Dendritic Growth Morphologies in Al-Zn Alloys—Part I: X-ray Tomographic Microscopy

Upon solidification, most metallic alloys form dendritic structures that grow along directions corresponding to low index crystal axes, e.g., $$\langle100\rangle$$ 〈 100 〉 directions in fcc aluminum. However, recent findings[1,2] have shown that an increase in the zinc content in Al-Zn alloys continuously changes the dendrite growth direction from $$\langle100\rangle$$ 〈 100 〉 to $$\langle110\rangle$$ 〈 110 〉 in {100} planes. At intermediate compositions, between 25 wt pct and 55 wt pct Zn, $$\langle320\rangle$$ 〈 320 〉 dendrites and textured seaweeds were reported. The reason for this dendrite orientation transition is that this system exhibits a large solubility of zinc, a hexagonal metal, in the primary fcc aluminum phase, thus modifying its weak solid-liquid interfacial energy anisotropy. Owing to the complexity of the phenomenology, there is still no satisfactory theory that predicts all the observed microstructures. The current study is thus aimed at better understanding the formation of these structures. This is provided by the access to their 3D morphologies via synchrotron-based X-ray tomographic microscopy of quenched Bridgman solidified specimens in combination with the determination of the crystal orientation of the dendrites by electron-backscattered diffraction. Most interestingly, all alloys with intermediate compositions were shown to grow as seaweeds, constrained to grow mostly in a (001) symmetry plane, by an alternating growth direction mechanism. Thus, these structures are far from random and are considered less hierarchically ordered than common dendrite

Some Consequences of Thermosolutal Convection: The Grain Structure of Castings

The essential principles of thermosolutal convection are outlined, and how convection provides a transport mechanism between the mushy region of a casting and the open bulk liquid is illustrated. The convective flow patterns which develop assist in heat exchange and macroscopic solute segregation during solidification; they also provide a mechanism for the transport of dendritic fragments from the mushy region into the bulk liquid. Surviving fragments become nuclei for equiaxed grains and so lead to blocking of the parental columnar, dendritic growth front from which they originated. The physical steps in such a sequence are considered and some experimental data are provided to support the argument

Simulations of experimentally observed dendritic growth behaviour using a phase-field model

An anisotropic phase-field model is used to simulate numerically dendritic solidification for a pure material in two dimensions. The phase-field model has been formulated to include the effect of four-fold anisotropy in both the surface energy and interfacial kinetics. The computations presented here are intended to model qualitatively experimentally observed dendritic solidification morphology. In particular, we simulate the growth into an undercooled melt of two dendrite tips which have formed as the result of a splitting event. The computation exhibits the competition between the two growing dendrite branches and the eventual predominance of one branch. Also, we simulate the effect of time-periodic forcing of an isolated dendrite tip on the mechanism of sidebranch formation. Although it is not yet computationally feasible to adequately verify convergence of the phase-field solutions, the phase-field simulations presented show many of the qualitative features observed in dendritic growth experiments