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 (0.2T_L). 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.