A General Formulation of Eutectic Silicon Morphology and Processing History

Abstract

Understanding the influence of rapid solidification conditions on the formed microstructure is important to determine how process conditions affect the final mechanical properties. In this work the rapid solidification of hypoeutectic and hypereutectic Al–xSi (x = 10, 18 wt%) alloys was achieved using Impulse Atomization (IA). The resultant microstructures were examined and from this analysis a wide range of eutectic Si morphologies and length scales were observed. These results were further analyzed using 2 mathematical approaches to help quantify the experienced eutectic growth kinetics (i.e. growth rate and cooling rate), along with the thermal history of the powders. In the case of the hypereutectic Al–Si alloy distinct microstructure zones were observed. These distinct zones were used to determine the nucleation undercooling using combined concepts of the coupled zone in Al–Si alloys and the hypercooling limit of a melt. Here, a region of fine eutectic, α-Al+Si, is observed and is believed to be the first solids that precipitated directly from the undercooled melt and grew during recalescence. Following recalescence, a coarser structure of Si particles and eutectic developed. Within this work, the solidification paths of both hypo and hypereutectic Al–Si alloys will be outlined and a generalized approach to relate the shape of the eutectic Si to processing conditions will be presented.