Structural Relationships between QC and Meta-stable Crystalline Phases in Melt Spun Zr 80 Pt 20 Ribbons
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Structural Relationships between QC and Meta-stable Crystalline Phases in Melt Spun Zr80Pt20 Ribbons X. Y. Yang, M. J. Kramer, E. A. Rozhkova and D. J. Sordelet Materials and Engineering Physics Program, Ames Laboratory (USDOE), Iowa State University, Ames, Iowa 50011
ABSTRACT Two competing meta-stable crystalline phases, a bcc hyperstoichiometric β-Zr(Pt) (Im3m) superstructure and a non-stoichiometric fcc ε-Zr6Pt3O (Fd3m) phase, have been observed to coexist with a quasicrystalline (i) phase, respectively, in as-spun Zr80Pt20 ribbons with low oxygen content (184ppm mass O) and high oxygen content (4737 ppm mass O). Transmission electron microscopy (TEM) results show that the β-Zr(Pt) superstructure and i phase have a well defined orientational relationship, good crystallographic match and nearly identical stoichiometry. Icosahedral two-fold axes coincide with , , and axes in β-Zr(Pt); {110}β-Zr planes register with icosahedral {211111} fivefold or {221001} twofold planes. The observed orientational relationship and the space group (Im3m) preclude β-Zr(Pt) as a cubic approximant to the i phase. Both β-Zr(Pt) and the i phase are distorted; β-Zr(Pt) maintains a basic β-Zr Bravais lattice with an aperiodic superlattice. Morphologies suggest that the i phase forms first, followed by an easy nucleation of the β-Zr(Pt) on the surfaces of the i phase. Also, a similar orientational relationship and lattice match between ε-Zr6Pt3O and i phase has been revealed by TEM.
INTRODUCTION The formation of nano-icosahedral quasicrystals in rapidly solidified Zr80Pt20 alloys has been reported [1-4]. Also, several reports described the formation of equilibrium crystalline phases of α-Zr, Zr5Pt3 and ZrPt in these alloys at slow cooling rates [2, 3]. However, so far no meta-stable crystalline phase associated with icosahedral phase has been observed in these alloys. In our very recent studies of as-spun Zr80Pt20 ribbons containing different oxygen levels, two very competing crystalline phases have been observed to grow epitaxially with the icosahedral phase, i.e., a hyperstoichiometric bcc β-Zr(Pt) (Im3m) superstructure in the ribbons with very low oxygen content (184 ppm mass O), and a non-stoichiometric fcc ε-Zr6Pt3O (Fd3m) phase in the ribbons with high oxygen content (4737ppm mass O) [4, 5]. In the present paper, we mainly focus on a detailed crystallographic relationship between a bcc β-Zr(Pt) (Im3m) superstructure and an icosahedral phase (i phase) by TEM, as well as that between fcc ε-Zr6Pt3O (Fd3m) and i phases. To the best of our knowledge, this is the first report on a crystallographic relationship between cubic crystals/QC phase in Zr-Pt system during rapid solidification.
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EXPERIMENTAL DETAILS Zr80Pt20 melt-spun ribbons were obtained using a graphite crucible and a wheel speed of 25m/s. Details on preparation of the ribbons and oxygen measurements were given elsewhere [4]. TEM was performed using a Philips CM30 equipped with energy-dispersive X-ray spectrometer (EDS) to characterize the microstructure and
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