Solidification in the system Al-Ge-Si: The phase diagram, coring patterns, eutectic growth, and modification
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INTRODUCTION
I N T E R E S T in the solidification of A1-Ge-Si alloys arose from previous studies of silicon modification by sodium, alkaline earth, and rare earth metals. E1,2,3iIt has been observed that morphological modification and more isotropic growth of the silicon are associated with the promotion of a high twin density in that phase: this could be explained by adsorption of the minor addition onto the faceted silicon surfaces and the promotion of twinning by adatoms having atomic radii within a certain size range. If the effect is peculiar to the diamond cubic structure, it might be expected to occur in other systems involving a phase with this structure, e.g., Ge in A1-Ge, Si in Ag-Si, and possibly III-V compounds growing from metallic solutions. Therefore, it seemed relevant to examine the solidification not only of A1-Ge alloys, in which some degree of modification has already been reported, t4] but also in ternary A1-Si-Ge alloys. As an introduction to this study, it was first necessary to confirm the expected form of the ternary phase diagram and to characterize the unmodified microstructures; in this connection, some interesting composition variations appeared which provided insight into the growth kinetics of the loosely coupled eutectic front by effectively "composition dating" the advance of each phase.
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EXPERIMENTAL
The impurity levels in the materials used were
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