Melting Point Measurements for Quasicrystalline Phases
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MELTING POINT MEASUREMENTS FOR QUASICRYSTALLINE PHASES J. A. KNAPP AND D. M. FOLLSTAEDT Sandia National Laboratories Albuquerque, NM 87185
ABSTRACT Melting transitions of metastable quasicrystalline phases of Al-Mn have been observed using rapid electron-beam heating of fine-grained icosahedral The congruent melting-point for icosahedral A1 8 0 Mn9 0 was surface layers. Heating to higher temperatures shows directly measured to be 910±20°C. another transition which is inferred to correspond to the liquidus of the The microstructure and formadecagonal phase at 965±20°C for 20 at.% Mn. tion kinetics of the decagonal phase are discussed, and its electron diffraction is described.
INTRODUCTION The discovery of rapidly quenched metallic phases which exhibit sharp but with disallowed symmetries such as diffraction like a crystal, icosahedral or decagonal, has prompted considerable research over the last two years.[l,2] These "quasicrystals" are under intense investigation, both and theoretically, in an effort to understand their experimentally structures and properties.[3,4] The best studied alloy system is Al-Mn, for which both an icosahedral and a decagonal phase can be formed under different quench conditions. Like most quasicrystals, these phases are metastable and transform to stable crystalline phases at 350-400°C.[1,5] Thus measurements of the high-temperature thermodynamic properties of these novel materials have not been made, even though such measurements are needed for a complete understanding of them. We have been using directed energy processes such as laser and electron beam heating and ion beam mixing to form surface layers of quasicrystalline phases on a variety of substrates.[5-8] We have recently used these techniques to measure directly melting temperatures for icosahedral Al-Mn and to deduce the liquidus for decagonal Al-Mn.[9] These measurements are possible because we heat and cool the alloys within -400 ps, and have control and accurate knowledge of their temperature histories. Here we will describe the technique, and present new information about the decagonal phase of Al-Mn obtained by this experimental approach. MELTING TEMPERATURE DETERMINATIONS The experimental procedure involves first using ion beam mixing to form a fine-grained surface layer of the icosahedral phase (I-phase) of Al-Mn[58], and then greating the layer with an electron beam[1O] to heat it rapidly (up to 5x10 K/s) to a known temperature. Solid-state transformations do not occur before the alloy reaches its melting point. Examination by either optical or electron microscopy shows abrupt transformations in microstructure occuring at specific peak temperatures which can clearly be associated with melting and resolidification. The starting material for the example shown in Fig. 1 was prepared by depositing 5 pairs of alternating layers of Al and Mn on a sapphire substrate. Thicknesses of -7.5 nm (Al) and 1.4 nm (Mn) gave a composition after mixing of 19.8±0.4 at.% Mn. A sapphire substrate was used for these studies because of its
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