Temperature Dependence of Plastic Yield Stress of Single Crystals of Magnesium Oxide

The temperature dependence of the plastic yield stress of cleavage slabs of Norton Company magnesium oxide, containing approximately 0.2%, impurity, has been investigated up to 1000° C. for bending under three-point loading. Previous thermal history has been found to be an important variable. In particular, annealing followed by a rapid quenching produces a marked softening, and at the same time causes a homogenization of the starting material. Such samples have been found to display inflections in strength versus temperature, corresponding to a minimum near 500°C. followed by a maximum near 700°C. At temperatures above those associated with the inflections the samples show a pronounced drop in load after yielding, and subsequent deformation occurs by jerky flow. The various observations have been discussed in terms of the interactions of dislocations with impurities that may be in precipitate or solution form, depending on past history and conditions at the time of testing. Some experimental information has also been obtained on the effect of strain rate on the plastic yield strength.