Creep of ceramics: Part 1 Mechanical characteristics

This paper is a brief review of the extensive literature relating to the formation of protective α—Al2O3 scales on alloys at high temperature. Emphasis is placed on the proposed mechanisms of scale growth based on observations of scale morphologies and microstructures, inert-marker experiments and the distribution of oxygen isotope tracers within thermally-grown oxides. Attention is also given to the determination of ionic-transport mechanisms by electrochemical methods and to the effects of reactive elements such as yttrium in modifying ionic-diffusion processes.

The formation of aluminum oxide scales on high-temperature alloys

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

Fully dense aluminas, prepared by hot-pressing, were found to swell during annealing at 1600°C in air, but not during annealing in a reducing atmosphere (po2= 10-7 Pa). The reaction followed the relation p - po = -K log t, where po and p are the initial and final densities, respectively, t is the time, and AT is a constant. The rate of swelling was enhanced by MgO solute. The reduction in density resulted from the nucleation and growth of grain-boundary pores. Pore formation was attributed to the reaction of carbon and sulfur impurities at the boundaries with oxygen, which had diffused down the grain boundaries from the ambient, to form CO/CO2 and SO2 gas at high pressures. Preliminary results indicate that this reaction can be avoided by preannealing powders in flowing oxygen prior to hot-pressing. The consequences of internal gas-forming reactions to other processes such as high-temperature creep and sintering are also discussed.

Swelling of Hot-Pressed A12O3

Ion conduction and redistribution at grain boundaries in oxide systems

Ionic and electronic conductivity and compressive creep of hot-pressed polycrystalline acceptor-dominated Al2O3 were measured as a function of oxygen partial pressure and grain size varying from 3 to 200 μm. Hole conduction shows a slight preference for grainboundaries; ionic conduction is slightly hindered by grain boundaries, indicating that fast oxygen grain-boundary diffusion involving charged species does not occur. Conductivity and creep are accounted for on the basis of a model in which there is fast grain-boundary migration by a neutral oxygen species.

Conductivity and creep in acceptor-dominated polycrystalline Al2O3

Partial ionic and electronic dc conductivities and compressional creep rate were measured for hot-pressed poly crystalline AI2O3 made from AI-isopropoxide (AI2O3(II)). The undoped material was found to contain 1.5×1018 cm−3 fixed valency acceptors (Mg). Properties of undoped material and material doped with Fe or Ti were investigated as a function of grain size, dopant concentration, oxygen pressure, and temperature. No fast ionic conduction along grain boundaries is found in either acceptor- or donor-dominated material. Absolute values of self-diffusion coefficients calculated from conductivity and creep indicate that both effects are limited by migration of AI, involving VAI“in donor-, AI,” in acceptor-dominated material. In creep, oxygen is transported along grain boundaries in a neutral form (Oip). The pO2 dependence of σt and σh are as expected on the basis of a defect model. That of creep is weaker for reasons that are not entirely clear. An ionic conductivity with low activation energy, observed at low temperature, is attributed to the presence of AI-silicate second phase.

High-Temperature Conductivity and Creep of Polycrystalline AI2O3 Doped with Fe and/or Ti

High-density samples of polycrystalline Al2O3 doped with iron retain their high density when annealed at high temperature in a reducing atmosphere, but pores are formed, both inside grains and at grain boundaries, and the density decreases upon annealing in oxidizing atmospheres. An explanation for these effects is proposed. The presence of pores and second-phase particles of FeAl2O4 is found to affect the physical properties and to slow grain growth. Grain size is proportional to the number of second-phase particles to the power −1/3 indicating that the number of second-phase particles per grain remains constant during grain growth.

H. A. Wang

Papers

Conductivity and creep in acceptor-dominated polycrystalline Al2O3

High-Temperature Conductivity and Creep of Polycrystalline AI2O3 Doped with Fe and/or Ti

Pore formation during oxidative annealing of Al2O3−∶Fe and slowing of grain growth by precipitates and pores