentration, magnetic moments of Mn atoms decrease and deviate gradually from the ab-plane to the c-axis for 1.20≤x≤1.22 at room temperature, which leads to a decrease of the Curie temperature T c and the spin reorientation temperature T SR . By doping with nonmagnetic elements, a substantial room-temperature magnetocaloric effect without magnetic hysteresis was achieved for Mn 1.12 Zr 0.1 Sb compound, which is highly beneficial to the practical magnetic refrigeration application.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Phase separated behavior in yttrium doped CaMnO{sub 3}

Highlights: • Electron doped manganite. • Magnetic phase separation observed. • Structural transition coupled with magnetic transition. - Abstract: The effect of electron doping on the structural, transport, and magnetic properties of Mn (IV)—rich Ca{sub 1−x}Y{sub x}MnO{sub 3} (x ≤ 0.2) samples have been investigated using neutron diffraction, neutron depolarization, magnetization and resistivity techniques. The temperature dependence of resistivity follows the small polaron model and the activation energy exhibits a minimum for x = 0.1 sample. A phase separated magnetic ground state consisting of ferromagnetic domains (∼7 μm) embedded in G-type antiferromagnetic matrix is observed in the sample, x = 0.1. The transition to the long-range magnetically ordered state in this sample is preceded by a Griffith’s phase. On lowering temperature below 300 K a structural transition from orthorhombic structure (Pnma) to a monoclinic structure (P2{sub 1}/m) is observed in the case of x = 0.2 sample. The ferromagnetic behavior in this case is suppressed and the antiferromagnetic ordering is described by coexisting C-type and G-type magnetic structures corresponding to the monoclinic and orthorhombic phases, respectively.

36 MATERIALS SCIENCE↗