Phase Relations and Ordering in the System ZrO ₂ ‐MgO

The Phase relationships in the system ZrO 2 ‐MgO were reinvestigated over a wide range of temperatures and compositions. The extent of the cubic solid solution field was determined with precise lattice parameter measurements and a high‐temperature X‐ray furnace using analyzed samples. DTA results show that the addition of MgO to ZrO 2 decreases the transition temperature for monoclinic ⇌ tetragonal ZrO 2 and 1 mol% of MgO is soluble in the monoclinic zirconia at ∼1070°C.The invariant eutectiod point is at 13.5 ± 0.3 mol% MgO at 1406°± 7°C, which is in fair agreement with previous results by Grain. The ordered phase Mg 2 Zr 5 O 12 (δ‐phase) can form metastably in cubic solid solutions at temperatures as low as 800°C after prolonged annealing. Evidence for the existence of the ordered phase MgZr 6 O 13 (γ‐phase) was obtained by electron diffraction technique. Conditions for the formation of this phase are described. The ordered phases in this system are metastable and their formation is an intermediate step in the eutectoid decomposition of the cubic phase.