Influence of Minor Cr Additions on Crystal Growth in Rapidly Solidified Al-20Zn Alloys.

Félix Royer,Michel Rappaz,Julien Zollinger,Bernard Rouat

doi:10.3390/ma13020379

Abstract

It has been discovered quite recently that Icosahedral Short-Range Order (ISRO) of atoms in the liquid phase of metallic alloys surrounding some trace elements added to the melt can influence both the nucleation and growth of the primary phase. In this work, Al-20wt.%Zn alloys without and with 0.1 wt.% Cr additions have been processed using a free-falling droplet technique. This technique allows to undercool the liquid droplet during its fall and thus to have rapid directional solidification conditions when it collides a copper-cooled substrate. Under such rapid solidification conditions, microstructural and EBSD analyses have shown that, under such rapid solidification conditions, Cr addition is responsible for the nucleation and growth of feathery grains (or twinned dendrites). This morphology specific to aluminum alloys has been discovered more than seventy years ago without a clear identification of its origin. The angular analysis between twinned dendrites indicates a behavior similar to those of the propagation of topological defects, through an ISRO-induced stacking fault mechanism.

Highlights

Alloys with cubic crystal structure usually solidify with their dendrite trunks and arms growing along h100i directions
Kurtuldu et al [3] have interpreted their result in terms of density of Icosahedral Short-Range Order (ISRO) motifs in the liquid and time for them to attach to the growing fcc phase
It has been shown that minor Cr additions leads to the formation of feathery grain in Al-Zn alloys rapidly solidified using the Free-Falling Droplet experiment

Summary

Introduction

Alloys with cubic crystal structure usually solidify with their dendrite trunks and arms growing along h100i directions. Aluminium alloys are known for exhibiting additional unusual dendrite growth morphologies, depending in particular on additional solute elements such as Zn [1] or Ge [2]. One dendritic growth morphology specific to aluminium alloys is the formation of twinned dendrites, within what are referenced as “feathery grains” [3,4,5,6]. The growth direction and propagation of twinned dendrites are fairly well understood: their trunk grows along a h110i direction and is split in its centre by a coherent {111} twin plane. The origin of such twinned structure remained unknown until the PhD work of Kurtuldu [7] on the effect of trace elements on the solidification of Al alloys brought a possible explanation. Multiple Twin (MT) relationships between nearest-neighbours could only be explained if one considers the geometry of an icosahedron (or interlocked icosahedron) template on which face-centered cubic (fcc) grains form with the following heteroepitaxy relationships: Materials 2020, 13, 379; doi:10.3390/ma13020379 www.mdpi.com/journal/materials

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