Dendritic Growth Morphologies in Al-Zn Alloys—Part II: Phase-Field Computations

Abstract

In Part I of this article, the role of the Zn content in the development of solidification microstructures in Al-Zn alloys was investigated experimentally using X-ray tomographic microscopy. The transition region between h100i dendrites found at low Zn content and h110i dendrites found at high Zn content was characterized by textured seaweed-type structures. This Dendrite Orientation Transition (DOT) was explained by the effect of the Zn content on the weak anisotropy of the solid-liquid interfacial energy of Al. In order to further support this interpretation and to elucidate the growth mechanisms of the complex structures that form in the DOT region, a detailed phase-field study exploring anisotropy parameters' space is presented in this paper. For equiaxed growth, our results essentially recapitulate those of Haxhimali et al. [1] in simulations for pure materials. We find distinct regions of the parameter space associated with h100i and h110i dendrites, separated by a region where hyperbranched dendrites are observed. In simulations of directional solidification, we find similar behavior at the extrema, but in this case, the anisotropy parameters corresponding to the hyperbranched region produce textured seaweeds. As noted in the experimental work reported in Part I, these structures are actually dendrites that prefer to grow misaligned with respect to the thermal gradient direction. We also show that in this region, the dendrites grow with a blunted tip that oscillates and splits, resulting in an oriented trunk that continuously emits side branches in other directions. We conclude by making a correlation between the alloy composition and surface energy anisotropy parameters.

Anomalous α-Mg Dendrite Growth During Directional Solidification of a Mg-Zn Alloy

“…Prior studies have observed seaweed-like structures during solidification either at near-isothermal conditions [24][25][26][27] or in deeply undercooled metallic melts. [28][29][30] Textured seaweed structures were also observed by Rappaz et al [8][9][10]19] in aluminum alloys when zinc was added at compositions of 25 to 55 wt pct. Our results in Mg-Zn, combined with those of Rappaz et al in Al-Zn, support Rappaz's hypothesis that the increase in interfacial energy anisotropy due to the addition of Zn alters the growth morphology.…”

“…In contrast, the anisotropy in hcp zinc is very large, as high as 30 pct between the c-direction and the growth directions in the basal plane. [9,10] Based on this, Rappaz et al hypothesized that the addition of Zn increased the anisotropy causing a dendrite orientation transition [8][9][10]19] in Al alloys. In terms of the influence of imposed thermal conditions, Pettersen et al [16,17] demonstrated that the growth direction of primary a-Mg dendrites in a directionally solidified (DS) AZ91 alloy (90.8Mg-8.25Al-0.7Zn-0.35Mn, wt pct) could be altered from h1120i at low G and high V to h2245i for high G and low V. In addition, the number of secondary arm side branches transformed from the expected six to only three.…”

“…The two most common ways to achieve new microstructure are as follows: 1. the addition of alloying elements, which significantly alter the interfacial surface tension anisotropy, c¢ sl ; 2. imposed thermal conditions (thermal gradient, G, and growth velocity, V). Rappaz et al [8][9][10]19] demonstrated that the addition of Zn alters the preferred growth direction in aluminum alloys, transforming a-Al dendrites from h100i to h110i as the Zn wt pct is increased from 5 to 90 pct. The addition of Zn to Al is thought to strongly increase c¢ sl .…”

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

“…al. [24] in directionally solidified Al-Zn alloys, in which they observe an extended transition between <100> growth and <110> growth with increasing Zn content. In the intermediate range, referred to as the 'Dendrite Orientation Transition, (DOT)', they find that competition between the differently directed growth anisotropies give rise to textured, seaweed-like structures.…”

The Origins of Spontaneous Grain Refinement in Deeply Undercooled Metallic Melts

“…Microstructure selection maps (in undercooling-anisotropy space) produced by Brener et al [31] using solvability theory indicate a range of such transitions should occur, while simulations by Haxhimali et al [32] have suggested that many common metals may lay close to the dendritic-"seaweed" transition. One such transition has been observed directly as a function of composition in Al-Zn alloys by Dantzig et al [33]. A range of undercooling mediated growth direction transitions have also been observed by Castle et al [34] in Cu-10at.% Ni alloy, in which 4-, 6-and 8-fold symmetric dendrites are observed as the undercooling is progressively increased, with 4-fold symmetry finally being recovered at the highest undercoolings achieved, providing strong evidence for velocity mediated transitions in the growth direction.…”