S. Spooner

Intercalibration of small-angle X-ray and neutron scattering data

Absolute calibration forms a valuable diagnostic tool in small-angle scattering experiments and allows the parameters of a given model to be restricted to the set which reproduces the observed intensity. General methods which are available for absolute scaling of small-angle X-ray scattering (SAXS) data are reviewed along with estimates of the degree of internal consistency which may be achieved between the various standards. In order to minimize the time devoted to calibration in a given experimental program, emphasis is placed on developing a set of precalibrated strongly scattering standards for the SAXS facilities of the National Center for Small-Angle Scattering Research (Oak Ridge). Similar standards have been developed previously for calibration of small-angle neutron scattering (SANS) data. Particular attention is given to standards which can be used for either SAXS or SANS experiments where each sample has been independently calibrated for both types of radiation. These calibrations have been tested via the theoretical relationships between the two cross sections. It has been found that specimens best suited for such intercalibration purposes are a glassy carbon specimen where the scattering arises from voids in a carbon matrix and a perdeuterated polyethylene where the scattering arises from periodic arrangement of the crystalline lamellae. In only these two cases could the identical specimen be used for both the neutron and X-ray scattering experiments.

Reduction of parasitic scattering in small-angle X-ray scattering by a three-pinhole collimating system

A series of experiments have been undertaken on the Oak Ridge National Laboratory (ORNL) 10 m small-angle X-ray scattering (SAXS) camera to provide quantitative data on the level of background (parasitic) scattering generated by different types of bevelled collimating slits. The addition of a third (guard) slit, positioned close to the sample, resulted in a reduction of over an order of magnitude in the parasitic background generated by the best two-slit combination of collimating slits. This made it possible to reduce the size of the beamstop, permitting useful data to be collected down to a value of the scattering vector Q = 4πλ−1sinθ ≃ 3 × 10−3 A−1, where λ is the wavelength, and 2θ is the angle of scatter. This permits the resolution of distances d ~ 2π/Q up to 2000 A.

Diffraction Peak Displacement in Residual Stress Samples Due to Partial Burial of the Sampling Volume

Near-surface measurement of residual strain and stress with neutron scattering complements and extends the surface residual stress measurements by X-ray diffraction. However, neutron diffraction measurements near surfaces are sensitive to scattering volume alignment, neutron beam wavelength spread and beam collimation and, unless properly understood, can give large fictitious strains. An analytic calculation and a numerical computation of neutron diffraction peak shifts due to partial burial of the sampling volume have been made and are compared with experimental measurement. Peak shifts in a strain-free nickel sample were determined for conditions where the sample surface is displaced so that the scattering gage volume is partially buried in the sample. The analytic and numerically computed peak shifts take into account the beam collimation, neutron source size, monochromator crystal mosaic spread and the collection of diffracted intensity with a linear position-sensitive counter.

Theory of the Peak Shift Anomaly due to Partial Burial of the Sampling Volume in Neutron Diffraction Residual Stress Measurements

A theory is presented to describe the anomalous peak shift encountered in neutron diffraction residual stress measurements as the specimen is translated into and out of the sampling volume, which is defined by a pair of masking slits inserted before and after the specimen. Analytical formulae for the anomalous peak shift were obtained for both position-sensitive-detector-based diffractometers and conventional scanning diffractometers. The results indicate that the observed peak shift is a complex function of many variables, including the in-pile collimation, slit widths, slit-to-axis distances, mosaic spread of the monochromating crystal, and mismatch in lattice spacing between the sample and the monochromator. Calculations based on the derived analytical formulae are in good agreement with experimental observations. It is shown that by the choice of appropriate experimental conditions, this peak shift anomaly can be suppressed or, in some cases, eliminated altogether.

