R. A. Page

Characterization of nickel titanate synthesized by sol–gel processing

Abstract Previous studies indicated that oxides of titanium and nickel might be effective solid lubricants. In the current work, we synthesized a range of titanium and nickel oxides including nickel titanate (NiTiO 3 ) via wet-chemical processing. Coatings and powders made from the sol–gel-derived precursor solutions were characterized by X-ray diffraction, ellipsometry, and pin-on-disk tests. Nickel titanate monoliths were also made by traditional powder processing and characterized by X-ray diffraction, ellipsometry, dilatometry and micro-hardness measurements. These data assisted the characterization of the sol–gel-derived materials. Nickel titanate proved to be a stable oxide at elevated temperatures and exhibited lubricity under certain conditions.

Deformation mechanisms in yttria-stabilized zirconia

Yielding behaviour and deformation modes are characterized for single-crystal and polycrystalline yttria-stabilized ZrO2 tested in compression from 23 to 800° C. The plastic flow of single-crystal specimens is shown to be orientation and temperature dependent, and is interpreted in terms of dislocation activity, transformation plasticity, and ferroelastic domain switching. Polycrystalline material deforms at room temperature by transformation plasticity, and at intermediate temperatures (∼800° C) by forming unstable shear bands, which flow via grain-boundary sliding and cavitation.

The initiation and growth of fatigue cracks in a titanium aluminide alloy

The micromechanics of small, naturally initiated fatigue cracks and large through-thickness fatigue cracks have been studied in the titanium aluminide alloy Super Alpha 2. The microstructure investigated had equal volume fractions ofα2 and Β phases. Crack growth rates were higher than through α-Β titanium alloys. Initiation of small cracks was found always to occur in theα2 phase, and small cracks grew belowΔKth, the minimum cyclic stress intensity required for growth of large fatigue cracks. A method previously proposed for reconciling the growth rates of large and small cracks is applied to these results.

Small-angle neutron scattering study of creep cavity nucleation and growth in sintered alumina

The early stages of creep cavitation in sintered alumina are characterized using small-angle neutron scattering (SANS). It is found that the initial cavity density is of the order of 1011 cm−3, and that the average initial pore is approximately 60 nm in radius. The incubation time for nucleating additional pores during subsequent creep is extremely short, in agreement with the theory based on the “precipitation” of grain-boundary diffusing vacancies. Pore density at constant stress and temperature is a linearly increasing function of time, as predicted by classical nucleation theory. However, a local stress of 10−2E is required to achieve the measured nucleation rate. Cavities are observed to lie primarily on two-grain junctions in linear arrays, with an average cavity radius of approximately 60 nm. It is hypothesized that the cavities nucleate at grain boundary ledges which provide the necessary local stress concentrations. Calculation of the individual cavity growth rate yields a zero or near zero value. This suggests a rapid transient growth period following nucleation which quickly decreases to a negligible growth rate.

Viscous cavity growth in ceramics under compressive loads

The cavity growth behavior of liquid phase sintered ceramics containing a continuous, amorphous grain boundary phase and subjected to compressive loads is examined. Based on the assumption that under compressive loads the creep-induced cavities nucleate and grow on grain boundaries which are approximately parallel to the loading axis, it is deduced that the local tensile stress which drives cavity growth in the viscous grain boundary film is likely induced as the result of sliding in the adjacent grain boundaries and, for compatibility reasons, controlled by the constraints and creep behavior of the matrix. On this basis, a viscous cavity growth model is developed by extending the analysis of Raj and Dang to compressive loading conditions through the incorporation of Raj and Ashbys grain boundary sliding analysis and Dysons concept of constrained cavity growth. The present model is used to predict the sizes and shapes of evolving cavities as a function of creep strain. The predicted results are discussed and compared with small-angle neutron scattering measurements of hot-pressed SiC.

