Microstructure evolution and solidification behaviour of Nb-16Si-22Ti-2Cr-2Al-6Hf alloy processed by directional solidification

The microstructure evolution and solidification behaviour of Nb-16Si-22Ti-2Cr-2Al-6Hf alloy processed by liquid metal cooled directional solidification were investigated. The temperatures (1850 and 1900°C) and withdrawal rates (1·2, 3, 6, 12 and 18 mm min^-1) were selected. The directionally solidified alloy was composed of primary Nb solid solution dendrites, silicide laths and two types of eutectic (fish bone-like and lamellar eutectic) aligned along the growth direction. As the withdrawal rate increased, the microstructure of the quasi-steady state growth region became finer and the volume fractions of (Nb,X)₅Si₃ laths and the fish bone-like eutectic (Nb,Ti)_SS+(Nb,X)₅Si₃ increased, which was contrary to lamellar eutectic (Nb,Ti)_SS+(Nb,X) ₅Si₃. The volume fraction of (Nb,X)₅Si ₃ laths in the whole directionally solidified rod decreased as the temperature increased from 1850 to 1900°C. In addition, the solidification paths were also discussed.