The microstructure and fracture properties of MgO crystals containing a dispersed phase

MgO crystals containing up to 40% by volume of magnesioferrite and up to 2% by volume of iron and nickel were produced by a diffusion technique followed by appropriate heat treatments. Magnesioferrite precipitate did not significantly change the effective surface energy of a crack as measured by the double cantilever beam technique. Iron and nickel precipitate was produced in the form of platelets lying on {100}_MgO planes whose orientation relationships were 001]_MgO ∥001]_Fe, 110]MgO ∥100]_Fe with a spread of +10°, approximately, and 001]_MgO ∥001]_Ni, 010]_MgO ∥010]_Ni with negligible spread. Despite the crystallographic orientation relationships, the metal-MgO interface appeared to be very weak; the reasons for this are discussed. The effect of the metal precipitate on crack propagation was to markedly increase the density of cleavage steps. For a volume fraction of precipitate of 0.02, this led to a small increase in the effective surface energy, on the order of 1 Jm⁻².