Grain Selection and Texture Evolution in Directionally Solidified Al-Zn Alloys

Abstract

In a recent contribution, we have reported that dendrites in the aluminum-zinc binary system can change their growth direction continuously from $$ {\left\langle {{\text{100}}} \right\rangle } $$ to $$ {\left\langle {{\text{110}}} \right\rangle } $$ as the concentration of zinc C Zn increases from 5 to 90 wt pct.[1] This was interpreted as an influence of the hcp Zn element on the low anisotropy of the solid-liquid interfacial energy of aluminum. Seaweed morphologies were even observed at the start and end of this so-called “dendrite orientation transition” (DOT), i.e., for C Zn ≈ 30 and 55 wt pct. In the present work, we analyze how this DOT influences the selection of grains and the evolution of the texture in directionally solidified (DS) specimens. The solidification texturing of $$ {\left\langle {hk{\text{0}}} \right\rangle } $$ dendritic specimens is shown to be similar to that occurring in normal $$ {\left\langle {{\text{100}}} \right\rangle } $$ specimens. It is analyzed on a similar basis by considering a random orientation distribution of nuclei at the surface of the chill plate and a minimum undercooling criterion. This produces $$ {\left\langle {hk{\text{0}}} \right\rangle } $$ textures with grains that look fairly “equiaxed” in transverse sections. For seaweed morphologies, which also exhibit an $$ {\left\langle {hk{\text{0}}} \right\rangle } $$ texture parallel to the thermal gradient, the grain selection is slower and the grains appear more elongated in transverse sections. Their elongation occurs along a (001) plane, i.e., along a $$ {\left\langle {1\ifmmode\expandafter\bar\else\expandafter\=\fi{2}0} \right\rangle } $$ direction for $$ {\left\langle {210} \right\rangle } $$ seaweeds (C Zn = 30 wt pct) and along a $$ {\left\langle {1\ifmmode\expandafter\bar\else\expandafter\=\fi{1}0} \right\rangle } $$ direction for $$ {\left\langle {{\text{110}}} \right\rangle } $$ seaweeds (C Zn = 55 wt pct). The scanning electron microscopy (SEM) observations reveal that this elongation is accompanied by a microsegregation pattern that is mainly parallel to (001) planes. This indicates that seaweeds at the onset and end of the DOT grow with some type of layered structures, but their detailed growth and grain selection mechanisms are still unknown.