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.

This paper presents experimental ductile fracture data for 2139-T8 aluminum alloy under various stress states for the ultimate purpose of probing the fracture envelope as a function of stress triaxiality and normalized Lode angle. A new ductile fracture model is also proposed and validated by experimental data, along with the evaluation of existing ductile fracture models. The data extracted from published literature on 2024-T351 aluminum alloy was also used as an additional experimental data set for validation. An extensive experimental program was implemented to produce data in a wide range of stress states, including tensile tests (with round smooth, round notched, and plate specimens), torsion, compression (with round smooth and round notched specimens), and shear-compression experiments (two different sizes). The combined effects of stress triaxiality and normalized Lode angle were used to define a 3D fracture envelope for fracture strain. A parallel finite element simulation (fine-tuned by the experimental results) has been performed for each experiment to evaluate the evolution of stress triaxiality and normalized Lode angle in the critical section of the specimens with complex geometries. Finally, these results were used to develop two fracture models whose predictions were compared with some of the existing models.

Perovskite La0.6Sr0.4Co0.8Fe0.2O3−δ (LSCF) powders have been successfully synthesized from oxide and carbonates based on the principle of gelcasting. Phase-forming temperature is very dependent on the ball-milling process during the suspension preparation. As the ball-milling time is increased, the temperature of phase formation decreases, therefore the perovskite powder obtained has a larger Brunaver–Emmett–Teller (BET) specific surface area. The grain sizes were around 1 μm at 1000°C and 2 μm at 1100°C from scanning electron microscopy (SEM) photographs. The perovskite powders have good sinterability: the sintering densities of ceramic bodies shaped with as-prepared powders were investigated. SEM photos show that sintered ceramics exhibit a well defined morphology in the packing and sintering of particles. The oxygen permeance of disc shaped samples, with a thickness ranging from 1.02 to 1.98 mm was 6.39 × 10−8–1.99 × 10−8 mol cm−2 s−1 at 900°C indicating that LSCF ceramics have high oxygen permeation. It can be concluded that gelcasting is a simple and effective method for preparing practical multicomponent perovskite powders.

Eutectic Al2O3/GdAlO3 composites in the form of fibers were produced by the laser heated pedestal growth technique at high pulling rates ranging from 48 to 240 mm/h. Fibers 0.8 mm in diameter and 25 mm in length, devoid of pores or cracks, were pulled in an air atmosphere without seeding or pedestal rotation. “Chinese script” and complex-regular microstructures were present in all the fibers. The Jackson–Hunt relationship, λ2 v = constant, applied very well to the eutectic Al2O3/GdAlO3 fibers. The modulus of rupture values varied from 800 to 1,780 MPa. The effects of high pulling rate, average phase spacing and modulus of rupture on the microstructure were studied.