Evidence for an extensive, undercooling-mediated transition in growth orientation, and novel dendritic seaweed microstructures in Cu–8.9wt.% Ni

Abstract Within the context of rapid solidification, a melt-fluxing technique has been employed to study the microstructural development and the velocity–undercooling relationship of a Cu–8.9 wt.% Ni alloy. A number of microstructural transitions have been observed up to undercoolings of 235 K. At the lowest undercoolings, single grain, 〈1 0 0〉 type dendritic structures are observed, which give way to a recrystallized grain structure as undercooling is increased. At intermediate undercoolings, twinned dendritic samples exhibiting features of mixed orientation are reported. Within this range, the dominant 〈1 0 0〉 character is evidenced by eightfold growth at low undercooling whilst the 〈1 1 1〉 character begins to dominate at high undercooling, accompanied by a switch to sixfold growth. It is believed that this range of undercooling represents an extended transition between fully 〈1 0 0〉 oriented growth at low undercooling and fully 〈1 1 1〉 oriented growth at high undercooling. It is suggested that an observed positive break in the velocity–undercooling relationship at high undercooling is therefore coincident with the transition to fully 〈1 1 1〉 growth, though this could not be confirmed microstructurally. The existence of competing anisotropies in the growth directions at intermediate undercoolings appears to be giving rise to a novel form of the dendritic seaweed structure, characterized by its containment within a diverging split primary dendrite branch. In addition, at the highest undercoolings achieved, an equiaxed-to-elongated grain structure is observed, in which an underlying dendritic seaweed substructure exists. We suggest that this structure may be an intermediate in the spontaneous grain refinement phenomenon, in which case dendritic seaweed appears to play some part.