Landau theory of crystallization and self-assembly of octagonal quasicrystals

The theory of crystallization of quasicrystal structures that does not use the concept of multidimensional crystallography for describing the quasicrystal order has been proposed. It has been shown using the structure of the MnSiAl octagonal quasicrystal as an example that the coordinates of the sites in the corresponding quasicrystal lattice can be calculated by conditional minimization of the Landau free energy. The abandonment of the unconditional minimization of the free energy has been justified by special features of the local atomic order in the considered structure. The proposed theory gives a new physical meaning to the traditional concepts of multidimensional crystallography and can also be used for explaining the formation of quasicrystal structures with other quasicrystal lattices.     

An Attempt to Prepare Metallic Glasses from Quasicrystals

Icosahedrons in supercooled liquids and glasses are considered to be of significance for the glass formation in alloy systems.Starting from the similarity of the local structure of quasicrystals to the icosahedrons in metallic glasses,a scheme is put forward to prepare metallic glasses based on a well-known quasicrystal Zr_(40)Ti_(40)Ni_(20).A series of(Zr_(40)Ti_(40)Ni_(20))_(100-x)Co_x metallic glasses are fabricated,and the optimized glass forming composition is determined at(Zr_(40)Ti_(40)Ni_(20))_(92)Co_8.The results show that the glass-forming ability of the alloys is closely related to the quasicrystalline phases.The mechanism of the enhanced glass-forming ability is discussed.     

Simulation study for atomic size and alloying effects during forming processes of amorphous alloys

1 Introduction Since amorphous alloys were successfully prepared by Duwez et al.[1] in the 1960s by rapid cooling from the eutectic alloy of Ag-Cu system, the investigation about atomic size and alloying effects during the formation process of amorphous alloys has become such an important area that many people paid a good deal attention. From the experiments that AgxCu1-x alloys formed amorphous structures, Takayama[2] has determined the content range of amorphous alloys, being x = 35—65, …     

How do quasicrystals grow?

Using molecular simulations, we show that the aperiodic growth of quasicrystals is controlled by the ability of the growing quasicrystal nucleus to incorporate kinetically trapped atoms into the solid phase with minimal rearrangement. In the system under investigation, which forms a dodecagonal quasicrystal, we show that this process occurs through the assimilation of stable icosahedral clusters by the growing quasicrystal. Our results demonstrate how local atomic interactions give rise to the long-range aperiodicity of quasicrystals.     

Structural relaxation of metallic glass forming melts

The fragility of superheated melts, M, for 13 kinds of metallic alloys has been evaluated from the data of the dynamic viscosity above their liquidus temperatures. The authors find that the glass forming ability of metallic melts depends on the fragility of superheated melts rather than on the value of viscosity. In the present work the value of fragility is less than 1 for good glass-forming melts but more than 1 for the other melts. The variation rate of atomic coordination number with temperature indicat...     

Charge density determination in icosahedral AlPdMn quasicrystal using quantitative convergent beam electron diffraction

Charge density distribution in icosahedral AlPdMn quasicrystal has been studied on a single-crystal specimen by using quantitative convergent beam electron diffraction (QCBED) technique. The QCBED systematic row method was used in the refinement of structure factors. To refine the low-order structure factors, the wave-mechanical formulation of electron diffraction dynamical theory was used in the calculation of electron diffraction intensities for the quasicrystal in fitting the experimental intensity line scan profiles. The shapes of atomic surfaces (occupation domains) were described with symmetry-adapted series of surface harmonics. An iterative procedure was used in determination of structure factors of the quasicrystal. The structure factors of nine strongest symmetry inequivalent reflections according to X-ray diffraction experiment were refined with QCBED technique. The average of refinement results for a given reflection performed on several CBED patterns, which were slightly different in orientation and sample thickness, and on different line scans, was taken as the value of structure factor for the reflection. The obtained structure factors for electrons were transformed into X-ray structure factors with Mott formula. The bonding charge density map for the quasicrystal was constructed with the obtained nine structure factors. Assuming that the atoms are spheres, the gain or loss of electrons for different atoms were calculated. It shows that identical atoms can have different valences at different kinds of positions. The bonding charge is localized along certain directions.     

Medium-Range Order Structure and Fragility of Superheated Melts of Amorphous CuHf Alloys

The structural factors of amorphous CuHf alloys at different temperatures are determined by using a high temperature x-ray diffractometer. It is found that not only the short-range order structure but also the medium-range order structure exists in amorphous CuHf alloys. The dynamic viscosities of CuHf alloy melts are measured by a torsional oscillation viscometer. The fragility of superheated melts of CuHf alloys is calculated based on the viscosity data. The experimental results show that the glass-forming ability of the CuHf alloys is closely related to the fragility of their superheated melt. The relationship between the medium-range order structures and the fragility of superheated melts has also been established in amorphous CuHf alloys. In contrast to the fragility of supercooled liquids, the fragility of superheated liquids promises a better approach to reflecting the dynamics of glass forming liquids.     

Structural origins of the excellent glass forming ability of Pd40Ni40P20

We report a hybrid atomic packing scheme comprised of a covalent-bond-mediated "stereochemical" structure and a densely packed icosahedron in a bulk metallic glass Pd40Ni40P20. The coexistence of two atomic packing models can simultaneously satisfy the criteria for both the charge saturation of the metalloid element and the densest atomic packing of the metallic elements. The hybrid packing scheme uncovers the structural origins of the excellent glass forming ability of Pd40Ni40P20 and has important implications in understanding the bulk metallic glass formation of metal-metalloid alloys.     

Ca，Be在镁合金中的阻燃作用

建立了镁合金的晶体,液态及其固/液界面模型.采用递归法计算了Ca,Be在α-Mg、固/液界面、镁液态中的环境敏感镶嵌能,定义并计算了Mg,Ca及Be与氧的原子亲和能.计算结果表明：Ca,Be在镁晶体中的环境敏感镶嵌能较高,不能稳定固溶于晶体中,因此在固体中的溶解度较小.合金凝固时Ca,Be扩散到环境能较低的液体中,向液面聚集.由于Ca,Be与氧的原子亲和能低于镁与氧的亲和能,聚集在液体表面的Ca,Be将优先与氧结合,生成致密的镁与合金元素的混合氧化物,阻止镁合金燃烧. 关键词： 电子结构 阻燃 Mg合金     

Atomic dynamics of the α-(Al,Si)CuFe alloy: A crystalline approximant of a quasicrystal...

The atomic dynamics of the Al_0.550Si_0.070Cu_0.255Fe_0.125 alloy with the structure that approximates the structure of an icosahedral quasicrystal with a similar chemical composition has been investigated using inelastic neutron scattering. The partial vibrational spectra of copper, iron, and aluminum atoms and the total spectrum of thermal vibrations of the compound have been directly reconstructed from the experimental data for the first time. A combined analysis of the results obtained and the data on the atomic dynamics of the i-AlCuFe icosahedral quasicrystal has been performed.