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Liquid Ni-31.7%Sn-2.5%Ge alloy was highly undercooled by up to 238 K (0.17T) with glass fluxing and drop tube techniques. The dendritic growth velocity of primary NiSn compound shows a power-law relation to undercooling and achieves a maximum velocity of 380 mm/s. The addition of Ge reduces its growth velocity as compared with the binary NiSn alloy. A structural transition from coarse dendrites into equiaxed grains occurs once undercooling exceeds a critical value of about 125 K, which is accompanied by both grain refinement and solute trapping. The NiSn intermetallic compound behaves like a normal solid solution phase showing nonfaceted growth during rapid solidification. [ABSTRACT FROM AUTHOR]