Directional solidification and microstructural development of Al ₂O₃/GdAlO₃ eutectic ceramic in situ composite under rapid growth conditions

Oxide eutectic ceramic in situ composites prepared by directional solidification from the melt in the Al₂O₃-Ln ₂O₃ systems are promising candidates for the manufacture of turbine blades because of their excellent mechanical properties. With high temperature gradient and rapid cooling rate, directionally solidified (DS) Al₂O₃/GdAlO₃(GAP) eutectic ceramics having smooth surface and full density are successfully prepared by laser zone remelting at the scanning rate of 240-720 mm/h. The microstructure development and solidification behaviour under rapid growth rate are investigated. Fine typical DS irregular eutectic structure of "Chinese script" consisting of interpenetrating α-Al₂O₃ and GAP phases is obtained at the scanning rate below 360 mm/h. Further increasing the scanning rate, the as-solidified Al₂O₃/GAP eutectic presents a combination of "Chinese script" and elongated colony microstructure with complex regular structure. Inside the colonies, the rod-type or lamellar-type eutectic microstructures with ultra-fine GAP surrounded by the Al₂O₃ matrix are observed. The interphase spacing is strongly dependent on the laser scanning rate, rapidly decreasing to the sub-micron range for the samples grown at the highest rate. Moreover, the formation condition and solidification mechanism of the particular microstructure of the ceramic composite during rapid solidification are discussed.