Undercooling-mediated growth transitions in rapidly solidified Cu-Ni alloys: Towards the elucidation of the fundamental mechanism behind ‘spontaneous grain refinement’ in undercooled metallic melts

Within the context of the rapid solidification of metals, an experimental investigation has been undertaken in order to elucidate the fundamental mechanism behind the ‘spontaneous grain refinement’ phenomenon; in which abrupt transitions between coarse columnar and fine equiaxed grain structures are observed with increasing solidification velocity. A melt fluxing technique has been employed to undercool and rapidly solidify a Cu-8.9 wt.% Ni alloy and a Cu-3.98 wt.% Ni alloy. This method permits in situ high-speed imaging of the recalescence front, allowing the solidification velocity to be calculated and studied as a function of undercooling. A B2O3 + Na2SiO3 glass flux has been identified as the optimal composition for use in the melt fluxing of Cu-Ni alloys, and undercoolings of up to 240 K have subsequently been achieved. Light microscopy, SEM, EBSD and XRD pole figure analysis has been performed on the as-solidified samples in order to study the evolution of microstructure and texture with increased undercooling/solidification velocity. An extended transition in growth orientation, from <100>-oriented at low undercooling to <111>-oriented at high undercooling, has been observed in both alloys. At intermediate undercooling, competitions between the two growth orientations are observed to give rise to mixed-orientation microstructures, a novel form of the dendritic seaweed structure and multiple twinning. In the high-Ni alloy, this mixedorientation regime appears to coincide with the low undercooling grain refinement transition, subsequently driving a recrystallisation and recovery process. In the low-Ni alloy, however, the mixed-orientation regime does not coincide with the grain refinement region, and grain refinement in this instance occurs via a dendrite fragmentation mechanism. Dendrite fragmentation is also observed to drive spontaneous grain refinement at high undercooling in this alloy. By contrast, at the largest undercooling achieved in the high-Ni alloy, a partially grain refined sample has been obtained, in which the majority of the substructure consists of dendritic seaweed –suggesting that seaweed is the most likely precursor to spontaneous grain refinement in this case. Thus, three separate grain refinement mechanisms have been identified in the two closely-related alloys; the selection of which appears to be dependent upon the original growth structures present, which are in turn determined by the closeness of the competitions between differently-directed anisotropies. It is suggested that the addition of Ni to Cu either increases the strength of the kinetic anisotropy, or decreases the strength of the capillary anisotropy (or both); leading to a closer competition between the two at higher Ni concentrations, thereby accounting for the differences in microstructure and subsequent grain refinement mechanisms.