T. J. Davies

A comparative study of superplastic deformation and cavitation behaviour in 3 and 8 mol.% yttria-stabilized zirconia

Superplastic deformation, grain boundary structure and cavitation behaviour during high temperature tensile testing in fine-grained 3 and 8 mol.% yttria-stabilized zirconia have been compared. The 8 mol.% yttria stabilized zirconia (8Y-CSZ) had a single cubic phase, while the 3 mol.% yttria-stabilized zirconia (3Y-TZP) was predominantly tetragonal. Extensive tensile ductility was obtained in 3Y-TZP, reaching ∼283% at 1673 K and 1×10−4 s−1, whereas under the same test conditions, tensile elongation in 8Y-CSZ was limited to ∼20%. These differences in elongation were related to differences in segregation of the yttria cation at grain boundaries. Transmission electron microscopy (TEM) observations with energy dispersive spectrometer (EDS) analyses revealed strong segregation of the yttria on grain boundaries in the 3Y-TZP; this segregation suppressed grain growth and lowered grain boundary mobilities. There was no evidence of preferential segregation of the yttria at grain boundaries in the 8Y-CSZ. Internal cavities developed in both materials during superplastic deformation. The results demonstrated that the extent of cavitation in 8Y-CSZ was much higher than in 3Y-TZP. In 8Y-CSZ, most of the cavities propagated in a direction perpendicular to the tensile axis as in most ceramics (with increasing strain these grew and interlinked to form cracks leading to failure at relatively low strains), whereas, in 3Y-TZP cavities were elongated parallel to the tensile axis as in metals. This difference arose from severe grain growth in 8Y-CSZ.

Diametral compression of silicon nitride

Abstract An experimental study of the diametral compression testing of silicon nitride is described. The effect of packing strips (shims) and specimen sizes were studied. A strength anomaly observed for discs of different sizes is considered to be due in part to different fracture modes. The results show that the volume effects is mainly responsible for the difference observed between the strength obtained using bending flexure and that obtained using diametral compression disc tests for sintered silicon nitride.

Superplasticity of ceramic materials-II. Effect of initial porosity and doping on the superplastic behaviour of alumina

Abstract The present work has demonstrated the ability of fine grained alumina containing small additions of copper oxide or nickel oxide to undergo superplastic tensile flow. The effect of small levels of porosity on superplastic behaviour has been examined using specimens subjected to HIP. This provides an effective way of controlling the density of ceramic materials while avoiding appreciable grain growth. Results showed that low density specimens tended to have higher superplastic elongations to failure than higher density specimens. In the latter case, long cracks developed during deformation and grew perpendicular to the tensile axis; this led to premature failure. The uniformly distributed pores in the lower density specimens provided sites for both crack initiation and crack arrest, leading to a large number of short cracks, and to higher failure strains. TEM observations gave evidence of significant dislocation activity in grain boundary regions of the deformed materials and this is tentatively interpreted in terms of the electronic contributions of the dopants to dislocation mobility.

Cavitation behaviour in fine grain 3Y-TZP during tensile and compressive superplastic flow

Studies of cavitation in Y-TZP during superplastic flow have been made for both tensile and compressive deformation conditions. It was observed that the morphologies of cavities near the fracture faces of tensile specimens varied markedly with testing conditions and in most cases differed from those near the gauge heads. Two quite different forms of cavitation behaviour were observed leading to high and low strains to failure, respectively. For optimum conditions of superplastic flow, of high temperature/low strain rate (low stress), when large elongations were observed, cavities were either spherical or elongated parallel to the tensile axis. Those near the fracture face interlinked in a plastic (necking) mode to give transverse cavities and subsequent failure. At high strain rate/low temperature (high stress), transverse intergranular cracking played a dominant role in failure at low elongations. For intermediate conditions of temperature/strain rate, elongated cavities developed parallel to the tensile axis, but near the fracture face these usually interlinked by transverse cracking. These conditions were associated with intermediate elongations to failure. For the assessment of cavity growth mechanisms, artificial pores were introduced into fine grain Y-TZP specimens and changes in their shape and size during tensile or compressive deformation were investigated. Results show that the change of pore volume, in the superplastic regime, is controlled by plastic deformation of the matrix and can be described by the relationship of dR/dɛ = ;ηR, where ɛ is the true strain, η the cavity growth rate parameter and R is the radius of the pore.

Proposed selection rules for suitable binders in cemented hard metals with possible applications for improving ductility in intermetallics

The results of a selective study of the literature on transition metal binder-hard metals and the parameters that determine the choice of suitable binders are reported. Thermodynamic and kinetic considerations have played a major role in the selection of suitable binders, with the electron theory of metals playing a minor role. It was deduced from the study that the electronic configuration of metals could be a major controlling factor in the thermo-kinetic reactions that take place during sintering in hard metal systems. A set of rules for binder metal selection is proposed based on this study. A modification of these rules would seem partly to explain ductilities currently obtained in certain intermetallics. The application of these rules would point to suitable binder metal(s) selection possibilities that are currently unexplored and predict alloy selection that could improve the ductility of brittle intermetallics.

