Abstract
The combined effect of strontium and bismuth on the flake-to-fibrous transition associated with eutectic silicon in near-eutectic Al-11.7Si-1.8Cu-0.8Zn-0.6Fe-0.3Mg alloys, in both normal and interrupted solidification, on the macro, micro and nano-scale has been investigated. The results revealed that an interaction occurred between Sr and Bi at a certain threshold weight ratio of Sr/Bi, whereby the fine fibrous Si changed into coarse flake-like morphology. This interaction led to a dramatic change of both the nucleation and growth modes of eutectic Si. Interrupted quenching and electron back-scatter diffraction observations showed that fibrous Si nucleates independently between aluminium dendrites whereas flake Si nucleates epitaxially adjacent to the dendrites. TEM observations showed a decrease of twin density and increase of twin spacing associated with the fibrous-to-flake transition. Moreover, eutectic coherency point revealed a shorter time for the growth of eutectic grain in fibrous silicon. Formation of the pre-eutectic ternary compound of Bi2Mg2Sr is proposed to be responsible for the deactivation of Sr and consequently fibrous-to-flake transition of silicon.
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