Grain‐Boundary Migration in Nonstoichiometric Solid Solutions of Magnesium Aluminate Spinel: I, Grain Growth Studies

The grain‐boundary mobility in magnesium aluminate spinel (MgO · nAl 2 O 3 ) of magnesia‐rich ( n < 1) and alumina‐rich ( n > 1) compositions has been measured from normal grain growth in dense, hot‐pressed samples. Over the temperature range 1200° to 1800°C, the mobility in magnesia‐rich compositions is found to be greater than that in alumina‐rich compositions by a factor of 10 2 to 10 3 . Within the alumina‐rich field, the mobility varies by less than a factor of 10 over the composition range 1 > n > 1.56. Nearly stoichiometric spinels (1 < n < 1.07) form a variety of sources and starting materials exhibit boundary mobilities within a factor of 5 at fixed temperature, showing an impurity tolerance which has not been found in other ionic solids.