High-Temperature Steady-State Creep Rate in Single-Crystal MgO

Abstract

Steady‐state, three‐point bending deflection rate measurements on single‐crystal MgO samples in the temperature range of 1450°–1700°C show that creep is a thermally activated process without an unique activation energy. The measured energies range from 3.5 to about 7 eV. This range of energies can be explained in terms of self‐diffusion data if it is assumed that the creep rate is limited by self‐diffusion. Small deviations from stoichiometry are shown to be very important in controlling the creep rate as they are in all ionic or electron transport processes. Understanding and control of these deviations are necessary to obtain a quantitative understanding of either steady‐state creep or diffusion. The variation of creep rate with stress can be described by a power law with an exponent varying from 4 to 7. This range of exponent lends some support to the theory based on edge dislocation climb over barriers as the rate limiting step. On the other hand, such barriers have not been observed in single crystals of MgO and other substances, so that some other mechanism in which diffusion is rate limiting may be operating. The creep rate is apparently independent of subgrain structure, precipitates observed at room temperature, surface roughness, spectroscopically determined impurity content, the valence state of the iron impurity, and dislocation strain pattern after bending.