Subsolidus Phase Equilibria and Ordering in the System ZrO 2 ‐Y 2 O 3

The subsolidus phase relations in the entire system ZrO 2 ‐Y 2 O 3 were established using DTA, expansion measurements, and room‐ and high‐temperature X‐ray diffraction. Three eutectoid reactions were found in the system: ( a ) tetragonal zirconia solid solution→monoclinic zirconia solid solution+cubic zirconia solid solution at 4.5 mol% Y 2 O 3 and ∼490°C, ( b ) cubic zirconia solid solutiow→δ‐phase Y 4 Zr 3 O 12 +hexagonalphase Y 6 ZrO 11 at 45 mol% Y 2 O 3 and ∼1325°±25°C, and ( c ) yttria C ‐type solid solution→wcubic zirconia solid solution+ hexagonal phase Y 6 ZrO 11 at ∼72 mol% Y 2 O 3 and 1650°±50°C. Two ordered phases were also found in the system, one at 40 mol% Y 2 O 3 with ideal formula Y 4 Zr 3 O 12 , and another, a new hexagonal phase, at 75 mol% Y 2 O 3 with formula Y 6 ZrO 11 . They decompose at 1375° and >1750°C into cubic zirconia solid solution and yttria C ‐type solid solution, respectively. The extent of the cubic zirconia and yttria C ‐type solid solution fields was also redetermined. By incorporating the known tetragonal‐cubic zirconia transition temperature and the liquidus temperatures in the system, a new tentative phase diagram is given for the system ZrO 2 ‐Y 2 O 3 .