Studies of RE5(SixGe1-x)4 alloys, where RE equals rare earth, have revealed a second-phase having a thin-plate morphology in essentially every alloy examined, independent of exact composition and matrix crystal structure. Identified as having a composition approximating Gd5(SixGe1-x)3 and a hexagonal crystal structure in the Gd-based system, it has been suggested that the observed thin-plate second phases seen in this family of rare earth alloys are all most likely of the form RE5(SixGe1-x)3. A number of interesting observations suggest that the formation of these second-phase plates is somewhat unusual. The purpose of this article is to investigate the stability of this second phase in Gd- and Er-based compounds. The stability was investigated as a function of thermal cycling and large-scale composition fluctuations. The results of scanning and transmission electron microscopy (SEM, TEM) studies indicate that the RE5(SixGe1-x)3 phase is extremely stable once it forms in a RE5(SixGe1-x)4 matrix.
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