The Thermal Stability of a Single-Grain Mg-Zn-Y Icosahedral Quasicrystal
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The Thermal Stability of a Single-Grain Mg-Zn-Y Icosahedral Quasicrystal Z.P. Luo1, Y.L. Tang1,2, D.J. Miller1, M.J. Kramer2, I.R. Fisher2 and P.C. Canfield2 1 Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, U.S.A. 2 Ames Laboratory, Iowa State University, Ames, IA 50011, U.S.A. ABSTRACT The stability of the Mg-Zn-Y icosahedral quasicrystal (IQC) has been studied by long-term annealing of a single grain IQC in quartz tubes. Decomposition of the IQC was observed after annealing at high temperatures (T≥773 K) sealed in Ar. During the decomposition process, the quasilattice parameter aR was found to decrease, associated with a decrease in Mg content of the IQC phase as confirmed by quantitative x-ray energy dispersive spectroscopy analyses. In addition, a new cubic approximant has been found in the annealed samples. This cubic approximant has a face-centered cubic (fcc) structure with lattice parameter of a = 1.276 nm, which is about (1/τ) times smaller than that of the fcc W'-(MgZnY) with a = 2.05 nm reported previously (where τ is the golden ratio). INTRODUCTION It is currently believed that the Mg-Zn-RE (RE: rare earth) icosahedral quasicrystals (IQCs) are thermodynamically stable [1-3], since they are formed from the melt directly upon slow solidification. However, their thermal stability has not yet been well studied due to the presence of secondary crystalline phases in polycrystalline materials. Earlier work showed that the IQC is stable during continuous heating [4, 5], while at high temperatures oxidation reactions occurred [4]. Later, Tsai et al. [6] reported that the IQC completely decomposed to a crystalline phase of (Mg, Zn)5Y at high temperature (900 K), whereas it was found to be stable at 750 K. However, the melting points of their samples were lower than 873 K based on their differential thermal analysis (DTA) measurements, suggesting that decomposition could have been due to melting and leaving the issue of stability of the IQC phase unanswered. More recently, Abe and Tsai [7] proposed that the IQC is only stable at high temperature (873 K) as a single phase, but is metastable at low temperature due to decomposition to a hexagonal phase at 773 K. In this work we studied the stability of the Mg30Zn60Y10 IQC in order to resolve these discrepancies and improve our understanding of their stability. In order to avoid the effects of secondary crystalline phases, we used a single large grain as the parent material for long-term annealing. EXPERIMENT Single large grains of Mg30Zn60Y10 with volumes of up to 0.5 cm3 were prepared by the selfflux method from the ternary melt [8]. Single grains were crushed to yield small fragments, which were then sealed individually into quartz tubes for annealing studies to evaluate the stability of the IQC. Small pieces of Mg were placed in the tube to provide Mg vapor during annealing and the tube was filled with Ar prior to sealing. The melting onset of the single Mg30Zn60Y10 is around 873 K, as determined by DTA, consistent with the earlier work
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