Stable Icosahedral Quasicrystals in the Cd-Based Alloys
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Stable Icosahedral Quasicrystals in the Cd-Based Alloys J. Q. Guo1, E. Abe and A. P. Tsai National Research Institute for Metals, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan 1 CREST, Japan Science and Technology Corporation, Kawaguchi, Saitama 332-0012, Japan ABSTRACT Stable icosahedral quasicrystals have been found in ternary Cd65Mg20RE15 (RE=Y, Gd, Tb, Dy, Ho, Er, Tm, Lu, Yb and Ca) and binary Cd85Ca15 and Cd84Yb16 alloys. These icosahedral quasicrystals have a primitive quasilattice. Their quasilattice parameters range from 0.5571 to 0.5731 nm. Formation mechanism of the quasicrystals is discussed. INTRODUCTION Since the discovery of quasicrystal in 1984, a limited number of stable quasicrystals have been found in Al-, Zn- and Ti-based alloys [1-9]. To reveal electronic structure and atomic structure of quasicrystal, much more stable quasicrystals are required. Recently, Guo et al. [10-13] substituted Cd for Zn in Zn-Mg-RE (RE=rare earth element) quasicrystals, finding a series of ternary Cd-Mg-RE quasicrystals. In the detailed study on the Cd-Mg-RE quasicrystals, the binary Cd-Yb and Cd-Ca quasicrystals were further discovered. In this paper, we report the results of these quasicrystals. EXPERIMENTAL DETAILS The stable quasicrystal can be considered as a Hume-Rothery electron compound. All the stable icosahedral phases have common valence concentration (e/a). For example, the e/a is 1.75 for the Al-transition metal group and 2.1- 2.2 for the Al-Zn-Mg group. Cd-Mg-RE quasicrystals belong to the Al-Zn-Mg group. Therefore, their e/a should be around 2.15. As the valences of Cd and Mg are +2, and those of most of the rare earth elements are +3, content of rare earth elements in the quasicrystals should be around 15 at%, so that valence concentration of the alloys (e/a) can be 2.15. According to the previous experimental results, the composition of alloys was determined to be Cd65Mg20RE15. These alloys were prepared from pure elements of Cd (99.99wt%), Mg (99.99wt%), RE=Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu (99.9wt%) and Ca (99.5wt%) using an induction furnace in Al2O3 crucibles sealed in quartz tubes under Ar atmosphere, followed by annealing at 673K K2.7.1
for 100 h in an Ar atmosphere. Phase identification of the samples was performed by X-ray diffraction with Cu-K UDGLDWLRQ Structure characterization of the quasicrystals was carried out by transmission electron microscopy (TEM). The specimens for TEM observations were powdered in alcohol and then transferred on to holed film. RESULTS AND DISCUSSIN
The icosahedral phase was found to form in as-cast Cd65Mg20Yb15 alloy and annealed Cd65Mg20RE15 alloys with RE=Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, kept at 673K for 100 h. Figure 1(a) shows powder X-ray diffraction patterns of the as-cast Cd65Mg20Yb15 alloy. All peaks of the alloy can be assigned as an icosahedral quasicrystal according to the Elser’s indexing method [14]. As shown in table 1, quasilattice parameters estimated from the (211111) peak for all the alloys range f
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