Grain Boundaries and the Mechanical Behavior of Magnesium Oxide
A comparison of the mechanical behavior of magnesium oxide single crystals and polycrystals at different temperatures is presented. Single crystals show increasing interpenetrability and multiplicity of slip systems at elevated temperatures. Above 1700°C, 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 1600°C, rearrangement of dislocations by polygonization is observed, and, in certain cases, recrystallization can be obtained. Polycrystals prepared by the recrystallization of single crystals show a brittle-to-ductile transition with increasing temperatures. At low temperatures, any structural discontinuity capable of blocking slip dislocations, including subgrain and grain boundaries, induces cleavage crack nucleation and brittle fracture. At intermediate temperatures (i.e., 1400 – 1700°C), more slip occurs, but it is not flexible enough for grains to conform to each other’s change in shape, and constraints develop which can be relaxed only by intergranular sliding. This leads to intergranular crack nucleation and brittle fracture. At elevated temperatures (i.e., above 1700°C), slip is very flexible, and the polycrystalline matrix deforms plastically and necks down to a ductile fracture. Polygonization and recrystallization of the deforming grains also contribute to relaxation of internal stresses. These high-temperature deformation processes are remarkably similar to face-centered cubic metals undergoing creep. Slight porosity causes an increase in the brittle-to-ductile transition temperature. While high-density, hot-pressed magnesia shows plasticity above 2000°C, it is limited by intergranular failure.
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