Magnesium oxide crystals show a wide variety of deformation and fracture modes under tension at high temperatures depending on the number of slip systems operating concurrently in a given volume. At low temperatures slip is confined to a single 110 (110) system and plasticity is limited by stress concentrations which develop where slip switches from one plane to another. At intermediate temperatures 110 (110) slip systems at 90 degrees to each other can interpenetrate but those at 60 degrees cannot. At high temperatures dislocations can interpenetrate on all systems and polygonization can occur. After easy glide the crystals work harden and elongate over 100 percent before fracturing in a completely ductile manner. The transition from one mode to another depends on strain rate. The relative ability for 90 or 60 degree systems to intersect is discussed in terms of dislocation interactions. (Author).

The paper describes high temperature tensile deformation of polycrystalline magnesia prepared by (a) single crystal recrystallization and (b) hot pressing. Recrystallized polycrystalline magnesia goes through a brittle-ductile transition at 1700C (strain rate 0.0001/ sec). The brittleness below 1700C is due to a lack of slip systems and grain boundary sliding. At 1700C grain boundary migration produces corrugations in the interface which interfere with sliding. About 1700C the matrix becomes sufficiently plastic through multiple slip and polygonization to accommodate any distortion. Polycrystalline specimens then neck down for completely ductile fracture. Hot pressed magnesia starts to go through a transition at 2200C, i.c., 500C higher. The increase is attributed to the presence of pores and impurity. Porosity is considered to promote grain boundary sliding by (a) providing the source for intergranular sliding, (b) decreasing the interfacial contact area, (c) preventing grain boundary migration and corrugation. These observations confirm that high temperature deformation occurs by dislocation glide and climb and grain boundary sliding and migration. (Author).

The paper compares the mechanical behavior of magnesium oxide single crystals and polycrystals at different temperatures. Single crystals show increasing interpenetratability and multiplicity of slip systems at elevated temperatures. Above 1700C slip is very flexible, i.e., slip systems with different Burgers vectors completely interpenetrate each other and slip occurs over a variety of slip planes. Furthermore, above 1600C rearrangement of dislocations by polygonization is observed and in certain cases recrystallization can be obtained.