Eutectic solidification in hypoeutectic Al-Si alloys: electron backscatterdiffraction analysis

Nucleation and growth of the eutectic, in hypoeutectic Al-Si foundry alloys has been investigated by the electron backscatter diffraction (EBSD) mappi ng technique using a scanning electron microscope (SEM). Sample preparation procedures for optimizing mapping have been developed. To obtain a suffici ently smooth surface from a cast Al-Si eutectic microstructure for EBSD map ping, an appropriate preparation technique by ion milling was developed and applied instead of conventional electropolishing. By comparing the orienta tion of the aluminum in the eutectic to that of the surrounding primary alu minum dendrites, the growth mechanism of the eutectic can be determined. Tw o different results were found, in isolation or sometimes together, but dis tinct for different strontium contents: (1) crystallographic orientations o f aluminum in eutectic and surrounding primary dendrites are identical, and (2) wide variation in orientations of the aluminum in the eutectic. (C) 20 01 Elsevier Science Inc. All rights reserved.