Formation of a Quasicrystalline Phase from the Glassy State in Pd-U-Si
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FORMATION OF A QUASICRYSTALLINE PHASE FROM THE GLASSY STATE IN Pd-U-Si
f. J. DREHMAN, S. J. POON, AND K. R. LAWLESS** JAmes Laboratory-USDOE, Iowa State University, Ames, University of Virginia, Charlottesville, 22901
IA 50011
ABSTRACT Metallic glasses of composition Pd 8-U2 0. S can be transformed to a metastable single phase with quasicrystall ne order by thermal annealing. The icosahedral symmetry of this phase was verified by both electron and x-ray diffraction. Although the metallic glass can be formed over a broad compositional range the homogeneity range of this icosahedral phase is less than 1%. This indicates strong chemical ordering in the quasicrystalline state. INTRODUCTION The discovery by Shectman et al. [11 of an Al-Mn phase exhibiting icosahedral point group symmetry (m0) and the initial work of Levine and Steinhardt [21 has led to theoretical studies of the quasicrystalline structure [4,5] and its stability [61. Efforts have also been made to determine the atomic sites [4,7,8]. Even lacking a complete structural model, the observed diffraction patterns can be adequately explained [2,5,91. Molecular dynamics simulations of undercooled metallic liquids indicate the existence of icosahedral bond-orientational order [101. This leads to the question of how quasicrystalline phases and metallic glasses are related. Previously we reported [11] the formation of a quasicrystalline phase during the devitrification of Pd 6 0 U20 Si 0 metallic glass. Here we report the formation of what appears to be a truly single phase quasicrystalline material. EXPERIMENTAL METHODS Pd-U-Si alloys were prepared by arc-melting the components under an argon atmosphere. Metallic glass ribbons were produced by melt spinning onto a copper wheel under 0.2 atm of helium gas. Ribbons of 15-20 Im thickness were obtained when the wheel surface speed was 50-75 m/s. The ribbons were checked for crystallinity by x-ray diffraction. They were then annealed in evacuated silica tubes which also contained zirconium turnings to act as a getter. The 0 tube section containing the zirconium was preheated to about 700 C. Annealed ribbons were thinned for transmission electron microscopy using an ion mill with a liquid nitrogen temperature cold stage. Specimens were initially inclined at 150 to the two argon ion beams with the final thinning taking place at about 100. The operating voltage was 3.5kV. After thinning the very brittle specimens were mounted between folding-grids and secured in a double-tilt stage of the electron microscope. The specimens were examined in the TEM at an accelerated voltage of 120kV. Because of the small grain size, convergent beam electron diffraction was used for all diffraction patterns with a spot size of 40 or 100 nm. A small condenser aperture was used to obtain good angular resolution. FORMATION OF THE QUASICRYSTALLINE PHASE Fully glassy (Pd 3 U) 1_xxSix ribbons were formed over the range 16% to 23% silicon. A smaller range of glass formation, 18% to 22% uranium, was found for Pd 8 0 _yUySi 20 . Glassy ribbons n
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