Directional Solidification of the ZrO ₂ ‐MgO Eutectic

The eutectic between ZrO 2 and MgO was located at 27 wt% MgO. Directional solidification of this composition yields MgO rods in a cubic ZrO 2 matrix. Plane front growth occurred at solidification rates of <2.0 cm/h. The interfiber spacing was proportional to the inverse square root of the solidification rate. In samples where cellular growth occurred, the colony size was not affected by the solidification rate. The room‐temperature fracture strength, ∼23,000 psi, which was not affected by the MgO fiber size, was controlled by the colony size. Annealing for 24 h at 1200°C increased the fracture strength by 50% as a result of a compressive stress in the matrix caused by the cubic‐to‐monoclinic transformation in the ZrO 2 .