Competitive nucleation and dendritic growth of βNb5Si3 and Nb3Si intermetallic compounds within liquid Nb-25 at. % Si alloy under electrostatic levitation condition

The rapid solidification mechanism of the peritectic Nb-25 at. % Si alloy was investigated via the electrostatic levitation technique, where liquid undercooling attained 486 K (0.2TL). The dendritic growth process of primary phase was observed in situ by high-speed photography. If liquid undercooling was below 443 K, the primary phase was characterized by coarse βNb5Si3 dendrites, and their maximum growth velocity measured 93 mm/s at 434 K. In the interdendritic zone, a small amount of Nb3Si phase was produced by the peritectic reaction and then decomposed through eutectoid transformation. Once liquid undercooling increased above the threshold of 443 K, peritectic Nb3Si phase would nucleate and grow preferentially. The growth velocity of Nb3Si phase increased rapidly with the rise of undercooling, attaining 248 mm/s at the maximum undercooling of 486 K. Because of the high cooling rates during electrostatic levitation, the allotropic transformation from βNb5Si3 to αNb5Si3 was suppressed.