The neutron time-of-flight diffractometer HIPPO (High Pressure Preferred Orientation) was conceived and constructed as a joint effort between the University of California and Los Alamos National Laboratory. The unique detector coverage of HIPPO, with a total of 1360 He-3 detectors arranged in 50 panels, allows measurement of the sample orientation distribution function with rotation around only one axis. This capability allows design of sample environments, such as furnaces or loads cells, to measure texture in situ at non-ambient conditions. The high flux of the LANSCE spallation neutron source shortens count-times in some cases to less than 10 minutes to collect enough data for the determination of the ODF, opening the possibility of time-resolved texture studies. The capabilities for texture measurements of HIPPO are outlined and some examples of experiments conducted with HIPPO, such as phase transformation textures, are given.

We attempted to characterize by neutron powder diffraction the monoclinic alpha' phase that is known to form at low temperatures in dilute Pu-Ga alloys. This attempt was unsuccessful, as we did not detect any transformation to the alpha' phase, but instead observed a line-broadening effect in the fcc delta phase. This effect is large enough to be visible in the raw diffraction data and is highly anisotropic in crystal space. The onset temperature of the line broadening (150 K) coincides with previous observations of the delta-alpha' transformation. Bulk alpha' was not observed. We believe that the development of alpha' nuclei creates a spatially inhomogeneous stress distribution in the delta matrix, which in turn exhibits an anisotropic response, governed by its elastic anisotropy. We have analysed this observation of anisotropic microstrains in terms of the fictive microstresses required to produce them by elastic deformation. During the course of this work, we found a pseudo-isotope effect in the room temperature lattice constants of Pu-Ga alloys. The alloys made from nominal Pu-242 isotope show systematically higher lattice constants than the corresponding Pu-239 alloys, and the size of the effect is proportional to the Ga concentration. We believe that this effect is associated with the higher levels of radiation damage from isotopic impurities in the Pu-242 alloys.

We report on the temperature variation of the interatomic distances in isostructural U2Ni2In and U2Pd2Sn, both of which order antiferromagnetically at low temperatures. Both compounds exhibit complex non-collinear arrangements of the magnetic moments confined to the tetragonal basal plane, which is perpendicular to the shortest interuranium distance along the c-axis at low temperatures. The different temperature dependencies of the shortest interatomic links between uranium and the transition metal (Ni or Pd) provide evidence for the dual nature of 5f–d hybridization in these two compounds. We argue that magnetic ordering in U2Pd2Sn arises due to increased 5f–d hybridization (promoting stronger exchange) while the reduced hybridization in U2Ni2In allows for the formation of stable U magnetic moments.

In this paper we describe the capabilities for texture measurements of the new neutron time-of-flight diffractometer HIPPO at the Los Alamos Neutron Science Center. The orientation distribution function (ODF) is extracted from multiple neutron time-of-flight histograms using the full-pattern analysis first described by Rietveld. Both, the well-established description of the ODF using spherical harmonics functions and the WIMV method, more recently introduced for the analysis of time-of-flight data, are available to routinely derive the ODF from HIPPO data. At ambient conditions, total count time of less than one hour is ample to collect sufficient data for texture analysis in most cases. The large sample throughput for texture measurements at ambient conditions possible with HIPPO requires a robust and reliable, semi-automated data analysis. HIPPO’s unique capabilities to measure large quantities of ambient condition samples and to measure texture at temperature and uni-axial stress are described. Examples for all types of texture measurements are given

The co-deformation of Cu-Ag or Cu-Nb composite wires used for high-field magnets has a number of important microstructural consequences, including the production of very-fine-scale structures, the development of very high internal surface-area-to-volume ratios during the drawing, and the storage of defects at interphase interfaces. In addition, the fabrication and co-deformation of the Cu and Ag or Nb, which differ in crystal structure, thermal expansion, elastic modulus and lattice parameter, lead to the development of short-wavelength internal stresses in both composites. In this paper, these internal stresses are characterized by neutron diffraction and transmission electron microscopy as a function of the imposed drawing strain. The internal stresses lead to important changes in the elastic-plastic response, which is related to both magnet design and service life. The second derivative partial derivative(2)sigma/partial derivative(2)epsilon of the stresses with respect to strain is used to describe the low-strain anelasticity of the composites. The internal stresses in Cu-Nb are higher than in Cu-Ag and, consequently, the absolute values of (partial derivative(2)sigma/partial derivative(2)epsilon)(Cu-Nb) are higher than those of (partial derivative(2)sigma/partial derivative(2)epsilon)(Cu-Ag) at low strains.

