Journal of Materials Science

The intergranular and intragranular resistivity components in β-alumina ceramics cannot be separated quantitatively by impedance analysis, it was concluded a few years ago in a previous article in this journal. This conclusion was based on use of the so-called parallel multi-element equivalent circuit to model the electrical properties of polycrystalline electrolytes. However, this model is shown to be inconsistent with the observation that the activation energy for the intergranular resistivity is independent of the size of that component for many compositions — both β- as well as β″-alumina. From this finding and others, the author infers that the separation of intra- and intergranular resistivities in sodium beta-alumina type ceramics is clean. Consideration of the separability question is greatly facilitated by an unconventional method of resistivity analysis. This alternative method involves essentially d.c. measurements on a set of specimens of the same composition but with different microstructures and resistivities. The method is described and its use illustrated.

Melt spinning of a nickel-base superalloy containing various amounts of boron up to 3.0 wt% has been carried out to explore the potential of extended boride alloyability through rapid solidification. More specifically, the melt-spinning castability, ribbon-solidification microstructure and heat-treatment precipitation were studied as a function of boron concentration by using analytical electron microscopy and a number of other techniques. Special attention was given to the boride structure, chemistry and thermal stability. The microstructural observations were then correlated to the ribbon bend ductility tested in as-cast and annealed conditions. On the basis of the present results, future investigation of superalloys using the rapid solidification process and the boride alloying concept is discussed.

Small-angle X-ray scattering was used to study phase decomposition of liquid-quenched Al-14 at. % Ag. Ageing was carried out at temperatures from 50 to 175° C. Though it is clear that ideal linear spinodal behaviour was not obtained, the general trend points to the early unmixing of this alloy as being characterized by spinodal decomposition. The activation energy for the process is that for equilibrium volume diffusion, and it is therefore concluded that quenched-in vacancies play no role in phase decomposition following liquid quenching.

It is possible to ion implant patterns in diamond crystals at fluences below that which would impart visible damage and then to reveal those patterns by electrostatic charging and dusting. The charge distribution — and therefore the dust attachement — is related to the difference in electrical conductivity between the implanted region and the rest of the crystal. The technique may have applicability for “fingerprinting” or personalizing diamond gemstones.

Amorphous precursors of LaNbO4 have been prepared by coprecipitation and mechanical alloying. Powder characteristics before and after calcination were studied by thermogravimetric analysis, differential thermal analysis, Fourier transform-infrared spectroscopy and X-ray diffraction, and the crystallographic changes during thermal treatment were documented by high-temperature X-ray diffraction. The tetragonal structure was stabilized at room temperature for the mechanically alloyed product; cells dimensions were determined from the X-ray diffraction patterns and volume change between monoclinic and tetragonal phase was evaluated and is discussed.