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

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, <100> 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 <100> character is evidenced by eightfold growth at low undercooling whilst the <111> 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 <100> oriented growth at low undercooling and fully <111> oriented growth at high undercooling. It is suggested that an observed positive break in the velocity–undercooling relationship at high und- ercooling is therefore coincident with the transition to fully <111> 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 sub-structure 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.