Precipitation in non-stoichiometric magnesium aluminate spinel

Abstract

Precipitation, in non-stoichiometric magnesium aluminate spinel (MgO: 3·5Al₂O₃) single crystals was studied by transmission electron microscopy. For crystals annealed at 850°C for various times, the following three-stage sequence of precipitation was confirmed: Stage I Stage II Stage III Pro-precipitate I Supersaturated (G.P. zones?) Intermediate phases Equilibrium solid solution Pre-precipitate II Type I and Type II α-Al₂O₃ phase (G.P. precipitates?) The phases forming in stages I, II and III are believed to be coherent, semi-coherent and incoherent, respectively. The mechanical properties of crystals aged to stage I or II are much better than the supersaturated single-phase crystals or the over-aged crystals represented by state III.