High undercooling and rapid solidification of Ni32.5% Sn eutectic alloy

Ni32.5% Sn eutectic alloy has been undercooled up to 397 K (0.283 T_E) by levitation melting and superheating within 45 special inorganic glasses. Its rapid solidification behaviour is studied by high speed infrared pyrometry and cinematography. It is found that interface heterogeneous nucleation may take place in preference to homogeneous nucleation, even though undercooling exceeds 0.2T_E. The transient solidification velocity during recalescence attains 784 mm/s at an undercooling of 302 K. If undercooling is below 100 K, an abundance of feathery lamellar eutectic grains form, and β-Ni₃Sn serves as a better nucleant for α-Ni phase than vice versa. As undercooling increases, both β-Ni₃Sn and α-Ni phases can nucleate primarily and independently, and a microstructure morphology transition from regular lamellar eutectic into irregular anomalous eutectic occurs. It is concluded that anomalous eutectic is the product of rapid solidification whereas lamellar eutectic forms under slow solidification conditions. An undercooling of 376 K leads to an extension of Sn solid solubility in α-Ni phase from the equilibrium limit of 19.0% to a maximum of 25.8%.