Persistence of 5:3 plates in RE 5 (Si x Ge 1-x ) 4 alloys
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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. I. INTRODUCTION 1–6
The unusual properties of RE5(SixGe1-x)4 alloys, where RE equals rare earth, have caused alloys of this type to be the focus of numerous studies in recent years.7 Such studies have included examination of alloys whose compositions and crystal structures ranged from stoichiometric RE5Si4 to RE5Ge4. In the course of these studies a second phase, having a thin-plate morphology, has been seen in essentially every alloy examined, independent of exact composition and matrix crystal structure. To date this includes RE5(SixGe1-x)4 samples (hereafter referred to as “5:4” alloys) based on Gd, Er, Dy, and Tb.8–11 In studies of Gd5Si2Ge2 and Er5Si4 that used energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM), this phase was identified as having a composition approximating RE5(SixGe1-x)3 and a hexagonal crystal structure.9,12 These results led the authors to suggest that the observed thin-plate second phases seen in the 5:4 family of rare earth alloys are all most likely of the form RE5(SixGe1-x)3 (hereafter referred to as “5:3”), based on the observed morphology and a consideration of the similarities between the phase diagrams of the studied systems. Subsequent studies of single crystal Gd5Ge4 identified the orientation and formation mechanism for the 5:3 phase in this particular system. In this study it became clear that the formation of the 5:3 occurred as a result of a rapid solid–solid transformation that the authors proposed as being “displacivediffusional” in nature.13 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0326 J. Mater. Res., Vol. 21, No. 10, Oct 2006
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A number of interesting observations suggest that the formation of the 5:3 plates is somewhat unusual. The presence of 5:3 thin-plates in as-solidified samples using a variety of techniques including arc-melting,8,9 induction melting,10 and single crystal growth11–13 indicates that formation is extremely rapid. The fact
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