Directional solidification and microstructural development of Al2O3/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–Ln2O3 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.

Research highlights
► Al2O3/GdAlO3 eutectic ceramic rods and plates are prepared by laser zone remelting.
► Studying the microstructure development and growth behavior at rapid growth rate.
► Ultra-fine irregular microstructure with eutectic spacing below100 nm is obtained.
► Complex-regular rod/lamellae structure are observed with increasing growth rate.
► Transition of irregular “Chinese script” to complex-regular structure is discussed.