Grain‐Boundary Migration in Nonstoichiometric Solid Solutions of Magnesium Aluminate Spinel: II, Effects of Grain‐Boundary Nonstoichiometry

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Abstract
The grain‐boundary chemistry of magnesium aluminate spinel solid solutions MgO·nAl2O3 in which grain growth measurements were reported in part I has been investigated in order to understand the mechanism of grain‐boundary migration. It is found that although segregation of impurity Ca and Si is common, much larger deviations in grain‐boundary stoichiometry are present. There is an excess of Al and O relative to Mg at grain boundaries in all compositions. Grain‐boundary migration appears to be rate‐limited by solute drag from intrinsic defects accommodating lattice nonstoichiometry, rather than by extrinsic solutes, consistent with the observed impurity tolerance of grain‐boundary mobility. Different rate‐limiting defects are proposed for magnesia‐rich and alumina‐rich spinels.