Structural Relationships between QC and Meta-stable Crystalline Phases in Melt Spun Zr80Pt20 Ribbons

Abstract

ABSTRACTTwo 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 <110>, <111>, <112> and <113> 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.