Microstructure development of unidirectionally solidified (Nb)/Nb3Si eutectic alloys

Unidirectional solidification experiments were performed on the (Nb)/Nb3Si eutectic alloys in the Nb–Si binary and Nb–Ti–Si ternary systems. The effect of solidification rates on microstructure development was studied using an optical floating zone melting furnace within the solidification rates from 10 to 200 mm/h. For the binary invariant eutectic alloy, a planar eutectic microstructure forms at 10 mm/h, while cellular eutectic microstructures develop at the higher solidification rates. The orientation relationship between Nb and Nb3Si formed upon planar eutectic solidification is determined as: {1 1 0}Nb//{1 1 0)Nb3Si, and 〈1 1 2〉Nb//〈0 0 1]Nb3Si. For the ternary univariant eutectic alloys, the formation of cellular eutectic microstructures is prevailing for all over the rates of solidification. The growth of Nb3Si is faster than that of (Nb). The size distribution of (Nb) particles spreads widely in an asymmetric manner. The cell size variation against solidification rates is in a reasonable agreement with the theoretical prediction.