Microyield and fracture in polycrystalline MgO

Abstract

Microyield and plastic flow occurring in polycrystalline MgO prior to fracture at room temperature is examined. It is shown that the initial dislocation activity occurs in the region of grain boundaries at a stress independent of grain size and below the microyield stress. The microyield stress corresponds to the stress necessary for propagating slip across the grain diameter but is below the stress necessary to produce dislocation activity in adjacent grains. The microyield stress obeys a Petch type of relationship with respect to grain size and this is attributed to the variation of dislocation density with grain size. The fracture stress — grain size relationship also follows the Petch equation but indicates that extensive work hardening has occurred prior to fracture.