Comparison of the surface morphology of dendrites of different growth orientation in Al-ge

Abstract

Dendritic solidification is a common growth form in metallic alloys and has a significant influence on the mechanical properties [1]. However, the anisotropy of the interfacial energy between solid and liquid, which influences the growth direction and shape of the dendrites, is not known for many alloys [2]. In some alloys, such as aluminium-germanium, a dendrite orientation transition can be observed, which is caused by the change in anisotropy with composition [2,3]. Three different growth forms of fcc-Al dendrites depending on the alloy composition were demonstrated by Becker et al. in X-radiography in-situ experiments, whereby the directions <100> at low germanium content, <110> at high and a mixture of both between the concentrations could be observed [3]. To analyze the morphology of equiaxed dendrites in greater detail, a time resolved X-ray tomoscopy experiment was carried out with thin samples (300µm) at the TOMCAT beamline of the Paul Scherrer Institut in Switzerland. Alloy compositions between 5% and 50% Al-Ge were used. The thin square-like samples of approximately 2.2 mm x 1.5 mm x 0.4 mm were contained in a slitted boron-nitrite (BN) cylinder held in place in a sandwich configuration by equally sized graphite-foil pieces. The BN-cylinder was heated by two infrared lasers from the side whilst continuously rotating around its vertical axis. Temperature was calibrated and measured by pyrometry on the BN-cylinder. Full tomograms with an effective voxel size of 2.75 µm were taken with a maximum speed of 20 tomograms per second. The dendrites were segmented and the interface properties were investigated with a focus on their growth shape [4]. This shape can be described by the local curvatures of the dendritic surface. An interface shape distribution map was used to visualize the results. This method allows to identify and compare differences in the morphology of dendrites with different crystallographic growth directions in aluminum-germanium alloys of different compositions.

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