Evidence for an extended transition in growth orientation and novel dendritic seaweed structures in undercooled Cu-8.9 wt%Ni

A melt encasement (fluxing) technique has been used to systematically study the microstructural development and velocity-undercooling relationship of a Cu-8.9 wt%Ni alloy at undercoolings up to 235 K. A complex series of microstructural transitions have been identified with increasing undercooling. At the lowest undercoolings a 〈1 0 0〉 type dendritic structure gives way to an equiaxed grain structure, consistent with the low undercooling region of grain refinement observed in many alloys. At intermediate undercoolings, dendritic growth returns, consisting of dendrites of mixed 〈1 0 0〉 and 〈1 1 1〉 character. Within this region, 8-fold growth is first observed at low undercoolings, indicating the dominance of 〈1 0 0〉 character. As undercooling is increased, 〈1 1 1〉 character begins to dominate and a switch to 6-fold growth is observed. It is believed that this is an extended transition region between 〈1 0 0〉 and 〈1 1 1〉 dendrite growth, the competing anisotropies of which are giving rise to a novel form of dendritic seaweed, characterised by its containment within a diverging split primary dendrite branch. At higher undercoolings it is suggested that a transition to fully 〈1 1 1〉 oriented dendritic growth occurs, accompanied by a rapid increase in growth velocity with further increases in undercooling. At the highest undercooling achieved, a microstructure of both small equiaxed grains, and large elongated grains with dendritic seaweed substructure, is observed. It is thought that this may be an intermediate structure in the spontaneous grain refinement process, in which case the growth of dendritic seaweed appears to play some part.