C. J. Sparks

Mosaic crystal X-ray spectrometer to resolve inelastic background from anomalous scattering experiments

Abstract We describe a mosaic crystal X-ray energy analyzing spectrometer to separate the inelastic scattering enhanced by radiation near an absorption edge from the elastic diffraction of interest for atomic site determinations. An important consideration is to minimize the sensitivity to changing source profile so that diffracted intensities are comparable over large angular ranges. Energy resolution limited mainly by fractional-mm source sizes is comparable to that with nearly perfect crystals. High spectrometer efficiency for the diffusely distributed X-ray scattering associated with amorphous materials and the weak Laue scattering from chemical order in crystalline solid solutions is achieved with a combination of sagittal and parafocusing from a mosaic graphite monochromator. Because of the mosaic nature of the crystal, the parafocusing geometry achieves meridional focusing for a magnification of 1 : 1, and a broad energy range proportional to the mosaic spread is dispersed along a linear position-sensitive detector for simultaneous detection. Energy resolution ΔE/E of 1 500 and lower is achieved in the 6–10-keV energy range tested. Though the sagittal radius is fixed, the spectrometer will operate effectively over an X-ray energy range of E ± 0.5E with fixed focal distances. This mosaic crystal spectrometer offers similar advantages for X-ray spectroscopy experiments.

Focussing optics for a synchrotron-based X-ray microprobe

Abstract X-ray microprobe experiments at third-generation synchrotron sources will provide trace element analysis of samples with 1 μm × 1 μm spatial resolution and femtogram sensitivity. For these experiments to be possible. X-ray optical elements need to be developed to focus the beam from a hard X-ray undulator to a micron spot size. In June 1991 several different optical elements were tested during a dedicated undulator run at CHESS. The undulator produced radiation similar to that which will be available at third-generation sources like the Advanced Photon Source. Both Fresnel zone plates and multilayer-coated spherical mirrors were tested. With the Fresnel zone plate a spot size of 8.5 μm × 30 μm was achieved in the first order and 6 μm × 20 μm in the second order. With a Kirkpatrick-Baez multilayer mirror system a spot size of 4 μm × 9 μm was achieved. Based on these results, some of the requirements for an optical system suitable for a dedicated microprobe beamline are given.

Formation of Cubic Ll 2 Phases from Al 3 Ti and Al 3 Zr by Transition Metal Substitutions for Al

Alloys of (Al x M 1−x ) 3 Ti where M is V, Cr, Mn, Fe, Co, Nb, W, Fe + Ga or Fe + Ga + Mn were fabricated to form the Ll 2 cubic phase, tested for ductility, and characterized by lattice parameter, microstructure, quantitative phase analysis, and phase composition. The elements V, Co, Nb, W, Fe + Ga and Fe + Ga + Mn are hereby newly found Ll 2 formers in Al 3 Ti. Both X-ray diffraction and microprobe analysis were used to identify the phases, their volume fractions and compositions. The amount of ternary addition found in the Ll 2 cubic phase varied between 4 and 12 atomic percent. This amount of ternary addition decreased as the atomic radius of the elemental addition increased. Pettifors Mendeleev number was a useful guide in the selection of elements to form the Ll 2 phase but did not correlate strongly with the amount of the ternary addition. For the DO 23 phase of Al 3 Zr, the elements V, Mn, and Co were added to the list of Ni, Cu, Fe, and Cr as elements known to form the Ll 2 phase. Little if any ductility was observed in these cubic Ll 2 phases which readily cleaved with transgranular failure. To achieve ductility and toughness, more than transformation of these compositions from a lower to a higher symmetry crystal structure will be required.

A simple cantilevered mirror for focusing synchrotron radiation

Abstract A large cantilevered mirror was constructed to focus the vertical divergence from a synchrotron radiation source. The advantages of this mirror are its compactness, simple bending device, simplicity of construction, and good thermal contact to structures outside the vacuum. The central portion of the mirror is supported with variable loading springs to reduce gravitational sag. The figure and thermal stability of the mirror have proven to be excellent, though the focusing is limited by the roughness of the mirror surface. This paper describes the design, construction, and performance of the mirror.

The effect of high magnetic field on phase stability in Fe-Ni

Identically prepared samples of Fe0.85Ni0.15 were annealed either in the ambient magnetic field or in a field of 29 T. Room temperature x-ray powder diffraction measurements that were performed after magnetic annealing showed that the ratio of the volume of the γ to α phase is decreased in the field-annealed sample by a factor of 2. First-principles calculations of the magnetic structure in the presence of a magnetic field are used to compute the resulting change in free energy. Analysis in terms of the phase diagram calculated with and without a magnetic field is in substantial agreement with the measurements.Identically prepared samples of Fe0.85Ni0.15 were annealed either in the ambient magnetic field or in a field of 29 T. Room temperature x-ray powder diffraction measurements that were performed after magnetic annealing showed that the ratio of the volume of the γ to α phase is decreased in the field-annealed sample by a factor of 2. First-principles calculations of the magnetic structure in the presence of a magnetic field are used to compute the resulting change in free energy. Analysis in terms of the phase diagram calculated with and without a magnetic field is in substantial agreement with the measurements.

