Energy-resolved neutron imaging options at a small angle neutron scattering instrument at the Australian Center for Neutron Scattering

Energy-resolved neutron imaging experiments conducted on the Small Angle Neutron Scattering (SANS) instrument, Bilby, demonstrate how the capabilities of this instrument can be enhanced by a relatively simple addition of a compact neutron counting detector. Together with possible SANS sample surveying and location of the region of interest, this instrument is attractive for many imaging applications. In particular, the combination of the cold spectrum of the neutron beam and its pulsed nature enables unique non-destructive studies of the internal structure for samples that are opaque to other more traditional techniques. In addition to conventional white beam neutron radiography, we conducted energy-resolved imaging experiments capable of resolving features related to microstructure in crystalline materials with a spatial resolution down to ∼0.1 mm. The optimized settings for the beamline configuration were determined for the imaging modality, where the compromise between the beam intensity and the achievable spatial resolution is of key concern.

Force chains in monodisperse spherical particle assemblies: three-dimensional measurements using neutrons

The full triaxial stress state within individual particles in a monodisperse spherical granular assembly has been measured. This was made possible by neutron imaging and computed tomography combined with neutron diffraction strain measurement techniques and associated stress reconstruction. The assembly in question consists of 549 precision steel ball bearings under an applied axial load of 85 MPa in a cylindrical die. Clear evidence of force chains was observed in terms of both the shape of the probability distribution function for normal stresses and the network formed by highly loaded particles. An extensive analysis of the source and magnitude of uncertainty in these measurements is also presented.

Shear stiffness in nanolaminar Ti3SiC2 challenges ab initio calculations

Nanolaminates such as the M(n + 1)AX(n) (MAX) phases are a material class with ab initio derived elasticity tensors published for over 250 compounds. We have for the first time experimentally determined the full elasticity tensor of the archetype MAX phase, Ti(3)SiC(2), using polycrystalline samples and in situ neutron diffraction. The experimental elastic constants show extreme shear stiffness, with c(44) more than five times greater than expected for an isotropic material. Such shear stiffness is quite rare in hexagonal materials and strongly contradicts the predictions of all published MAX phase elastic constants derived from ab initio calculations. It is concluded that second order properties such as elastic moduli derived from ab initio calculations require careful experimental verification. The diffraction technique used currently provides the only method of verification for the elasticity tensor for the majority of new materials where single crystals are not available.

Diffraction thermometry and differential thermal analysis

A unit-cell parameter anomaly observed during the precipitation and growth of Ti3SiC2from a Si-substituted TiC phase is interpreted as the release of latent heat. The observations are used to propose a powder diffraction method for conducting differential thermal analysis as part ofin situphase transition studies.

Preferred orientation in Debye–Scherrer geometry: interpretation of the March coefficient

The March function is a widely used preferred-orientation correction function that, in flat-plate geometry, often closely approximates the pole-density profile of axially symmetric textures. It is shown that in Debye–Scherrer geometry, the assumption that the pole-density profile of a powder specimen can be described by a March function with coefficientR, leads to an intensity correction factor that can be approximated quite well by another March function, with coefficientR−1/2. This result validates the use of the March function correction in Debye–Scherrer geometry, facilitates the comparison of results obtained in the different geometries and should prove useful in some studies of axially symmetric textures and in residual-stress analysis.

Measurement of single-crystal elastic constants by neutron diffraction from polycrystals. Addendum and erratum

Correction is made to an equation in a paper by Howard & Kisi [J. Appl. Cryst.(1999),32, 624–633] and additional references are cited.

Measurement of single-crystal elastic constants by neutron diffraction from polycrystals

The relationships of diffraction averaged elastic compliances for an ideally random polycrystal to the single-crystal elastic compliances are given, within the Reuss approximation, for crystal systems with orthorhombic and higher symmetry. For anisotropic materials, these diffraction elastic compliances are dependent on the reflection indexhkl. Expressions for the conventional elastic constants (Young's modulus, Poisson's ratio) are also given. A connection is made to the `X-ray elastic constants' used for diffraction-based measurements of residual stress. The relationships are used to calculate diffraction averaged constants for comparison with neutron diffraction data recorded from samples under applied uniaxial stress. The Reuss approximation works well for materials with the capacity for plastic deformation, such as metals and transformation toughening ceramics, whereas for other materials the Voigt–Reuss–Hill approximation gives better results. Based on the given relationships and experimental determinations of the diffraction elastic compliances for polycrystalline materials, a method is developed for determining the single-crystal elastic constants. The method for estimating single-crystal compliances is demonstrated here by application to extant data on Ni–Cr–Fe and Ti–6 wt% Al–4 wt% V alloys, and new measurements on cubic zirconia. It has been applied very recently [Kisi & Howard (1998).J. Am. Ceram. Soc.81, 1682–1684] to determine the previously unknown elastic constants for a tetragonal zirconia.