Effect of Microstructure on Deformation of Polycrystalline MgO

Six types of polycrystalline MgO, one nominally fully dense and the others having porosities of ∼1 to 2%, were tested in compression up to 1400°C. At 1200°C and above, all materials deformed plastically; two of the materials, both porous, also exhibited plastic flow at temperatures down to 800°C, and a third at 1000°C. A qualitative analysis of the microstructures of these materials indicated that the differences in behavior arose primarily because of variations in the size and distribution of pores and in the concentration of impurities at the grain boundaries. It is suggested that the following factors aid plasticity below ∼1200°C: (1) Strong grain boundaries, in the absence of excessive impurities, permit buildup of stress concentrations with consequent nucleation of slip on the {100} system and the extension of slip across the boundaries and (2) clusters of very fine pores within the grains allow some mass accommodation.