Precipitation in non-stoichiometric spinel crystals

Abstract

Microstructural changes preceding the formation of the equilibrium (α-Al₂O₃) phase in non-stoichiometric magnesium-aluminium spinel have been examined by electron microscopy. Two intermediate phases have been identified which nucleate on paired partial dislocations in the spinel matrix. Both phases have monoclinic symmetry but differing morphologies. The first phase precipitates as thin plates with {113} habit and orientation relation {113} matrix ‖ {001} precipitate, ⟨110⟩ matrix ‖ ⟨010⟩ precipitate. The second phase grows as thin laths parallel to ⟨100⟩ matrix directions and orientation relation ⟨100⟩ matrix ‖ ⟨010⟩ procipitate, ⟨110⟩ matrix ‖ ⟨001⟩ procipitate. The laths coalesce on long ageing to produce composite plate shapes formed from laths in twin related orientation. Image contrast arises mainly from the mismatch between precipitate and matrix lattices and takes the form of interface fringes or matrix strain contrast. Thc extinction criterion for contrast is consistent with a lattice mismatch which is smallest in the direction of maximum growth rate. Both phases have crystal structures formed by changes in periodicity of order of the positive ion sub-lattice with only small distortions of the matrix sub-lattice of oxygen ions.