Precipitation and Hardening Behavior of Czochralski Star Sapphire

Sapphire with 0.11‐wt% titania in solution (mainly as TiO_1.5) was aged isothermally in air up to 21 days at 1100°–1600°C. Optical studies showed that the solubility limit of the titania was exceeded at 1400°C in air, and consequently needle‐shaped precipitates were developed. For aging temperature between 1100° and 1300°C, precipitation initiated at grown‐in dislocations and at subboundaries. This phenomenon was attributed to rapid oxidation along these defects. Following this initial stage, precipitates formed uniformly throughout the crystals, and they coarsened with time. The variation of average needle length with time was measured. Knoop microhardness measurements showed that Ti⁴⁺ ions plus the attendant defects in solution caused measurable hardening of sapphire, but the greatest changes resulted from precipitation. Significant hardening resulted from aging at 1300°C for 13 days. The hardness anisotropy in the basal plane of sapphire, i.e., $\mathrm{KH}{〈}_{\text{101̄0}}\mathrm{〉>}\mathrm{KH}{〈}_{\text{112̄0}}〉$ , was not altered by precipitation.