Modulated Structures, Nanocrystals and Quasicrystals in Mg-Zn-Y Alloys
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Modulated Structures, Nanocrystals and Quasicrystals in Mg-Zn-Y Alloys Anandh Subramaniam and S. Ranganathan Department of Metallurgy, Indian Institute of Science, Bangalore- 560012, India. FAX: (0091)-(80)-3609472 email: [email protected] ABSTRACT The formation of the icosahedral phase, BCC (Body centred cubic) phase and nanocrystals are seen in the as-cast alloy with nominal composition of Mg4Zn94Y2. FCC (face centred cubic) phase and modulated structures are formed in the alloys with higher Y content (10% and 25% Y respectively). These phases are analysed keeping in view their relation to the quasicrystals of the Mg-Zn-Y system. INTRODUCTION Pioneering work in the elucidation of the phase diagram of Mg-Zn-Y system was done by Padezhnova et al [1], wherein they reported three ternary phases. The startling realisation by Luo et al [2] that the equilibrium Mg3Zn6Y phase is actually an Face Centred Icosahedral (FCI) quasicrystal has opened new vistas. Tsai et al [3] reported a list of rare earth elements, which form the FCI phase with a highly ordered structure. Subsequently the decagonal phase was also reported to occur in this system [4]. Hexagonal phases and their structural relation to the quasicrystals of this system have been studied [5,6]. The structure of the decagonal phase can be explained in terms of a covering of tiles [7]. Superlattices to one of the hexagonal phases were also reported [8]. It was seen that the icosahedral phase can reversibly transform to the hexagonal crystal [9]. These discoveries brought forth the important realisation that the quasicrystals in this system do not have large clusters [10]. Crystalline phases in this system have received considerable attention as well. Luo et al [11] had shown the existence of an 18R modulated structure in this system. Further Luo et al [12] had identified Mg7Zn3 as an 1/1 approximant to the icosahedral phase. Recently Luo and Hashimoto [13] have found W’ as an ordered structure to be an approximant phase and Sterzel et al [14] have reported a cubic approximant in the Zn-Mg-Er system. The recent phase diagram work by Tsai et al [15] has updated our understanding of the Mg-Zn-Y system. It is important to note that the hexagonal quasicrystal is conspicuous by its absence. Currently, Mg3Zn6Y (Icosahedral quasicrystal, Fm53, aR = 5.19Å), Mg38Zn60Y2 (Decagonal quasicrystal, P105/mmc, a = 2.27Å, c = 2.55Å), Mg24Zn65Y10 (S/Z, P63/mmc, a = 14.58Å, c = 8.68Å), Mg2Zn3Y3 (W, Fm3m, a = 6.83Å; W', Fm3m a= 20.5 Å), Mg15Zn70Y15 (H, a = 7.76Å, c = 9.2Å) and Mg12ZnY (X, FCC modulated structure, a = 4.5Å) phases are documented. In the present work, using TEM (Transmission Electron Microscope) as the main tool, the formation of nanocrystals, quasicrystals, modulated structures and crystals is shown in the as-cast alloys of Mg-Zn-Y system.
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EXPERIMENTAL DETAILS Mg (99.9%), Zn (99.9%) and Y(99.9%) were used for sample preparation and the compositions chosen were melted in an induction furnace back-filled with Ar, in highalumina crucibles. JEOL compute
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