Structure and Defects of Quasicrystals

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9732567WidomThis grant supports theoretical research into the properties of quasicrystalline alloys. Aluminum-rich alloys containing transition metals form intricate crystal structures and quasicrystals. These alloys display unusual mechanical properties such as low coefficients of friction, great hardness at low temperatures, and ductility at high temperatures. Quasicrystals already strengthen surgical alloys, and may eventually coat surfaces of conventional metals subject to wear by friction, such as frying pains and automobile engine parts. The research conducted here will attempt to understand the atomic interactions, atomic structure and resulting mechanical properties of bulk quasicrystals, their defects and surfaces. Electronic structure based pair and many-body interaction potentials for intermetallic alloys allow reliable total energy calculations for complex crystalline and aperiodic structures. This research evaluates and applies these interactions for such alloy families as Al-Mn, Al-Cu-Co, Al-Ni-Co and Al-Cu-Fe. With the interactions in hand, we will evaluate mechanical and thermodynamic stability for complex crystalline structure and decagonal and icosahedral quasicrystals. We will improve current understanding of the atomic positions, and understand the energetic preference for particular structures.To understand the equilibrium thermodynamics of these phases we will evaluate low energy excitations such as vacancies, chemical disorder and atomic vibrations. Quasicrystalline materials possess additional types of structural excitations known as phason fluctuations. The precise meaning of a phason fluctuation in terms of atomic displacements is presently not well known. One stategy for modeling thermodynamic properties involves deriving tiling Hamiltonians. These are approximations for calculating the free energy of large aperiodic structures based on careful evaluation of energy and entropy for small unit cell structures. With the tiling Hamiltonian we can study phason eleasticity and solid state phase transformations.Forefront theoretical and experimental research addresses mechanical properties of quasicrystals, including the role of defects. This research examines structures and long range strain fields at dislocations and energetics of grain boundaries and twin boundaries. Structural modifications at interfaces between quasicrystals and vacuum or other metals require attention. Beyond these purely structural questions, the low energy excitations discussed above influence dynamical properties such as atomic diffusion and dislocation motion. This grant studies such defects. Starting with simplified tiling and Lennard-Jones atomistic models, we eventually link with the tiling Hamiltonians and atomic structure modeling.%%%This grant supports theoretical research into the properties of quasicrystalline alloys. Aluminum-rich alloys containing transition metals form intricate crystal structures and quasicrystals. These alloys display unusual mechanical properties such as low coefficients of friction, great hardness at low temperatures, and ductility at high temperatures. Quasicrystals already strengthen surgical alloys, and may eventually coat surfaces of conventional metals subject to wear by friction, such as frying pains and automobile engine parts. The research conducted here will attempt to understand the atomic interactions, atomic structure and resulting mechanical properties of bulk quasicrystals, their defects and surfaces. ***