Structure and Defects of Quasicrystals

9732567 widom该基金支持准晶合金性能的理论研究。含过渡金属的富铝合金形成复杂的晶体结构和准晶体。这些合金表现出不同寻常的机械性能，如摩擦系数低，低温下硬度大，高温下延展性好。准晶体已经增强了外科手术用合金的强度，并可能最终涂覆在易受摩擦磨损的传统金属表面，如油炸零件和汽车发动机部件。在这里进行的研究将试图了解原子相互作用，原子结构和由此产生的大块准晶体的力学性能，它们的缺陷和表面。基于电子结构的金属间合金对和多体相互作用势为复杂晶体和非周期结构提供了可靠的总能量计算。本研究对Al-Mn、Al-Cu-Co、Al-Ni-Co和Al-Cu-Fe等合金族进行了评价和应用。随着相互作用的掌握，我们将评估复杂晶体结构和十面体和二十面体准晶体的力学和热力学稳定性。我们将改进目前对原子位置的理解，并了解特定结构的能量偏好。为了理解这些相的平衡热力学，我们将评估低能量激发，如空位、化学无序和原子振动。准晶材料具有附加类型的结构激发，称为相位波动。就原子位移而言，相子涨落的确切含义目前还不是很清楚。模拟热力学性质的一种策略包括推导平铺哈密顿量。这些是计算大型非周期结构自由能的近似方法，基于对小单元胞结构的能量和熵的仔细评估。利用平铺哈密顿量，我们可以研究相弹性和固态相变。前沿的理论和实验研究解决了准晶体的力学特性，包括缺陷的作用。本研究考察了晶界和孪晶界位错和能量学的结构和远程应变场。准晶体与真空或其他金属之间的界面结构变化需要引起注意。除了这些纯粹的结构问题之外，上面讨论的低能激发还会影响原子扩散和位错运动等动力学性质。这项拨款研究这些缺陷。从简化的平铺和Lennard-Jones原子模型开始，我们最终将平铺汉密尔顿和原子结构建模联系起来。这笔经费用于准晶合金性能的理论研究。含过渡金属的富铝合金形成复杂的晶体结构和准晶体。这些合金表现出不同寻常的机械性能，如摩擦系数低，低温下硬度大，高温下延展性好。准晶体已经增强了外科手术用合金的强度，并可能最终涂覆在易受摩擦磨损的传统金属表面，如油炸零件和汽车发动机部件。在这里进行的研究将试图了解原子相互作用，原子结构和由此产生的大块准晶体的力学性能，它们的缺陷和表面。＊＊＊

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. ***