Metastable Phase Formation and Its Transformation in Highly Undercooled Ni75B25 Alloy

Ni₇₅B₂₅ alloy was solidified at various undercooling. The formation and subsequent transformation of metastable Ni₂₃B₆ 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 Ni₃B phase. At larger undercooling, metastable Ni₂₃B₆ phase primarily forms in the melt but then is decomposed into α-Ni and Ni₃B 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/Ni₃B eutectoid is partially remelted if temperature exceeds the eutectic temperature during the second recalescence. Then, residual Ni₃B grows into coarse round grains while the remaining liquid re-solidifies into α-Ni/Ni₃B 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 Ni₃B 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 Ni₃B and Ni₂₃B₆ phases in the alloy melt is nucleation controlled. The heterogeneous site in Ni₇₅B₂₅ alloy melt is a better nucleation substrate for Ni₂₃B₆ phase than for Ni₃B phase.

Graphical abstract