Abstract
Compressive stress‐strain curves for several types of polycrystalline MgO specimens were correlated with those for single crystals and analyzed as a function of grain size and grain‐boundary character at 1200° and 1400°C for several strain rates. The results for fully dense specimens were explained in terms of grain‐boundary sliding and intergranular separation in addition to slip. The modification of grain‐boundary nature concurrent with heat treatment for grain growth, caused by residual LUF, was associated with enhanced grain‐boundary sliding and intergranular separation. For grain sizes <30 μm, it was concluded that the von Miss criteria for ductility could be relaxed by the Occurrence of dislocation climb and, to a limited extent, by intergranular separation. Yield drop corresponding to dislocation multiplication occurred when grain‐boundary sliding was initially promoted. Specimens with a liquid phase of adequate viscosity also indicated plasticity accompanied by high strength. Specimens with clean grain boundaries exhibited ductility and normal strain hardening with no intergranular separation.
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