Halo formation in directionally solidified Al2O3-Er3Al5O12 off-eutectic in situ composite ceramics

Directionally solidified Al2O3-Er3Al5O12 binary off-eutectic composite ceramics have been fabricated by laser floating zone melting. Microstructure morphologies and halo formation behaviors were explored. At hypereutectic composition (Al2O3-22.5 mol% Er2O3), Er3Al5O12 primary phase formed but no Al2O3 halo structure has been detected at normal solidification rate (similar to 200 mu m/s). When the eutectic spacings have refined down to 100-150 nm, only thin layer of Al2O3 halo structure could form around the Er3Al5O12 primary phase. At hypoeutectic composition (Al2O3-13.5 mol% Er2O3), coarse Er3Al5O12 halo phase can form around the Al2O3 primary dendrite even at low solidification rate (50 mu/s). The formation behavior of halo structures has been analyzed on the basis of competitive growth between primary dendrite phase, halo phase and eutectic.