Asymmetrical contribution of eutectic growth kinetics on the coupled growth behaviour in rapid eutectic solidification

Abstract

The interface undercooling in classical eutectic growth theories consists of solute and curvature undercooling with different expression formats. However, a significant difference in interface kinetic coefficients arises for a disordered non-facetted solid solution and an ordered facetted intermetallic compound in that different growth kinetics govern the attachment kinetics at the solid–liquid interface, which correspond to a typical eutectic reaction with a solid solution and an intermetallic compound as its terminal eutectic phases. Following the pioneering work of Jackson and Hunt (Trans. Metall. Soc. AIME 236 1129 (1966) [1]), the kinetic undercooling is supplemented to interface undercooling and two eutectic phases are considered separately so as to diagnose the effect of the asymmetrical contribution of kinetic undercooling on the coupled eutectic growth behaviour. Further analysis indicates that it is the asymmetrical contribution of kinetic undercoolings of the facetted and non-facetted phases that enables the coupled eutectic composition shift to the facetted phase side so as to weaken the solute undercooling of the facetted phase and balance the kinetic contribution in the rapid solidification of coupled eutectics.

Acknowledgements

Partial work was completed when one of the authors (M. Li) held the Foreign Research Fellowship from the Japan Aerospace Exploration Agency (formerly the Institute of Space and Astronautical Science, which was renamed the Japan Aerospace Exploration Agency after it was merged with the National Space Development Agency of Japan and National Aerospace Laboratory of Japan). The authors are quite grateful to Prof. W. Kurz in Switzerland for valuable suggestions. Thanks are also due to Dr. K. Nagashio for stimulating discussions.