Influence of Minor Cr-Additions to the Growth of Columnar Dendrites in Al-Zn Alloys: Influence of Icosahedral Short Range Order in the Liquid

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

“…It has to be mentioned that, out of three experiments for each alloy, all of those containing Cr exhibit feathery grains while binary Al-Zn alloy have shown only regular columnar grains. This also confirms the results of [3] obtained for DS samples: they observed feathery grains in only one out of ten specimens without Cr, while with Cr eleven specimens out of twelve exhibited twinned dendrites. As illustrated in Figure 3, nucleation of a twinned dendrite seems to occur when a "regular" growing crystal has one direction 110 closely aligned with the direction of the thermal gradient.…”

“…Based on the DS results of Al-Zn alloys presented in the previous section, it is clear that minor Cr additions influence the columnar microstructure in several ways. Although "regular" dendritic grains clearly exhibit a 100 texture in the FFD experiment with or without Cr, Kurtuldu et al [3] have shown that, at lower speed, dendrite trunks grow along 110 with Cr, and 100 without Cr. This Cr-induced change of dendrite growth direction, from 110 at low speed to 100 at high speed, is clearly linked to an attachment kinetics effect rather than a modification of the solid-liquid interfacial energy.…”

“…This Cr-induced change of dendrite growth direction, from 110 at low speed to 100 at high speed, is clearly linked to an attachment kinetics effect rather than a modification of the solid-liquid interfacial energy. Kurtuldu et al [3] have interpreted their result in terms of density of ISRO motifs in the liquid and time for them to attach to the growing fcc phase. Cr being a peritectic-type element, the density of Cr in the liquid ahead of a dendrite tip decreases, and so does the density of ISRO motifs.…”

“…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 110 direction and is split in its centre by a coherent {111} twin plane.…”

“…In two recent papers [3,17], ISRO has been shown to also influence the growth directions of dendrites during solidification. In Al-20wt.%Zn alloys [3], 100 dendrites are replaced by 110 trunks at intermediate growth rate (<200 µm/s) when 1000 ppm Cr is added. At higher growth speed (1-2 mm/s), 100 trunks are retrieved but with the concurrent formation of 110 twinned dendrites.…”