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Licensed Unlicensed Requires Authentication Published by De Gruyter December 7, 2021

Microstructure and modification mechanisms of Si phase in as-thixoformed alloy A356

Mikrostruktur und Modifikationsmechanismen von Si-Phasen in einer thixogeformten Legierung A356
K. Y. Wen and G. Gottstein

Abstract

Heavily faulted Si flakes, which can be considered as crystallization products of eutectic droplets embedded in an Al matrix, were for the first time observed in an as-thixoformed A356 alloy, which contains 0.3 wt.% Sr. Large isolated particles as well as clusters of fine Si particles were observed. Evidence is provided that the faults on {111} planes of Si are not twins. Sr and Na are known to modify the morphology of the Si phase from flake to fiber morphology. Contrary to common perception, it is shown that the twin plane re-entrant edge mechanism does not operate during fiber growth. It is proposed that the modification process occurs during the nucleation of the Si phase by adsorption of modifying elements on twinned nuclei of Si. They prevent nucleus growth into stable twinned crystals, which is a prerequisite for the growth of Si flakes.

Abstract

In der thixogeformten Legierung A356 wurden erstmalig Si-Erstarrungsphasen untersucht. Sie haben gewöhnlich eine flockenförmige Morphologie und sehr hohe Defektdichten und kommen als größere Einzelteilchen oder fein verteilte Dispersion vor. Durch die Beimengung von Sr oder Na wird die Morphologie von flockenförmig zu faserförmig verändert, was Auswirkungen auf die mechanischen Eigenschaften hat. Die Untersuchungen zeigen, dass die Modifikation der Morphologie, entgegen geläufiger Meinung, nicht auf das Zwillingswachstum, sondern auf die Keimbildung der Si-Phase zurückzüführen ist. Den modifizierenden Elementen kommt dabei die Rolle der Wachstumshemmung in der Keimbildungsphase von Zwillingskristallen zu.


Dedicated to Professor Dr. Haël Mughrabi on the occasion of his 65th birthday

The authors like to thank Prof. J. Mayer of the Gemeinschaftslabor für Elektronenmikroskopie of RWTH Aachen for critically reading the manuscript and for valuable comments. We are grateful to the Collaborative Research Center 289 of the Deutsche Forschungsgemeinschaft “Thixoforming” for providing the alloy.

Prof. Dr. G. Gottstein Institut für Metallkunde und Metallphysik RWTH Aachen, D-52056 Aachen Tel.: +49 241 80 268 60 Fax: +49 241 80 226 08

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Received: 2002-02-18
Published Online: 2021-12-07
Published in Print: 2021-12-07

© 2002 Carl Hanser Verlag, München