Surface Alloys of Icosahedral AlMnSi with Phason Distortions
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SURFACE ALLOYS OF ICOSAHEDRAL AlMnSi WITH PHASON DISTORTIONS D. M. FOLLSTAEDT and J. A. KNAPP Sandia National Laboratories, Division 1112, Albuquerque,
NM 87185-5800
ABSTRACT The microstructures produced by electron-beam melting and by ion-beam The mixing Al/Mn and Al/Mn/Si layers on Si substrates are examined. treatments were found to incorporate Si from the substrate into the surface Several phases formed, depending on treatment, including a- and alloy. f-AlMnSi, p-AlMn (epitaxial on Si(lll)), and amorphous and icosahedral The observed microstructures relate the novel icosahedral phase to AlMnSi. Diffraction patterns other phases and elucidate its formation kinetics. from large icosahedral grains (up to 5 Am) show distortions in the position and shape of weak (but not strong) reflections, as predicted for phason defects in a quasicrystalline lattice, one of the structures proposed for icosahedral phases. INTRODUCTION two years [1] are The icosahedral phases discovered in the last receiving much attention because they exhibit long-range order and sharp, diffraction symmetry even though their icosahedral crystal-like diffraction, We have previously used ion is inconsistent with translational periodicity. beam mixing and melting of deposited layers to form alloys of icosahedral AlMn on Al and Fe substrates [2,3] and have thereby gained important insight into its formation kinetics and thermodynamic relationships to other phases. We have since used other substrates to obtain different temperature histories that produce larger grains for detailed examination and to provide Silicon was included because its melting point is above new information. that of the Al layers, and if up to 6 at.% Si dissolved into the alloy, it would stabilize icosahedral AlMn over the competing decagonal phase [4], which was prominent in our alloys on Fe substrates [3]. In this study, we formed large icosahedral grains by reacting Al/Mn and Al/Mn/Si layers on Si substrates, but the fraction and type of accompanying This variation is due to reaction between phases varied between treatments. the alloy and the Si substrate, and the incorporation of differing amounts The lack of control over the Si content limits our of Si into the alloy. ability to know which phase will form with a given treatment, and has led us Nonetheless, the microstructures observed on Si to use other substrates. provide important new information relating the icosahedral (I) phase to the Furthermore, formation kinetics. other phases and elucidates its to weak diffraction patterns from the large grains show distortions effects of phason defects in the which agree with predicted reflections quasicrystalline structure proposed for I phases [5]. Typically six sets of alternating layers were deposited on or Torr to produce total thicknesses Si wafers in a vacuum of - 5 x 10 of - 0.1 pm. The layers were then either melted with one of ty electron Xe/cm at beam treatments, or were alloyed by ion beam mixing with 1 x 10 400 keV. Most of the Xe passed through the layer, and this ine
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