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

Abstract

Upon solidification, most metallic alloys form dendritic structures that grow along directions corresponding to low index crystal axes, e.g., h100i 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 h100i to h110i in {100} planes. At intermediate compositions, between 25 wt pct and 55 wt pct Zn, h320i 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 dendrites.

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.…”

“…Understanding and controlling microstructural evolution in Mg alloys, including dendritic growth orientation and morphology during solidification, is critical to obtaining optimal mechanical properties. [4] Microtomography has proven to be a powerful tool for investigating dendritic growth and morphology for bcc and fcc alloys, with prior studies primarily focussed on binary aluminum alloys containing copper [5][6][7] or zinc, [8][9][10] which grow with a preferred orientation of [100] and form a four-fold symmetric structure. [4,11] Mg alloys are hcp, with a crystal structure that has six-fold symmetry.…”

“…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 .…”

“…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.…”

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

“…IN a companion article, [2] X-ray tomographic microscopy was used to characterize the microstructures that form in directionally solidified (DS) and quenched Al-Zn alloys for various Zn compositions ranging from 5 to 90 wt pct. The results obtained in that study were in agreement with those previously found by Gonzales and Rappaz [3,4] on DS (but not quenched) Al-Zn alloys.…”