Creep of Dense, Polycrystalline Magnesium Oxide

Abstract

Creep of polycrystalline MgO was studied using four‐point transverse bending at 1380° to 1800°K and stresses from 1000 to 5000 psi. The effects of temperature, stress, and grain size on the creep rate were determined for grain sizes from 2 to 20μ. Activation energies for creep decreased sharply with increasing grain size from 96,000 cal/mole at 2μ to 54,100 cal/mole at 5.5μ and then remained constant over the grain‐size range 5.5 to 20μ. Creep was attributed in part to a stress‐directed diffusional mechanism controlled by extrinsic oxygen ion diffusion in the 5.5 to 20μ grain sizes, although the calculated ionic self‐diffusion rates were higher than those predicted by the Nabarro‐Herring theory. It is suggested that the discrepancy may be due to a vacancy formation mechanism, which is consistent with the observed formation of dislocation substructure and preferentially distributed porosity during creep, as well as with the observed decrease in creep rate with increasing creep strain.