Characterization of H defects in the aluminium- hydrogen system using small-angle scattering techniques

Aluminium foils (99.99% purity) and single crystals (99.999% purity) were charged with hydrogen using a gas plasma method and electrochemical methods, resulting in the introduction of a large amount of hydrogen. X-ray diffraction measurements indicated that within experimental error there was a zero change in lattice parameter after plasma charging. This result is contradictory to almost all other face-centred cubic (f.c.c.) materials, which exhibit a lattice expansion when the hydrogen enters the lattice interstitially. It is hypothesized that the hydrogen does not enter the lattice as an interstitial solute, but instead forms an H–vacancy complex at the surface that diffuses into the volume and then clusters to form H2 bubbles. Small- and ultra-small-angle neutron scattering (SANS, USANS) and small-angle X-ray scattering (SAXS) were primarily employed to study the nature and agglomeration of the H–vacancy complexes in the Al–H system. The SAXS results were ambiguous owing to double Bragg scattering, but the SANS and USANS investigation, coupled with results from inelastic neutron scattering, and transmission and scanning electron microscopy, revealed the existence of a large size distribution of hydrogen bubbles on the surface and in the bulk of the Al–H system. The relative change in lattice parameter is calculated from the pressure in a bubble of average volume and is compared with the experimentally determined value.

A finite element model for residual stress in repair welds

This paper describes a three-dimensional finite element model for calculation of the residual stress distribution caused by repair welding. Special user subroutines were developed to simulate the continuous deposition of filler metal during welding. The model was then tested by simulating the residual stress/strain field of a FeAl weld overlay clad on a 2{1/4}Cr-1 Mo steel plate, for which neutron diffraction measurement data of the residual strain field were available. It is shown that the calculated residual stress distribution was consistent with that determined with neutron diffraction. High tensile residual stresses in both the longitudinal and transverse directions were observed around the weld toe at the end of the weld. The strong spatial dependency of the residual stresses in the region around the weld demonstrates that the common two-dimensional cross-section finite element models should not be used for repair welding analysis.

Characterization of welding residual stresses with neutron diffraction

Welding residual stresses are a key concern in the fabrication and use of structural components containing welds. Residual stresses in welds are caused by non-uniform expansion and shrinkage of differently heated zones during the thermal transient of a weld pass. In some alloys, solid state phase transformations occurring during the welding transient contribute additional residual stresses. Manufacturing problems arising from welding residual stresses include cracking and dimensional distortion. During use, tensile stresses in the welded zone limit the fatigue resistance of the component under cyclic loading. In an aggressive environment, tensile welding residual stresses also create a necessary condition for stress-corrosion cracking to take place.

Residual stresses due to processing of composite tubes

X-ray and neutron diffraction were used to characterize residual stresses in composite tubing of a corrosion-resistant clad alloy on carbon steel. A useful X-ray method, based on the measurement of the fcc (3 1 1) reflection using Cr K{sub {beta}} radiation, was developed which allowed precise determination of surface residual stresses in the textured clad layer. Neutron diffraction measurements were carried out in both the carbon steel core and the clad layer, using the bcc (2 1 1) and fcc (3 1 1) reflections, respectively. The neutron diffraction results are consistent with surface residual stresses determined with X-ray diffraction. However, the through-thickness stress profiles established by X-ray and neutron diffraction do not agree with elastic calculations based on the thermal expansion mismatch between the carbon steel and clad alloy. The differences between the calculation and experimental results are discussed.

Residual Stress Distribution in Feal Weld Overlay on Steel

Neutron diffraction was used to measure the residual stress distribution in an FeAl weld overlay on steel. It was found that the residual stresses accumulated during welding were essentially removed by the post-weld heat treatment that was applied to the specimen; most residual stresses in the specimen developed during cooling following the post-weld heat treatment. The experimental data were compared with a plasto-elastic finite element analysis. While some disagreement exists in absolute strain values, there is satisfactory agreement in strain spatial distribution between the experimental data and the finite element analysis.

Kinetic Investigations of Alkoxysilane Sol-Gel Processing

Effective control of sol-gel glass processing requires a detailed understanding of the kinetic and mechanistic aspects of the process. We have investigated structural evolution at the molecular level in the tetramethoxysilane (TMOS) hydrolysis reaction by various spectroscopic techniques. Development of structure at the macromolecular level and evolution of particle/network dimensionality have been studied by small-angle x-ray scattering (SAXS) in both hydrogenated and fully deuterated reactions for silica macromolecular structural development.