The role of the grain boundary in the elevated temperature fracture behaviour of magnesia

Abstract
The elevated temperature shear strength of MgO bicrystal grain boundaries has been shown to be strongly orientation and temperature dependent. Boundaries of all misorientations studied showed a catastrophic drop in strength at some critical temperature. This temperature was found to be about 1200°C for twist boundaries compared to 1400°c for tilt type boundaries. Tests on tricrystal specimens showed that the weaker twist boundary controlled the fracture behaviour of the body. It is suggested that the sudden onset of grain boundary fracture is caused by a critical crack formation in the vicinity of a restraining jog as grain boundary sliding occurs, the twist type boundaries being more viscous and sliding at lower temperatures and/or stresses.

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