Ion Irradiation Induced Metastable Phase Formation in the Al-Zr System
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ION IRRADIATION INDUCED METASTABLE PHASE FORMATION IN THE AL-ZR SYSTEM DAVID A. LILIENFELD and J. W. MAYER Dept. of MaterialsScience, Cornell University, Ithaca, NY 14853
ABSTRACT The Al-Zr system was investigated in an effort to form quasicrystals. While no quasicrystals were found, a metastable phase previously formed only by annealing of a metastable solid solution was observed. This phase was formed by thermal annealing or by elevated temperature ion irradiations of thin film specimens. This work is the first observation of ion irradiation induced formation of this metastable phase. INTRODUCTION Quasicrystalline materials exhibit long range order and non-allowed crystalline symmetries, five-fold or six-fold symmetry. The formation of such material has been the subject of intense theoretical and experimental research[I]. A primary experimental concern is the determination of new systems which exhibit quasicrystalline behavior. Several criteria for the formation of quasicrystalline phases have been advanced[2,3]. Testing these criteria leads to better understanding of quasicrystal formation and more reliable formation criteria. One of the primary criteria in the search for quasicrystal forming systems[2] is the similarity of the phase diagram to that of previously discovered quasicrystalline systems. All known binary quasicrystal systems have a descending series of peritectic reactions and complex phases in the quasicrystalline forming composition region. Additionally atomic volume ratios between elements for the first known systems were approximately the same, for instance the atomic volume of the transition element to the atomic volume of Al was s 0.73. A system which has a similar phase diagram to the Al-Fe system, a known quasicrystal forming system, is the Al-Zr system. The most Al-rich phase, however, is not nearly as complex in the Al-Zr system as the Al-Fe system. The crystal structures present are t116 for the AI3 Zr phase and mCl06 for the AI 3Fe phase. Thus the Al-Zr system allows one to test the requirement of a complex phase. From an atomic volume point of view, the Al-Zr system is also of interest because the ratio of Al atomic volume to that of Zr is 0.73 and thus also the Zr-rich end of the phase diagram might be expected to produce quasicrystals. If the formation mechanism of quasicrystals is similar to that of the Al-Fe system, then quasicrystals will form in the Al-rich end, but if atomic size is of primary importance then the quasicrystals will form in the Zr-rich end. Our investigations in the Al-Zr system had the following results: None of the samples ever transformed to a quasicrystalline state. The Zr-rich samples were crystalline after deposition and could only be amorphized at 77K. Coevaporated samples at the Al-rich end were amorphous. Elevated temperature ion irradiations of the Al-rich samples, while not producing quasicrystals, did produce a metastable phase that has been observed previously only by precipitation from metastable solid solutions.
EXPERIMENTAL Three types of Al-Zr sa
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