Studies of Icosahedral Quasicrystals
JOHNS HOPKINS UNIV BALTIMORE MD DEPT OF MATERIALS SCIENCE AND ENGINEERING
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The five manuscripts reproduced in this progress report cover the research of the Principal Investigator, Dr. Dan Shechtman, and his collaborators under the DARPAONR sponsored research program on icosahedral quazicrystals formed by rapid solidification of alloys. Microscopic evidence for quasi- periodicity in an alloy with long-range icosahedral order is presented. Local 5- fold rotational axes are observed to be uniformly distributed down to the atomic scale. In a high resolution electron microscopy study on rapidly solidified aluminum manganese alloy ribbons the relevant characteristics of quasi- periodicity were demonstrated. The indexing of icosahedral quasiperiodic crystals is described including the step necessary to prove, by diffraction, that an object is quasiperiodic. Various coordinate systems are discussed and an explanation is given for choosing one aligned with a set of three orthogonal two-fold axes. Using this coordinate system the main crystallographic projections are presented and several analyzed single-crystal electron diffraction patterns are demonstrated. Experimental evidence for quasiperiodic crystals is provided. Neutron diffraction was used to investigate the icosahedral phase of aluminum-manganese alloys. The research results furnish additional strong documentation for the existence of the icosahedral quasicrystals. The work advances our knowledge and understanding of quasicrystals and their relationship to chemical composition and solidification conditions.
- Metallurgy and Metallography