The mechanism for spontaneous grain refinement in undercooled pure Cu melts

2014

Metals

Phase-field modeling of rapid alloy solidification, in which the rejection of latent heat from the growing solid cannot be ignored, has lagged significantly behind the modeling of conventional casting practices which can be approximated as isothermal. This is in large part due to the fact that if realistic materials properties are adopted, the ratio of the thermal to solute diffusivity (the Lewis number) is typically 10 3 -10 4 , leading to severe multi-scale problems. However, use of state-of-the-art numerical techniques, such as local mesh adaptivity, implicit time-stepping and a non-linear multi-grid solver, allow these difficulties to be overcome. Here we describe how the application of such a model, formulated in the thin-interface limit, can help to explain the long-standing phenomenon of spontaneous grain refinement in deeply undercooled melts. We find that at intermediate undercoolings the operating point parameter, σ*, may collapse to zero, resulting in the growth of non-dendritic morphologies such as doublons and 'dendritic seaweed'. Further increases in undercooling then lead to the re-establishment of stable dendritic growth. We postulate that remelting of such seaweed structures gives rise to the low undercooling instance of grain refinement observed in alloys.

Section: Introductionsupporting

confidence: 68%

The Origins of Spontaneous Grain Refinement in Deeply Undercooled Metallic Melts

2014

Metals

“…Their model is further supported by experimental observations of a 'trapped-in' seaweed-like structure in samples of highly undercooled pure Cu [18].…”

Section: Introductionmentioning

confidence: 56%

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

Castle

2014

Acta Materialia

Self Cite

Paper:Castle, EG, Mullis, AM and Cochrane, RF (2013) Evidence for an extensive, undercooling-mediated transition in growth orientation, and novel dendritic seaweed microstructures in 8-fold growth at low undercooling whilst <111> character begins to dominate at high undercooling, accompanied by a switch to 6-fold 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 undercooling 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, characterised 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.

“…However, although there is plentiful evidence of an initial decrease in microstructural length scale in the low undercooling region, it has proved almost impossible to make a continuous extension of such an analysis into the high undercooling regime where the presence of a local minimum might be inferred. Even for systems where a single dendritic phase exists over the whole undercooling range, such as Ni-Cu [39] or Cu-O [44], the intervention of remelting and/or recrystallisation effects such as spontaneous grain refinement [43,45,46] make the estimation of the original tip radius during dendritic growth impossible. Given that the local minimum in the tip radius moves to lower undercooling as the Lewis number is increasing we consider, on the balance of probabilities, that at higher Lewis numbers than those studied here it may still be the case that a local maximum in the tip radius will be observed.…”

Section: Resultsmentioning

confidence: 99%

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

2011

Phys. Rev. E

We use a phase-field model for the growth of dendrites in dilute binary alloys under coupled thermo-solutal control to explore the dependence of the dendrite tip velocity and radius of curvature upon undercooling, Lewis number (ratio of thermal to solutal diffusivity), alloy concentration and equilibrium partition coefficient. Constructed in the quantitatively valid thin-interface limit the model uses advanced numerical techniques such as mesh adaptivity, multigrid and implicit time-stepping to solve the non-isothermal alloy solidification problem for materials parameters that are realistic for metals. From the velocity and curvature data we estimate the dendrite operating point parameter, σ*. We find that σ* is non-constant and, over a wide parameter space, displays first a local minimum followed by a local maximum as the undercooling is increased. This behaviour is contrasted with a similar type of behaviour to that predicted by simple marginal stability models to occur in the radius of curvature, on the assumption of constant σ*.