Cavity nucleation at grain boundary ledges

Abstract The kinetics of cavity nucleation at grain-boundary ledges is examined for ceramics subjected to compressive loads. By considering grain boundary sliding and diffusion as concurrent processes, the analysis shows that sliding of faceted grain boundaries can induce time-dependent stress concentrations of sufficient magnitude to cause cavity nucleation at the ledges. The transient stress concentration has been found to depend on the ledge height and spacing, and to decrease rapidly with time and with distance from the ledge. Whether or not cavity nucleation occurs at the ledge depends on the local stress, the shape of the critical nucleus, and the characteristic time for cavity formation. The theoretical results are discussed in conjunction with nucleation rate measurements obtained for a sintered alumina using small-angle neutron scattering.

Nucleation and early-stage growth of creep cavities in hot-pressed silicon carbide

Abstract Fine-grained silicon carbide with a continuous second phase grain boundary film was crept under compressive loading at 1600°C. The resultant nucleation and growth of creep cavities was characterized using small-angle neutron scattering. It was found that nucleation occurred within approximately the first 5% of the lifetime, so that the modeling of failure essentially involves the treatment of cavity growth and ultimate coalescence. The results suggested that cavity nucleation and growth occurred entirely within the viscous grain boundary film. However, the cavities do not grow, crack-like, across the grain boundary facets, but rather nucleate as lens-shaped pores which gradually transform, as the film thickens, to very slowly growing spheroidal cavities. The results are shown to contrast with similar experiments on an alumina with clean grain boundaries, in which pore nucleation, rather than pore growth, controlled creep life.

Relative stress corrosion susceptibilities of alloys 690 and 600 in simulated boiling water reactor environments

The relative susceptibilities of alloys 600 and 690 to intergranular stress corrosion cracking (IGSCC) in pure water and a simulated resin intrusion environment at 288 °C were evaluated. A combination of creviced and noncreviced slow-strain-rate, and precracked fracture mechanics tests were employed in the evaluation. Susceptibility was determined as a function of dissolved oxygen content, degree of sensitization, and crevice condition. The results indicated that alloy 600 was susceptible to various degrees of IGSCC in oxygen containing pure water when creviced, and immune to IGSCC when uncreviced. Alloy 690 was immune to IGSCC under all pure water conditions examined. Alloy 600 and alloy 690 were both susceptible to cracking in the simulated resin intrusion environment. Alloy 690, however, exhibited the greatest resistance to SCC of the two alloys.

The effect of hydrogen source on crack initiation in 4340 steel

The crack initiation site and the corresponding incubation time were determined as a function of notch radius in 4340 steel for both internally and externally supplied hydrogen. The source of hydrogen was found to affect both the crack nucleation site and the incubation time. Hydrogen cracking in cathodically charged 4340 steel initiated near the elastic-plastic boundary with incubation times which exhibited a linear dependence on notch radius. Hydrogen cracking in an aqueous solution initiated near the notch surface with incubation times which were relatively independent of notch radius. Short time diffusional flow models which include a stress dependent critical hydrogen concentration were found to predict incubation times reasonably for internally supplied hydrogen. Cherepanovs solution for the diffusion at the tip of a semi-infinite linear slit when applied in the context of a finite notch root radii problem was found to predict incubation times adequately for externally supplied hydrogen.

Inelastic response of confined aluminium oxide under dynamic loading conditions

Previous efforts to explore the compressive strength of ceramics as a function of confining pressure at high strain rates have been limited by the maximum hydrostatic pressure that could be achieved within the experimental apparatus. An alternate procedure, using an autofrettaged confinement ring, has been designed to achieve higher confining pressures. A 6.2 GPa stress pulse, of approximately 20 μs duration, was used to load a 99.5% pure cylindrical aluminium oxide specimen under a hydrostatic load of approximately 650 MPa. The specimen remained intact and showed no evidence of fracture under scanning electron microscopy (SEM). Transmission electron microscopy (TEM), however, showed extensive evidence of plastic flow; the microcracks that were observed were associated with dislocation arrays. Static and dynamic yield strengths as a function of strain rate are compared.