Abstract In powder-diffraction measurements made by elastic scattering of thermal neutrons, diffuse scattering shows up as a weak, oscillating component in the scattering data. For years, we observed such diffuse scattering from plutonium (Pu) and its alloys, but we did not know how to analyze it. This analysis is important for any material to account for all scattering, and it is especially important for Pu because of its role in the nuclear weapons program. We used neutron pow-der diffraction at Los Alamos Neutron Science Center (LAN-SCE) to help us discover the cause of diffuse scattering in our experiments. We analyzed the data with tools recently incorporated into the General Structure Analysis System (GSAS)—a powder-diffraction data-analysis package used to refine and determine crystal structure. We found that the observed diffuse scattering comes from lattice vibrations (not from some other exotic mechanism) and is quite an ordinary, non-Pu-specific phenomenon.

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The use of severe plastic deformation techniques such as equal channel angular pressing, has been shown to refine metal microstructures giving advantageous mechanical properties. Metals and alloys subjected to ECAP procedures can have very high yield strengths while maintaining substantial ductility, a unique and attractive combination. However, the implicitly large deformations (the application of repeated shear strains of similar to1 are typical) make prediction of the resulting mechanical properties difficult. In particular, modeling the polycrystalline texture evolution and microstructural strain response is challenging. In this paper, results are presented from a neutron diffraction study on aluminum, copper, nickel and beryllium processed by ECAP. Specific attention is given to the evolution of the bulk texture after one pass and the effect of the initial texture. The neutrons probed volumes on the order of cubic centimeters and therefore provided texture and strain information averaged over the bulk of the sample. The results are discussed in the context of a visco-plastic self-consistent model.

Rietveld refinements using neutron powder diffraction data were used to locate H atom positions and obtain a more precise crystal structure refinement for akaganeite [Fe7.63+Ni0.42+O6.35 (OH)(9.65)Cl(1.25)(.)nH(2)O]. Difference Fourier maps clearly showed H atoms positions near those O atoms at the midpoints of the tunnel edges. The O-H vectors point toward the Cl sites at the center of the tunnel, and weak hydrogen bonds likely form between the framework 0 atoms and Cl. The Cl position is near the center of a prism defined by the eight hydroxyl H atoms. The Cl atoms fill similar to2/3 of the tunnel sites, suggesting an ordering scheme in a given tunnel with every third tunnel site vacant. Such an arrangement allows the Cl anions to increase their separation distance along a tunnel by displacing away from one another toward their respective adjacent vacancies. The Fe-O octahedra in akaganeite are distorted with Fe-(O, OH) distances ranging from 1.94 to 2.13 Angstrom and show three longer and three shorter Fe-O distances; as expected the longer distances are associated with the OH- anions.Temperature-resolved synchrotron X-ray powder diffraction data and Rietveld refinements were used to investigate changes in the akagandite structure and its transformation into hematite as it was heated from 26 to 800 degreesC. Rietveld refinements revealed surprising consistency in all unit-cell parameters between room temperature and similar to225 degreesC, resulting in nearly zero thermal expansion of the akaganeite structure over a 200 degreesC interval. Above similar to225 degreesC, the unit-cell volume gradually decreased, primarily in response to decreases in c and b, and an increase in the P angle. The a parameter remained nearly constant until similar to225 degreesC and increased thereafter. Akaganeite started to transform to hematite in the temperature range 290 to 310 degreesC with no evidence for maghemite as an intermediate phase.

. We present a feasibility study to extract quantitative texture and precise crystal structure information of polycrystalline monoclinic NiTi shape-memory alloys from a simultaneous refinement of 52 time-of-flight neutron-diffraction patterns taken in 13 orientations in the diffractometer. The multiple-data-set capabilities and the generalized spherical harmonic texture model of the GSAS program system were employed.

The multiple-data-set capabilities of the Rietveld refinement program GSAS was used to extract a quantitative texture description from a simultaneous refinement of 52 time-of-flight neutron diffraction patterns taken from a NiTi polycrystalline sample held in a variety of orientations in the diffractometer. The patterns were analyzed using a generalized spherical-harmonic model for the preferred orientation. With this method the accuracy of the available crystal structure parameters of the monoclinic B 19’ phase of near stochiometric NiTi (50.14 atomic \% Ni) system at room temperature has been improved dramatically.

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We are using the techniques of neutron and x-ray powder diffraction to learn more about the driving forces for structural transformations in Pu metal. This research is currently being conducted in three areas: (1) understanding the low melting point of Pu, (2) measuring the intergranular stresses that arise in solid-solid phase transitions of Pu, and (3) determining whether the idiosyncratic crystal structures of Pu are stabilized by Pu atoms in different valence states at different crystallographic sites.

