Abstract
Single and polycrystalline samples of magnesium oxide have been doped with calcium and zirconium oxides and heat-treated to produce grown-in whiskers of calcium zirconate or stabilised zirconia. The directions of growth of the whiskers are those in which the misfit between matrix and whisker along the whisker axis is small. Examination of cleaved surfaces indicates that the principal effect of the whiskers on crack propagation occurs when the crack velocity has fallen below the critical value for plastic relaxation at the crack tip. Cleavage and slip steps are then produced at the points of intersection of the crack front with whiskers but the energy absorbed by this process appears small compared to the energy dissipated when a crack traverses a grain boundary.
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