X-ray diffuse scattering from a nitrogen-implanted niobium film

A 2500‐A niobium single‐crystal film was deposited onto a sapphire substrate and subsequently implanted with nitrogen to an average concentration of 0.5 at. %. Synchrotron radiation was used to measure the difference between the implanted and an unimplanted film to isolate the diffuse scattering from the implanted film near two Bragg reflections. This diffuse intensity arises mainly from elastic displacement fields about radiation‐damage‐related loops located on (211) planes. A small contribution of the scattering is calculated from the displacements about single interstitial nitrogen in octahedral sites. The Burgers vector of the loops is along the [111] direction and makes an angle of 62° with the loop plane giving a dominant shear component. Vacancy loops have a radius ∼5 A while interstitials are somewhat larger ranging from 10 to 15 A. The number of vacancies and interstitials are nearly the same.

Phase and Microstructure of Fe Modified A1 3 Ti

Alloys of Al/sub 75-x/, where x = 0 to 12 at. % Fe, were fabricated by splat cooling and examined primarily with x-ray diffraction to determine the crystallographic phases present, their composition, volume fraction, lattice parameters, and sublattice occupation. The tetragonal DO/sub 22/ structure of Al/sub 3/Ti dissolved 0.8/+-/0.4 at. % Fe, with additional iron partitioned to the Ll/sub 2/ structure containing 6.2/+-/0.4 at. % Fe, and completed the transformation to the Ll/sub 2/ structure with the addition of about 6 at. % Fe. The lattice parameters of both phases were independent of iron concentration up to about 6 at. %. At 8 at. % Fe, the iron was found to be predominantly distributed on the nominal aluminum sublattice which also contained about 5 at. % Ti. The Al/sub 3/Ti composition was similarly found to contain about 5 at. % Ti on the aluminum sublattice. All compositions, including a single crystal of Al/sub 67/Fe/sub 8/Ti/sub 25/, were found to be brittle. 11 refs., 4 figs., 1 tab.

Diffuse scattering measurements of static atomic displacements in crystalline binary solid solutions

Diffuse x-ray scattering from crystalline solid solutions is sensitive to both local chemical order and local bond distances. In short-range ordered alloys, fluctuations of chemistry and bond distances break the long-range symmetry of the crystal within a local region and contribute to the total energy of the alloy. Recent use of tunable synchrotron radiation to change the x-ray scattering contrast between elements has greatly advanced the measurement of bond distances between the three kinds of atom pairs found in crystalline binary alloys. The estimated standard deviation on these recovered static displacements approaches ± 0.001 a (± 0.0001 nm), which is an order of magnitude more precise than obtained with extended x-ray absorption fine structure measurement. In addition, both the radial and tangential displacements can be recovered to five near neighbors and beyond. These static displacement measurements provide new information that challenges the most advanced theoretical models of binary crystalline alloys.

Experimental measurement of local displacement and chemical pair correlations in crystalline solid solutions

The measurement of near-neighbor atomic arrangements in crystalline solid solutions is well established and provides meaningful values for the chemical preference of atoms for their near neighbors to beyond the first ten neighboring shells. Static displacements (atomic size) between these atom pairs has mostly been either ignored in the recovery of the local pair preferences or removed by making use of the displacement scattering dependence on momentum transfer. With intense and energy tunable x-ray synchrotron sources, our ability to recover these static displacements between atoms has greatly improved. Data taken with multiple x-ray energies to obtain the contrast necessary to separate like from unlike neighbor pair distances is discussed for the two cases studied to date: a locally ordered Ni77.5Fe2.25 crystal and a locally clustered Fe53Cr47 crystal. An analysis of the experimental parameters and data gives the systematic and statistical errors on the recovered parameters. Meaningful atomic displacements from the mean lattice can be measured and recovered. These displacements help us understand material properties and will provide theorists with tests for their calculations.

Microstructural features and shape-memory characteristics of melt-spun Ni-Al-Fe-B ribbons

This paper summarizes our recent study of NiAl+Fe+B alloy ribbons containing 4 to 20 at. % Fe and doped with 300 wt ppm B. Alloy ribbons were successfully fabricated by rapid solidification via melt spinning. The alloys with {ge}8% Fe and {le}34% Al-equivalent (=Al%+(Fe%)/2) showed the best bend ductility at room temperature. The ribbons exhibited a reversible martensite (body-centered orthorombic structure) to B2 transformation as evidenced by DSC, X-ray and TEM studies. The shape-memory effect, as characterized by measuring the recovery of bend angles with temperature, is sensitive to alloy composition, with the best recovery observed in SMA015 (B-doped Ni-27% Al-14% Fe). Annealing at 600{degrees}C causes aging embrittlement; in particular, in alloys containing {le}12% Fe.