Ni 75 B 25 alloy was solidified at various undercooling. The formation and subsequent transformation of metastable Ni 23 B 6 phase were clearly identified. If undercooling prior to nucleation is less than a critical value of 240 K (240 °C), the alloy solidifies completely into Ni 3 B phase. At larger undercooling, metastable Ni 23 B 6 phase primarily forms in the melt but then is decomposed into α-Ni and Ni 3 B through a eutectoid reaction. The decomposition simultaneously triggers the rapid solidification of residual liquid, due to which a second temperature recalescence occurs. The α-Ni/Ni 3 B eutectoid is partially remelted if temperature exceeds the eutectic temperature during the second recalescence. Then, residual Ni 3 B grows into coarse round grains while the remaining liquid re-solidifies into α-Ni/Ni 3 B eutectic structure in the remelted region. In the case that the eutectic temperature is not reached, the eutectoid product with dot α-Ni distributing in Ni 3 B matrix is retained in the solidification structure. A longer delay time between the two temperature recalescence events means less residual liquid, lower recalescence temperature and thus depressed remelting. The formation competition between Ni 3 B and Ni 23 B 6 phases in the alloy melt is nucleation controlled. The heterogeneous site in Ni 75 B 25 alloy melt is a better nucleation substrate for Ni 23 B 6 phase than for Ni 3 B phase. Graphical abstract