Influence of Zinc Anisotropy on Solidification Microstructures: Experimental Results and Phase-Field Simulations

Due to a distortion of its hexagonal unit cell along the c-axis compared to hexagonal-close-packed metals (c/a = 1.856 instead of 1.633), zinc is characterised by a large anisotropy of its solid-liquid interfacial energy. While previous studies have focused on the use of zinc alloys for galvanic coating of steel sheets, there has been relatively few investigations of the effect of this anisotropy on the formation of dendritic microstructure in Zn-Al bulk specimens. The aim of this thesis is thus to understand the effect of the large solid-liquid interfacial energy anisotropy on the growth morphologies of Zn-Al alloys. This investigation is of great scientific interest as zinc dendrites grow along <10-10> directions with an edgy tip. Given the importance of the solid-liquid interfacial energy on the solidification microstructure, in situ observations of liquid alloy droplets in a solid matrix under near-equilibrium conditions were performed and reported here for the first time. The setup used has been found relatively accurate (with a 10% error) for measurements of large anisotropies. However, due to the limited resolution of the X-ray tomography beamline, low anisotropy measurements have fairly large errors. In order to study dendrite morphologies in Zn-4wt.%Al-0.17wt.%Pb, specimens were prepared in two ways: (i) Bridgman growth in a fixed temperature gradient and constant speed of 0.067 mm/s and (ii) directional solidification from a water-cooled steel chill, with a solidification velocity on the order of 0.3 mm/s. The samples were then studied by optical microscopy, SEM and EBSD as well as by X-ray tomography. Detailed phase-field simulations were used to understand the dendrite morphologies that formed in these experiments. Both experimental observations and phase-field simulations confirmed that, at low velocity, Zn dendrites grow preferentially along <10-10> with short secondary arms along <0001>. The growth morphology of <10-10> dendrites is very peculiar, with the presence of doublets/triplets of primary trunks in a transverse section: it can be accounted for by the very large solid-liquid interfacial energy anisotropy of zinc (and thus the presence of forbidden orientations). As c-axis secondary arms grow, their lateral surface start to have a normal which is no longer allowed by the equilibrium shape. Therefore, they start bending along the closest growth direction, i.e., <10-10> and eventually form new primary trunks. However, under faster solidification conditions, the microstructure displays a mix of grains with two different principal growth directions, namely <10-10> and <0001>. The gradual transition of Zn dendrites from <10-10> to <0001> with increasing solidification speed is suspected to be due to the fact that the dendrite tips along these two directions do not exhibit the same shape and radii of curvature. Because of the large solid-liquid interfacial energy anisotropy, <10-10> dendrites must have a sharp tip (i.e., a slope discontinuity) perpendicular to the basal plane and a smooth radius of curvature in the basal plane, the angle of the edge being almost constant. On the other hand, <0001> dendrites have two equal radii of curvature. Thus, when the growth rate is increased, <10-10> dendrites have only one adaptable radius, whereas <0001> dendrites have two. At some velocity, it becomes easier for <0001> dendrites to reject solute compared to <10-10> dendrites.


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