Phase selection of undercooled solidification of Ni-4.5 wt% B alloy

Abstract

Solidification of undercooled Ni-4.5 wt% B alloy melt was investigated by glass fluxing and cyclic superheating. A maximum melt undercooling up to ΔT_p = 283 K has been achieved. If ∆T_p < 175 ± 10 K, the primary solidification is L → Ni₃B; the structure consists of Ni₃B dendrite + lamellar eutectic; the phase sizes and fractions depend on ∆T_p. If ∆T_p ≥ 175 ± 10 K, the primary solidification is L → Ni/Ni₂₃B₆; the structure consists of the dot-phase region + the anomalous eutectic/network boundary; the phase fractions mainly depend on ∆T_r; the dot phases are determined as rod eutectic and dot precipitates, while the network boundary is the divorced eutectic. The solidification pathways show that there is a common critical nucleation temperature, 1227 ± 10 K, for metastable eutectic reaction in hypoeutectic and hypereutectic Ni-Ni₃B alloys.