Alumina precipitation in rutile single crystal saturated with alumina under thermal tensile stress

Abstract

Alumina precipitation behavior in rutile single crystals saturated with alumina was investigated using diffusion couples composed of rutile a-plane bonded to corundum c-plane. During annealing at 1200–1300 °C, alumina dissolved into rutile according to the defect reaction, Al₂O₃ → 2Al′_Ti + 3O_O ^× + V_O^⋅⋅. Subsequent cooling generated supersaturation of alumina and, simultaneously, produced significant tensile stress along the rutile c-axis due to the mismatch in thermal contraction between rutile and corundum. This stress induced dislocation formation having slip system, such as (011)[011]. In the air-quenched samples, supersaturated oxide-ion vacancies reacted with screw dislocations directed [101] and [101] on the (100) plane, to form rectangular helical and/or zigzag dislocations. Alumina precipitated along these modified dislocations as corundum. The direction of the rectangular helical and zigzag modifications associated with the pseudo-hexagonal oxide-ion sublattice of rutile. In contrast, slow cooling at 10 °C/min produced four-lobed corundum precipitates aligned with the rutile a- and b-axes, corresponding to half of the eight-lobed morphology. These results demonstrate that thermal stress, cooling rate, and crystallographic orientation jointly control modification of dislocation structures and alumina precipitation morphology in alumina-saturated rutile.