We report some high P-T diffraction experimental results on molybdenum using synchrotron x-ray and time-of-flight neutron-diffraction techniques. Unit-cell dimensions, measured up to P = 10 GPa and T = 1475 K, were derived from the refinement results and fitted to a high-temperature third-order Birch-Murnaghan equation of state. The derived thermoelastic parameters for molybdenum are: isothermal bulk modulus K-T0 = 266(9) GPa with a pressure derivative of bulk modulus partial derivative K/partial derivative P = 4.1(9), temperature derivative of bulk modulus partial derivative K/partial derivative T = -3.4(9) X 10(-2)GPa/K, volumetric thermal expansivity alpha = a + bT having a = 1.32(14) x 10(-5) K-1 and b = 1.26(15) X 10(-8) K-2. Further, all of the previous experimental data involving shock wave, ultrasonic, thermal-expansion measurements were also analyzed and fitted to a high-temperature Birch/Vinet equation of state (EOS) and the thermal pressure approach of Mie-Gruneisen EOS, respectively. With the greatly extended P-V-T data coverage, we refined the thermoelastic parameter set for molybdenum as isothermal bulk modulus K-T0 = 268(1) GPa with pressure derivatives of bulk modulus partial derivative K/partial derivative P = 3.81(6), partial derivative K-2/partial derivative P-2 = -1.41(13) X 10(-2), temperature derivative of bulk modulus partial derivative K/partial derivative T = -2.13(31) X 10(-2) GPa/K, volumetric thermal expansivity alpha = a + bT having a = 1.31(10) x 10(-5) K-1 and b = 1.12(11) X 10(-8) K-2, and an invariant of alpha K-T,= 5.43(5) X 10(-3) GPa/K over a wide P-T range. We have conducted a neutron-diffraction study at simultaneous high pressures and high temperatures. Thermal vibrations of atoms (Debye-Waller factors) of molybdenum were derived as a function of pressure and temperature. The experimental results of thermoelastic equation of state parameters are compared with previous experimental data derived from shock wave and ultrasonic elasticity measurements.

High pressure Raman and neutron scattering study of carbon blacks and highly oriented pyrolytic graphite is reported. It is found that carbon black particles are composed of graphitic nanocrystallites and amorphous carbon. Pressure-induced order in inter-atomic distances within nanocrystallites is completely reversible. Relative concentration of amorphous carbon decreases slightly with increased pressure. This process differs from temperature induced transformation of amorphous carbon into ordered carbon. Post-production treatment at high temperatures results in lateral and vertical growth of graphitic crystallites, and at sufficiently high temperatures almost all amorphous carbon is transformed into graphitic structures. Within the pressure range under study, 5 GPa, only a small fraction of amorphous carbon is transformed into ordered structures. Pressure induced frequency shift of the E2g bands of various carbon blacks is explained in terms of a modified intermolecular potential.

We report a high-resolution synchrotron X-ray powder diffraction study on HfV(2), hafnium divanadium, at low temperatures. In this work we show, for the first time, a complete sequence of structural phase transitions of HfV(2) from cubic (Fd3m) to tetragonal (I4(1)/amd) to orthorhombic (Imma) in succession as temperature decreases. Peak splitting and extra diffraction peaks owing to lattice distortion can be clearly distinguished for the low-symmetry phases. The atomic positions and lattice parameters were obtained by Rietveld refinement. The bond lengths and angles of the HfV(2) crystal structure at the low-symmetry phases were correctly determined from the structure refinement. The face-centered cubic (Fd3m) unit cell (Z = 24) transforms to a body-centered tetragonal (I4(1)/amd) phase with a 45 degrees rotation relative to the cubic cell and with a reduced number of atoms (Z = 12) in the unit cell at a temperature of T = 112 K. The orthorhombic phase occurs at T = 102 K and it keeps the body-centered symmetry (Imma) and Z = 12 in the unit cell. The refinement results indicate that there may be a small amount of untransformed cubic phase left over in the lower symmetry phases. The abnormal thermal contraction of both tetragonal phase and orthorhombic phase marks the significance of structural change in HfV(2).

In an earlier paper (Lawson et al., 2000, Phil. Mag. B, 80, 53) we described the measurement and interpretation of Debye-Waller factors in a delta -phase Pu0.98Ga0.02 alloy. We concluded that the measurements could be described using a Debye model with a strongly temperature-dependent Debye-Waller temperature. In those experiments, we observed significant diffuse scattering in the neutron powder diffraction patterns. In this paper we describe a new technique for analysing diffuse scattering based on Rietveld analysis, and apply the technique to data for F-phase Pu0.98Ga0.02 and for Pb. The results from the correlations among lattice vibrations show that the diffuse scattering is Warren-Borie scattering and that the diffuse scattering is in basic agreement with that expected from the Debye model except that the scattering from the nearest-neighbour correlations in Pu0.98Ga0.02 is anomalous at high temperatures. We also discuss the observation of significant anisotropic broadening of the diffraction peaks for Pu0.98Ga0.02 at low temperatures.

By combining high-resolution X-ray powder diffraction data and stereochemical restraints, Rietveld refinement of protein crystal structures has been shown to be feasible. A refinement of the 1261-atom protein metmyoglobin was achieved by combining 5338 stereochemical restraints with a 4648-step ( d min = 3.3 Å) powder diffraction pattern to give the residuals R wp = 2.32%, R p = 1.66%, R ( F 2 ) = 3.10%. The resulting tertiary structure of the protein is essentially identical to that obtained from previous single-crystal studies.