Mullite formation from ethyl silicate and aluminium chlorides

The formation of mullite via gels prepared from technical ethyl silicate and aluminium chlorides has been studied. Normally, gels prepared specifically with the oxide stoichiometry of mullite (3Al2O3·2SiO2) do not form the mineral mullite on firing to 1200° C in the absence of a mineralizer. However, when the stoichiometric gel is homogeneous (achieved by acidic or neutral catalysts during the gel preparation) firing at 1200° C can lead to an almost quantitative yield of mullite. For a homogeneous gel, the presence of strontium or caesium salts, or an organo-tin compound such as dibutyltin diacetate or dibutyltin oxide during the gel preparation promotes almost quantitative conversion to mullite at about 1000° C. There is a threshold concentration under which conversion to mullite is incomplete, some cristobalite being formed. For the organo-tin compounds, the type of aluminium chloride is unimportant and the way in which water for the hydrolysis step is added is also unimportant. When the gel is non-homogeneous, the product obtained on firing contains cristobalite andα-alumina orγ-alumina, with little mullite, even if strontium or caesium salts, or organ-otin compounds are present. A ceramic bond is formed from alumina and some other refractory grains during firing.

Preparation and properties of some alumina-chrome refractories

Preparative procedures, grain growth, densification and change in corundum lattice dimensions in sintered compacts made from chromium (III) oxide/aluminium (III) oxide mixtures were evaluated in a study of some alumina-chrome refractories. High-energy milling using “A17 reactive” alumina gave densification at low temperature and rapid grain growth, also a low variability in modulus of rupture. X-ray diffraction studies confirmed solid solution formation, the corundum lattice dimensions decreasing with 7 wt% chromium (III) oxide, other compositions giving lattice expansion. The modulus of rupture at 1150 °C for a series of ethyl silicate-bonded alumina-chrome refractories containing 5 to 12 wt% chromium (III) oxide was also minimal at 7 wt% chromium (III) oxide. The composition of this series is typical of ethyl silicate-bonded alumina-chrome refractories used in the steel industry.

Superplasticity in ceramic materials—I. The observation of a “superplastic partition” in ceramics

Abstract Recent developments in the understanding of the mechanical behavior of ceramics assessed for superplastic deformation indicate that stoichiometric changes at interfaces and an associated “metallic” behaviour can account for the superplastic behaviour in the absence of a grain boundary glassy phase. Based on the experimental work carried out in Manchester and elsewhere, a “superplastic partition” is observed in ceramics which corresponds to a direct relationship with stoichiometric changes and a related “metallic” behaviour in ceramics. These changes in stoichiometry and the associated “metallic” behaviour can provide mechanisms for resistance to cavity nucleation and propagation, respectively. When this observation is linked with the Gifkins core-mantle concept and the findings of Mott, Cottrell and Gilman on insulator-metal transitions, this yields an improved appreciation of the superplastic partition in ceramics.

Preparation, characterization and uses of mullite grain

The properties were compared of grain formed at low temperature from a homogeneous mullite precursor gel, and grain formed from the manufactured material produced at high temperature by fusion or by sintering. A homogeneous precursor gel with the oxide stoichiometry of mullite was prepared by treating technical ethyl silicate with aluminium chlorohydrate, with or without dibutyltin oxide as hydrolysis/gelation catalyst. The gel was dried and heated for 5 h at 700 ° C to remove organic residues then ground to 8 μm nominal size to obtain a mullite precursor grain. Crystallites form even at this moderate temperature. Commercially produced fine mullite grain manufactured by fusion or by high temperature sintering was also ground to a give a grain of 8 μm nominal size to provide a comparative standard with well known material. Each of the three grain materials, homogeneous precursor gel, fused mullite and sintered mullite, each ground to 8 μm nominal size, representative of the ‘fines’ fraction of a grain mix to be used in producing refractory shapes, was made into compacts which were sintered, and, when cold, tested for compressive and bend strength. Materials of this size were chosen for comparison because of their significance in applications where properties at temperature are important. The precursor gel compacts were 95% crystalline mullite and reached 86% of theoretical density. At 1500 ° C, heated in a 90 deg min−1 schedule, they had strength comparable to sintered mullite; both materials were much stronger than fused mullite. The results show that sintering procedure has a profound effect on strength, and indicated that in the absence of binder, fused mullite is less reactive than sintered mullite. Some properties of refractory shapes and bricks made from the sintered or fused mullite grain are discussed and some uses in refractory shapes are considered.

Effect of the electronic state, stoichiometry and ordering energy on the ductility of transition metal-based intermetallics

The planar defects associated with deformation in ordered intermetallic compounds namely, the antiphase boundary, superlattice intrinsic stacking fault, and complex stacking fault are non-equilibrium structures corresponding to a state of disorder within the ordered structure of the lattice and therefore affect both the electronic energy states and the Brillouin zone structure. It is possible that a relationship exists between the antiphase boundary energy, γAPB, and the sum of the number of unfilled outermost d-state electrons in the transition metals on which this class of intermetallics is based. If this hypothesis is taken in conjunction with a set of rules for improving ductility in intermetallics proposed previously, a coherent explanation of recently observed ductilities in transition metal-based intermetallics would seem to be feasible.