Dendrite Growth Morphologies in Aluminium Alloys

摘要

Aluminium is known to have a weak anisotropy of the solid-liquid interfacial energy. As a result, it can exhibit different dendrite morphologies, depending on the alloy composition and solidification conditions. Besides regular <100> dendrites typical of fcc and bcc metallic alloys, twinned dendrites, also called "feathery grains", can form under high thermal gradient and solidification rate in several alloys (Al-Mg, Al-Cu, etc.). With the help of automated Electron Back-Scattered Diffraction (EBSD), it has been shown that these dendrites have grown along <110> directions. This growth morphology favours the rapid extension of (111) twins among [101] trunks and probably involves the formation of split dendrite tips or doublons. Under directional solidification or Bridgman conditions, <110> non-twinned dendrites, mixed <110>/<100> dendrites and even <211> curved dendrites have been produced. In the case of Al-Zn alloys, EBSD measurements have indicated that 20 fan-thick liquid films deposited by hot dipping on steel sheets solidify with a <320> dendrite growth morphology, regardless of the interactions with the surfaces of the film. The same growth directions have been identified in bulk specimens. Within a given grain, subgrain misorientations as large as 30 deg. have been measured. These misorientations have been interpreted in terms of the very specific microsegregation pattern associated with this growth morphology, although differential thermal contraction between the solid film and the substrate upon cooling cannot be entirely eliminated. This contribution presents the various dendrite microstructures observed in aluminium alloys and outlines some of the underlying mechanisms.