A. H. Clauer

Indentation fracture of WC-Co cermets

Indentation fracture of a series of well-characterized WC-Co cermets was studied with a Vickers diamond pyramid indenter. The resulting crack length-indentation load data were analysed in terms of relations characteristic of radial (Palmqvist) and fully developed radial/median (half-penny) crack geometries. The radial crack model gave a better fit to the data on all the alloys studied. Crack shapes determined by repeated surface polishing confirmed the radial nature of the cracks. An indentation fracture mechanics analysis based on the assumption of a wedge-loaded crack is shown to be consistent with the observed linear relation between the radial crack length and the indentation load. The analysis also predicts a simple relation among the fracture toughness (Klc), the Palmqvist toughness (W) and the hardness (H) of the WC-Co alloys.

The influence of stoichiometry on compression creep of uranium dioxide single crystals

Hyperstoichiometric uranium dioxide single crystal specimens have been creep tested in compression as a function of temperature, stress and composition. The dependence of steady-state creep rates on stress and temperature varies with O/U ratio. At a stress of 10 000 psi creep activation energies decrease from 135 kcal/mole to 57 kcal/mole as the O/U ratio increased from “stoichiometric” to 2.062. For O/U values above 2.001 creep rates can be fit by a power law stress dependence of the form g3 ∝ σn where n ≈ 17. At lower O/U levels n decreases to a value of 7. In the stoichiometry region of constant stress exponent, creep rates increase with the square of the oxygen excess (.e ∝ x2) where a; is given by UO2+x. At high temperatures (above 1250 °C) and low stress levels (whose values depend on stoichiometry) creep rates for UO2+x single crystals have been found to vary linearly with stress. In this region creep rates are nearly independent of O/U ratio, and at a stress of 3 000 psi the activation energies for creep are approximately 130 kcal/mole for “stoichiometric” UO2 and UO2.01.

Creep of CoO Single Crystals

Cobalt monoxide single crystals having a [100] orientation were creep tested in compression over ranges of temperature, stress and oxygen pressure. The creep curves were S-shaped and only the inflection creep rate, ε2, was analysed. In the range of 1000 to 1200° C, 850 to 1700 psi and 10−3 to 1 atm oxygen, ε2 was given by ε2=A po20.45σ7.1exp(− Qc/RT) where Qc=87±6 kcal/mol at 0.01 atm O2 and 100±16 kcal/mol at 1 atm O2. Slip occurred on two orthogonal {011} 〈0¯11〉 slip systems. The presence of subboundaries was observed by optical and transmission electron microscopy. It is suggested that the creep rate is controlled by oxygen diffusion.

The influence of stoichiometry on compression creep of polycrystalline UO2+x

Compressive creep tests have been performed on polycrystalline hyperstoichiometric uranium dioxide, as a function of temperature, stress and deviation from stoichiometry. Under low stresses measured steady-state creep rates for UO2+x increase linearly with applied stress, σ, and with oxygen excess, x. Creep rates for UO2+x tested at high stresses follow a power-law stress dependence of the form e ∝ σn where 4<n<7, and increase with xm where 1.75<m< 2. The transition stress, σt, between linear and power-law stress dependence behavior decreases with increasing OU ratio. For polycrystals tested at high stresses, the creep activation energy, Qc, decreases with increasing x in the same manner as found for uranium dioxide single crystals. Creep activation energies also decrease with increasing x in the linear stress-dependence region, but values for Qc are lower at a given x than are values obtained in the power-law stress region. It is suggested that different diffusion processes control the observed creep behavior in the two different stress regimes.

Effects of composition and microstructure on the slurry erosion of WC-Co cermets

Abstract Cemented carbides of optimum composition and microstructure are the preferred materials for resisting the severe erosion encountered in components handling erosive slurries. They possess an attractive combination of erosion resistance and fracture toughness required for structural reliability. In this paper the results are presented of an investigation of the effects of composition and microstructure of well-characterized WC-Co cermets on their erosion wear as assessed in a slurry jet impingement test. The effects of binder volume fraction and microstructural parameters such as WC grain contiguity and mean carbide grain size are rationalized in terms of a phenomenological analysis of erosion in the two-phase microstructure. Quantitative predictions of the semiempirical analysis are shown to be consistent with the experimental observations.

Erosive wear of advanced ceramics in coal-slurry streams

Abstract Erosive wear of advanced ceramics and cemented carbides was evaluated in a coal-slurry jet test that simulates the conditions in letdown valves in liquefaction plants. The results indicate...

Erosion-resistant materials for critical areas of coal liquefaction and coal gasification systems

Erosion has been encountered and has caused severe problems in a number of the prototype plants used to evaluate liquefaction and gasification processes. Analysis of the causes of these problems and of the available engineering systems solutions has shown that, in many cases, the rate of loss can be reduced to acceptable levels by careful attention to process control, component design, and materials selection. Nevertheless, some areas of recurring erosion problems are likely to remain, especially in the areas of raw material preparation and handling, and in effluent handling and disposal systems. Components such as valves and pumps are probably the most critically affected because of their importance to process control and safety, and because they are high cost items. Some areas of the associated piping, especially bends, and solids clean-up equipment such as cyclones and hydrocyclones may also be included in this category, but are more amenable to maintenance.Such critical areas can usually justify the application of the most erosion-resistant materials available, which are cermets or ceramics. These materials exhibit orders of magnitude better erosion resistance than the best metallic systems, but are also considerably more expensive and more difficult to apply. The WC-Co-base cermets have shown good performance as small inserts in valves in severe service in liquefaction plants. Laboratory studies in an erosion rig designed to simulate essential aspects of these service conditions have revealed relationships between erosion rates and materials properties such as hardness, fracture toughness, and microstructural parameters. It appears that a minimum erosion rate occurs at intermediate values of the materials properties, the reasons for which are discussed.Ceramic materials offer potentially greater erosion resistance and reduced costs, especially for larger sizes, compared to the WC-Co cermets. Rig test results confirm the potential for reduced erosion, but illustrate the profound effects of microstructure on the erosion behavior of a given ceramic. This has significant implications for both the manufacture of components and for techniques used to apply ceramics in, for instance, critical areas of valve and pump internals. Essentially, candidate ceramics exhibited one of two types of erosion behavior in the rig tests; ceramics within each group were found to show some relationship between hardness and erosion resistance. In practice, the application of ceramic materials will require revised handling and possibly operating procedures, and may require redesign of components to accommodate their low fracture toughness.

The Influence of Stoichiometric Defects on the Creep of Oxides with the Fluorite Structure

Studies have been conducted at this laboratory and elsewhere to establish the effects of varying the oxygen/metal ratio on the steady-state creep behavior of oxides that crystallize in the fluorite-cubic structure, including UO2, PuO2, CeO2, and mixed oxides of these compounds. Creep activation energies have been found to vary with stoichiometry, the maximum values being measured at or near the stoichiometric composition. The observed changes in activation energy with O/M level occur both in the power-law stress regime and when a diffusional creep mechanism is operative. The absolute values for the creep activation energies differ for these two cases however. The manner in which creep rates vary with O/M ratio suggests that the creep process is controlled by diffusion of cations via cation vacancies for hyperstoichiometric compositions and by diffusion of cation interstitials for hypostoichiometric specimens.