Creep of ceramics: Part 1 Mechanical characteristics

This review analyses a wide range of experimental data on the creep of ceramic materials and reveals many similarities with the creep of metals. It is demonstrated that there are two important differences in the creep behaviour of ceramics: (1) there is an enhanced role of diffusion creep, and (2) in the power-law regime, ceramics divide into two categories with stress exponents of ∼ 5 and ∼ 3, respectively. It is concluded that the behaviour with an exponent of ∼ 5 represents fully ductile behaviour as in f cc metals, whereas the behaviour with an exponent of ∼ 3 is due to dislocation climb from Bardeen-Herring sources under conditions where there is either a lack of five independent slip systems or, if five independent slip systems are available, a lack of interpenetration of these systems.

Creep of ceramics

Geological significance of recovery and recrystallization processes in quartz

An updated bibliography of diffusion data in oxides is provided for the materials scientist who requires a convenient source of published results. The scope of the review has been enlarged to include data for the diffusion of the host and impurity species in both binary and multiple oxides. Brief descriptions of terminology, diffusional behaviour and new measurement techniques are followed by tables of selected results and associated experimental details.

Self-diffusion and impurity diffusion in oxides

The deformation and cracking behavior of oxide scales formed in air on four heat-resistant steels and on nickel 99.6 have been studied in constant-extension-rate tests at 800°C. The strain rates in the experiments ranged between 10−6 and 10−9 s−1. Acoustic emission (AE) was used as an instrument for detecting the beginning of scale cracking. Additionally, metallographic, SEM, and micro-probe investigations were performed which supported the results from the AE measurements. The strain-to-cracking of the scales did not exceed 0.5% except when lateral growth effects in the oxide scales occurred, leading to critical strains of up to nearly 2.5%. Also the crack distribution in the scales was measured. The deformation and cracking behavior of the scales investigated could be explained by model like considerations.

Deformation and cracking behavior of protective oxide scales on heat-resistant steels under tensile strain

The creep of uranium dioxide has been investigated as a function of grain size. At high stresses, when creep is controlled by dislocation movement, grain boundaries exert a strengthening effect and this strengthening is correlated with the Hall-Petch equation. The degree of strengthening diminishes with increases in temperature. At lower stresses, when creep is controlled by mass transport, grain boundaries exert a weakening effect owing to the reduction in diffusion path length as grain size is reduced. In this range behaviour is correlated with the Nabarro-Herring equation with stress σ replaced by an effective stressσ
E=σ−σ0 whereσ
0 is a threshold stress for diffusional creep associated with the limitation of the ability of boundaries to emit and absorb vacancies.σ
0 appears to decrease as grain size is increased.

The influence of grain size on the creep of uranium dioxide

The estimation of the diffusion coefficient of oxygen in Cr2O3 from creep measurements

Hot tensile (or compression) testing, where the stress developed in a material is measured under an imposed strain rate, is often used as an alternative to conventional creep testing. The advantages of the hot tensile test are that its duration can be more closely controlled by the experimenter and also that the technique is more convenient, since high precision testing machines are available. These factors can be particularly important when extensive testing programmes of radioactive samples are involved. The main disadvantage is that the interpretation of results is more complex. Confusion can easily arise when attempts are made to extend the use of parameters which satisfactorily categorize behaviour at lower temperatures, into regimes where concurrent thermal recovery can occur. The present paper relates the parameters which are measured in hot tensile tests, to physical processes which occur in materials deforming by a variety of mechanisms. For cases where no significant structural changes occur, as in viscous or superplastic flow, analytical expressions are derived which relate the stresses measured in these tests to material constants. When deformation is controlled by recovery processes, account has to be taken of the structural changes which occur concurrently. A wide variety of behaviour may then be exhibited which depends on the initial dislocation density, the presence of second-phase particles and the relative values of the recovery rate parameters and the velocity imposed by the testing machine. Numerical examples are provided for simple recovery models.

The physical interpretation of the parameters measured during the tensile testing of materials at elevated temperatures

Creep failure generally occurs either by the attenuative loss of area as flow proceeds (geometrical failure) or by the development of internal cavities. The former mechanism dictates the ultimate creep life of a material. Criteria are developed for this upper limit to life for a variety of creep mechanisms. The effects of both primary creep and necking are discussed and a brief comparison is made between failure criteria for cavitation failure and geometrical failure.

The upper limit to the creep life of solids under a tensile force

Models for the growth of grain boundary voids may be extended to describe the strain-time response of a stressed material containing an array of gas-filled grain boundary bubbles. Above a critical value of applied stress the bubbles grow indefinitely until failure of the material occurs. Below this critical value significant anelastic strain may be observed following a stress change. The strain is not easily formulated explicitly in terms of time but may be evaluated using numerical techniques.

B. Burton

Papers

The influence of grain size on the creep of uranium dioxide

The estimation of the diffusion coefficient of oxygen in Cr2O3 from creep measurements

The physical interpretation of the parameters measured during the tensile testing of materials at elevated temperatures

The upper limit to the creep life of solids under a tensile force

On the anelastic properties of materials containing gas-filled grain boundary bubbles