Ion Induced Quasicrystalline Transformation in the Al-Cr-Er System
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ION INDUCED QUASICRYSTALLINE TRANSFORMATION IN THE Al-Cr-Er SYSTEM DAVID A. LILIENFELD Dept. of Materials Science, Cornell University, Ithaca, NY 14859
ABSTRACT The Aluminum-rich end of the Al-Cr-Er ternary system was investigated in an effort to find new quasicrystalline alloys. Composition, dose dependence and substrate temperatures during ion irradiation were varied. A quasicrystal alloy with the Al-Cr intensity pattern was found at low Er content. Another phase was observed that did not index to any known equilibrium structure. This new phase has d spacings and diffraction line intensities which are similar to those of the Al-Li-Cu quasicrystalline phase. INTRODUCTION Ion irradiation has been used to form quasicrystals in a number of binary alloy systems including the Al-Mn system[1,2], the Al-Cr system[3,4], the Al-Fe system(5] and the Al-Rh system[6]. Recently our group has reported work on the ternary systems Al-Mn-Cu, Al-Mn-Ni and Al-Mn-Zn[7]. Quasicrystal formation by ion irradiation has been reported in the the Al-Mn-Si[8] as well. The present work is an attempt to extend the ion irradiation formation of quasicrystals to new systems. The Al-Cr-Er system was chosen because the binary Al-Cr system forms quasicrystals for aluminumrich samples. Most of the Al based quasicrystals found to date have contained 80 at % or more Al. Exceptions to this occur when one of the alloy elements can occupy Al sites. Chrome and erbium were not expected to be able to occupy Al sites and thus quasicrystals might be expected to form in the above 80 at % Al composition range. Al20 Cr 2 Er which contains a large amount of inherent icoshaedral clustering is present in the Al-rich section of the ternary system[9]. The presence of such phases as has been suggested by Henley[10] and Rao[Ill] is an indicator that the system may form quasicrystals A summary of the results of this work is: Quasicrystals have been found in the AI-Cr-Er system. The presence of small amounts of Er reduces the quasicrystal transformation temperature for the AI-Cr system. For larger Er contents a new phase is observed that is not a known equilibrium structure, has quasicrystalline diffraction spacing and an approximate intensity of the Al-Li-Cu quasicrystal phase. This phase is metastable and shows 5-fold diffraction. It is tentatively identified as a quasicrystal. The composition region for the formation of this new phase is dose dependent.
EXPERIMENTAL All of the Al-Cr-Er samples were produced by depositing multilayer samples onto cleaved NaCI single crystals. The compositions of the samples was determined by the relative ratios of the individual layer thicknesses. All samples were produced in a ion pumped vacuum system with a single e-gun. The pressure in the system during deposition < 5xlO-Ttorr. Specimens for ion irradiation were made by cleaving the NaCl substrates, floating off the films in deionized water and picking them up onto Transmission Electron Microscope (TEM) grids. Composition of these samples was determined by Rutherford Backscattering
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