Directional solidification and microstructural development of Al 2O 3/GdAlO 3 eutectic ceramic in situ composite under rapid growth conditions

Abstract

Oxide eutectic ceramic in situ composites prepared by directional solidification from the melt in the Al 2O 3–Ln 2O 3 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) Al 2O 3/GdAlO 3(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-Al 2O 3 and GAP phases is obtained at the scanning rate below 360 mm/h. Further increasing the scanning rate, the as-solidified Al 2O 3/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 Al 2O 3 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.