The defect structure and mechanical properties of spinel single crystals

Abstract

Abstract The grown-in defect structure of non-stoichiometric magnesium aluminate spinels (of compositions between Al2O3/MgO = 3·5 and 1·5) has been analysed by electron microscopy. Dislocations in all crystals are composed of pairs of partial dislocations bounding a fault in the cation stacking sequence. Non-stoichiometric crystals with ⟨100⟩, ⟨110⟩ and ⟨111⟩ compression axes have deen deformed in the range 1300–1520°C. Measurements of flow stress for the three orientations together with observations of slip lines, birefringence patterns and the direct observation of dislocations by electron microscopy determine a principal glide plane of the type {110} and Burgers vector direction a/2⟨110⟩. Dislocation glide also occurs on {111} planes at higher resolved shear stresses such that τ111/τ110 = 1·5 for crystals of composition Al2O3/MgO ≈ 3·5. This ratio decreases with decreasing deviation from stoichiometry. Stoichiometric crystals with ⟨110⟩ compression axes undergo very small plastic strain before fracture (in the temperature range 1520–1620°C) at comparatively high stress. Dislocations nucleated in stoichiometric electron microscope specimens, by beam heating, glide on {111} planes. Glide dislocations for all compositions are composed of coupled pairs of partials of Burgers vector a/4⟨110⟩ enclosing a cation stacking-fault ribbon of width which increases with deviation from stoichiometry. Slip flexibility is limited by a difficulty of glide band intersection such that the dislocation sub-structure after large strains consists of dislocation boundaries stabilized by interaction of dislocations on {110} systems.