Anisotropic Small-angle Neutron Scattering Research Articles

The rate of bubble deformation during tungsten wire drawing

Rods and wires from K–Al–Si doped tungsten containing residual porosity and K filled bubbles were studied from 6 to 0.2 mm in diameter ( d) by means of anisotropic small angle neutron scattering (ASANS) method. This novel technique has been developed for monitoring shape and size of microscopic pores, bubbles, second phase particles in deformed PM materials. In case of mixture of different populations, e.g. uniformly elongated bubbles and spherical ones in tungsten, the morphological parameters for each population can be obtained from the quantitative analysis of ASANS data. Such parameters are: relative number density, diameter, aspect ratio of elongation. It was found that during wire drawing the average aspect ratio of elongated bubbles increases much slower (∼1/ d) than it was expected from the usual theory (∼1/ d 3). Instead of “constant volume” assumption, a “constant length” assumption seems to be reliable. The occurrence of spherical bubbles after several steps of wire drawing can also be revealed.

Evaluation procedure for anisotropic SANS

A brief description of an improvement of a special evaluation procedure for treatment of data from anisotropic Small-Angle Neutron Scattering (SANS) is presented. A spatial orientation distribution of particles, the improved calculation of a size distribution, the option to fit a data from a sample containing two types of particles as well as the raw-data treatment are the main extensions. The application of the procedure is demonstrated on data taken from investigation of precipitation in single-crystal nickel-base superalloys.

Small-angle neutron scattering investigations of the structure of thixotropic dispersions of smectite clay colloids

The structure and interparticle interactions in colloidal dispersions of montmorillonite and Laponite RD (a model hectorite-type clay) have been investigated by small-angle neutron scattering (SANS). Measurements have been made under static conditions and during shear, using a Couette-type cell, for concentrations in the range (5 × 10–3)–(6.5 × 10–2) g cm–3, where time-dependent gelation and thixotropic behaviour occur. It is shown that in dispersions of low ionic strength ( < 10–3 mol dm–3) spatial and orientational correlations between the plate-like particles can develop. Such self-organised structures, which have a limited range, as is evident from the anisotropic scattering behaviour, depend on the dispersion concentration and are also influenced by the particle size and shape. Thus orientational correlation is more extensive for the larger, or more anisotropic montmorillonite particles; here it appears that some preferential alignment may persist at distances 103A under equilibrium conditions. The effect of shear on such structures has been investigated by anisotropic SANS. Thus under low shear, montmorillonite dispersions show preferential alignment of particles in the direction of flow. This orientation occurs over long range, giving rise to anisotropic scattering; furthermore, spatial correlations persist. Under high shear (ca. 104 s–1) a breakdown in structure occurs and only preferential alignment is observed. These microstructural changes are discussed in the context of the bulk rheological and thixotropic properties of the clay dispersions.