Order hardening of MgO by large precipitated volume fractions of spinel particles

Abstract

Experiments on MgO single crystals containing high amounts of Fe 2O 3 described in order to examine the theory of order hardening. After isothermal aging, coherent and stress-free magnesiaferrite particles with the spinel structure precipitate. The particle size varies between 10 and 1200 nm and the volume fraction between 0.72 and 62%. With increasing volume fraction the following shape sequence was observed: octahedrons → dendritic particles → globular particles. The critical resolved shear stress (CRSS) was measured in compression between room temperature and 1900 K. Up to 1000 K the CRSS is independent of temperature and of deformation rate. Above 1000 K the CRSS decreases rapidly. The CRSS normalized by the square root of the volume fraction decreases with increasing particle size, exhibits a minimum and then increases again at very large particle sizes. In agreement with theory the observed decrease in CRSS is attributed to pairwise cutting of the spinel particles, while the increase for very large particles can be interpreted by the transition from cutting by strongly coupled dislocation pairs to Orowan looping of single dislocations. The significant effect of various particle shapes on the CRSS can be explained quantitatively. The present approach is an attempt to analyse the CRSS of materials with a more complex morphology on a theoretical basis.