Microstructure control, competitive growth and precipitation rule in faceted Al2O3/Er3Al5O12 eutectic in situ composite ceramics prepared by laser floating zone melting

Abstract

Microstructure control and competitive growth of Al2O3/Er3Al5O12 eutectic/off-eutectics are explored over wide ranges of solidification rates and compositions. Gradual transformation phenomenon of microstructure morphology from complete eutectic to eutectic + coarse Er3Al5O12 phase and to eutectic + Er3Al5O12 dendrite is observed and the corresponding influence factors are evaluated. Competitive growth between single-phase Al2O3 (or Er3Al5O12) dendrite and eutectic is analyzed and coupled growth zone is mapped through comparing interface temperatures of different patterns of microstructures. The complete eutectic microstructure could be obtained at Al2O3/Er3Al5O12 hypoeutectic (Al2O3-17.5 mol% Er2O3) under fast solidification rate and the onset growth rate (˜0.94 × 104 μm/s) estimated from the measured eutectic spacing (˜150 nm) fits well with the result calculated on the basis of competitive growth (˜1.27 × 104 μm/s). Transformation of microstructure from irregular eutectic to regular eutectic and probable adjustment mechanism of eutectic spacings are discussed when the eutectic spacings refined from micron-scale (<10 μm) to nano-scale (˜20 nm).