Formation of Icosahedral Al(Mn) by Pulsed Surface Melting
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FORMATION OF ICOSAHEDRAL Al(Mn) BY PULSED SURFACE MELTING D. M. FOLLSTAEDT AND J. A. KNAPP Sandia National Laboratories, Albuquerque, NM 87185 ABSTRACT Surface melting of alternating Al/Mn layers on Al and Fe substrates by pulsed electron beams and laser is found by TEM to produce icosahedral Al(Mn). Interpreting the observed microstructures in terms of the calculated temperature histories places limits on the melting point of the phase (660 0 C < T < 9600C), the time needed for its nucleation from the melt at 6600C (t n < 50 ns), and the rate of quenching from thg liquid needed to suppress formation of the competing T phase (-T > - 1 x 10 K/s). Melt spinning of AI(Mn) alloys containing 14-20 at.% Mn was found last year to produce a phase which exhibits long-range order with icosahedral orientational symmetry [1,2]. This observation attracted immediate interest because this symmetry is inconsistent with lattice translational invariance. The structure of the "icosahedral phase" is still in question [1-3], but many investigators believe that it may be a new type of atomic arrangement called a "quasicrystal". The unique symmetry of this phase makes its formation and properties very much of interest. We have applied directed energy processes to Al(Mn) surface alloys to study the thermodynamics and formation kinetics of the icosahedral phase [4]. We have demonstrated that the phase can be formed by surface melting techniques; this result is important because the temperature history of alloys quenched with this method can be accurately calculated. The microstructures observed can then be interpreted to place quantitative limits on the melting point of the icosahedral phase and on the kinetic parameters leading to its formation. Our observations on Al(Mn) alloys quenched on both Al and Fe substrates indicate that the melting point of the icosahedral phase exceeds 6600C. The surface alloys were formed by vapor depositing Al/Mn lqyers on electropolished substrates of Al or Fe in a vacuum of - 5 x 10 1 Torr. Typically 16.4 nm of Al and 2.3 nm of Mn were alternately deposited at - 0.1 nm/s to produce alloy layers 100-150 nm thick with average concentrations of 16 ± 1 at.% Mn. Nuclear 1 peacti n analysis shyged that such deposited layers contain at most 1.7 x 10'u O/cm and 3.5 x 10 C/cm-. To insure that the layers adhered to the 1 ubstratg, the samples were "ionbeam stitched" by irradiating with 2 x 10 Xe/cm at 400 keV. Nomarski optical microscopy of the surfaces also showed features exhibiting flow, which indicates that melting occurred. Samples for transmission electron microscopy (TEM) examination were prepared by jet electropolishing from the substrate side of the sample with a nitric acid solution. Large, freestanding areas of the Al(Mn) surface alloy were exposed and could be examined without interference from the substrate. Our line source electron-beam annealing (LEBA) treatment focuses a sheet beam of electrons tQ a line 1 mm x 20 mm, which is incident upon a sample being swept perpendicular to the line. For sweep spe
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