Experimental Evidence for Dendrite Tip Splitting in Deeply Undercooled, Ultrahigh Purity Cu

Abstract

In a recent paper we used a phase-field model of solidification in deeply undercooled pure melts to show that a kinetic instability could result in dendrite tip splitting, and we speculated that such tip splitting could give rise to the phenomenon of spontaneous grain refinement. Here we present evidence, from the as-solidified microstructure of deeply undercooled ultrahigh purity Cu, of what appears to be dendrite tip splitting during recalescence. The significance of this finding in a nongrain refined sample is discussed in terms of the current theories for spontaneous grain refinement.

The Origins of Spontaneous Grain Refinement in Deeply Undercooled Metallic Melts

“…Specifically, we have suggested that a tip-splitting instability at the dendrite tip leads to the growth of an unstable "dendritic seaweed" structure, which subsequently remelts to give the observed grain-refinement. Phase-field modeling of the thermally controlled solidification of a pure material at high undercooling [29] appear to support this suggestion, as does the observation, in deeply undercooled ultra-high purity Cu, of a "frozen in" seaweed morphology [30] above some critical undercooling. Moreover, above this critical undercooling a discontinuity in the velocity-undercooling curve was observed identical in character to that found in materials that display spontaneous grain refinement.…”

Prediction of the operating point of dendrites growing under coupled thermosolutal control at high growth velocity

“…marginal stability, growth at the extremum and even constant ρ appear to yield growth velocities displaying a power law dependence on Δ). Moreover, although experimental data [39,40,41,42 ] is only available for free dendritic growth in 3-dimesions, experimental velocityundercooling curves for a wide range of materials show a very similar type dependence.…”

The Role of Recrystallization in Spontaneous Grain Refinement of Rapidly Solidified Ni3Ge

“…In the case of Ni 3 Ge, it appears that recrystallization is not suppressed until the cooling rate exceeds 42,000 K s À1 , wherein the observed primary solidification morphology is dendritic seaweed. Secondly, we note that [23] observed recrystallization grain refinement at low undercooling while in contrast [22] observed it at high undercooling but only at undercoolings higher than those required for the transformation to seaweed morphologies. In contrast, in this study we observe recrystallization grain refinement at lower undercoolings than the transformation to dendritic seaweed.…”

“…Conversely, in the immediately smaller size fraction (c: 53 to 38 lm, 42,000 to 62,000 K s À1 ) we see the clear development of dendritic seaweed, a morphology observed only at very large departures from equilibrium. [22,23] The histogram of grain orientations for the material in the 75 to 53 lm sieve fraction is shown in Figure 5(a), from which it is clear that the majority of grain boundary misorientations are either <10 deg or close to 60 deg. This is not the distribution that would be expected due to randomly nucleated grains.…”