The solidification of undercooled Ni–3.3 wt% B alloy was studied by high-speed video analysis and microstructural analysis. For moderate initial undercooling (ΔTp = 75 K), the solidification interface for primary phase transformation manifests a shape of a planar dendrite, and possesses an constant growth velocity, for eutectic transformation whereas the interface presents multi-dendrite shape and spasmodic growth, so that a constant velocity cannot describe the interface exactly. These differences suggest that primary phase solidification is controlled by far-distance diffusion while eutectic solidification by short-distance diffusion. For large initial undercooling (ΔTp = 262 K), a kinds of large “white dendrites”, which is in fact composed of multiple phases, were found in the microstructure, from inside to outside of which, the eutectic phase changes from dot phases (anomalous structure) to irregular eutectic and then to regular eutectic, indicating that the center of “white dendrites” may be the nucleation zone of eutectic reaction.

The growth velocity during solidification of an undercooled melt of a Co-Cu alloyprocessed by electromagnetic levitation was measured using a high speed video camera.Applying a model of local non-equilibrium solidification, theoretical predictions ofdendrite growth velocity and dendritic growth radii are compared with high-accuracymeasurements of the growth kinetics. As the undercooling ΔT reaches a critical valueconsistent with the dendrite growth velocity being equal to the atomic diffusion speedVD in bulk liquid,ΔT =ΔT(VD),the velocity-undercooling relationship exhibits a break-point. A distinct change in thedendritic growth mechanism exists with the onset of complete solute trapping andchemically partitionless solidification of the core of the main stems of the dendritesoccurs. A complete transition to the thermally controlled growth of dendrites occurs atΔT =ΔT(VD)that leads to essential changes in the microstructure of dendritic patterns The phenomenonof dendritic fragmentation in Co-Cu melts, solidifying at ΔT <ΔT(VD),is discussed.

Solidification of undercooled Ni–4.5 wt% B alloy melt was investigated by glass fluxing and cyclic superheating. A maximum melt undercooling up to ΔTp = 283 K has been achieved. If ∆Tp < 175 ± 10 K, the primary solidification is L → Ni3B; the structure consists of Ni3B dendrite + lamellar eutectic; the phase sizes and fractions depend on ∆Tp. If ∆Tp ≥ 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 ∆Tr; 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.