ZIRCONIA‐MAGNESIA SPINEL SYSTEM*

ABSTRACT An investigation of the zirconia‐spinel system, ZrO 2 ‐MgAl 2 O 4 , was made, using the criteria of thermal expansion, density, and microstructure. The samples were fired for six hours at 1650°C. No stable compounds or solid solutions were formed. An unstable solid solution, possibly based on a zirconia‐spinel compound, was formed, however, apparently extending from pure spinel to 30 mol. % of zirconia. The solid solution gradually decomposed on repeated heating to 1300°C., and the system ultimately behaved as a series of simple aggregates of zirconia and spinel. The zirconia transformation took place at 1110°C. throughont the system. On cooling, the transformation temperature was lowered by the presence of spinel from 990°C. for the zirconia‐rich samples to fi30°C. at the spinel end of the system.