Dislocation Structures in Creep-deformed Polycrystalline MgO

Secondary creep of polycrystalline MgO with grain sizes of 100 and 190 μm was investigated at 1300° to 1460°C under compressive loads of 2.5 to 5.5 kgf/mm2. The dependence of creep rate on load follows a power law with an exponent of 3.2±0.3. The process is thermally activated, with an activation energy of 76 ± 12 kcal/mol. The creep rate is independent of grain size. The dislocation structure was investigated by transmission electron microscopy. The total dislocation density follows the relation, σ=bG√ρ, commonly found for metals. The dislocations form a 3-dimensional network in which many dislocation segments lie in their slip or climb planes. On the basis of this structure, a model is proposed in which glide is the principal cause of deformation but the rate-limiting process, i.e. annealing of the network, is diffusion-controlled. Theoretical estimates and experimental results agree within 1 order of magnitude.