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

Abstract

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)₅Si₃ 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.

Keywords:

• Nb–Si based alloy
• Directional solidification
• Microstructure
• Solidification path