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.     

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.     

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, …     

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.     

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...     

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 a <Emphasis Type="Italic">d</Emphasis>-AlNiFe decagonal quasicrystal

The atomic dynamics of an Al_71.3Ni₂₄Fe_4.7 decagonal quasicrystal has been investigated using the isotopic contrast method for inelastic neutron scattering. The partial vibrational spectra of the Ni, Fe, and Al atoms and the spectrum of the thermal vibrations of the alloy have been reconstructed directly from the experimental data without any model assumptions. The cutoff energies and the positions of the main features of the spectra have been determined. It has been revealed that the average binding energy of the nickel atoms in the quasicrystal under investigation is lower than that of the iron atoms and the vibrational spectrum of the aluminum atoms is noticeably harder than the spectrum of the pure metal. The results obtained for the d-AlNiFe decagonal quasicrystal have been compared with the previously published data for an i-AlCuFe icosahedral quasicrystal.     

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.     

Identification of the atomic structure of the fivefold surface of an icosahedral Al-Pd-Mn quasicrystal: helium diffraction and scanning tunneling microscopy studies

High resolution He diffraction and scanning tunneling microscopy images of the fivefold surface of a single-grain i-AlPdMn quasicrystal are obtained showing an almost perfect quasicrystal order. Observed configurations can be identified within the framework of polyhedral models. The terrace terminations are found to be Al-rich planes and successions of step heights agree with distances between dense Al planes in the model. This shows the ability of recent 6D polyhedral models to describe real quasicrystalline atomic configurations.     

熵在非晶材料合成中的作用

熵作为系统的状态函数,对于真实物质体系而言是一个极为重要的物理量.在非晶态合金的制备过程中最具代表性的指导原则有"混乱原理"和井上三原则,二者皆与熵有着紧密的联系.在过去很长一段时间内,这些经验准则指导了大量新型非晶体系的发现,但近些年的实验结果对这些理论提出了质疑.除组元数目之外,还有其他尚待研究的因素也影响着合金体系的玻璃形成能力.本文总结了玻璃转变过程中熵在热力学条件、动力学条件和结构条件中所扮演的角色,阐述了其对玻璃形成能力产生的或正或反的影响.特别是对近几年发展起来的高熵非晶体系的研究有助于开发出临界尺寸更大的非晶合金,也有助于进一步探索多组元合金和非晶形成能力之间的关系.     

镁合金稀土阻燃机理电子理论研究

通过计算机软件建立镁合金的晶体,液态及其固/液界面模型.采用递归法计算了稀土元素在α-Mg、固/液界面、镁液态等原子环境中的环境敏感镶嵌能,定义并计算了Mg,La及Y与氧的原子亲和能.计算结果表明：La,Y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.合金凝固时稀土元素扩散到环境能较低的液体中,向液面聚集.由于稀土与氧的原子亲和能低于镁与氧的亲和能(镁、稀土与氧的亲和能分别为Mg-O：-14.9338 eV,La-O：-19.0608 eV,Y-O：-19.5050 eV 关键词： 电子结构 阻燃 Mg合金     

铀基非晶合金的发展现状

铀基非晶合金是非晶家族中的特殊成员,受限于铀元素的高活性与放射性特点,目前这类非晶材料的研究极不充分.本文结合非晶合金的最新发展动态简要介绍了铀基非晶发展历史,较系统地总结了本团队的最新铀基非晶研究工作:首先较详细地介绍了新型铀基非晶的制备技术、成分体系、形成规律与晶化行为,澄清了其形成机制与热稳定性;结合高分辨电镜分析展示了其微观结构特点;采用纳米压痕技术揭示了这类非晶的微纳力学性能;利用电化学测试方法评估了其耐腐蚀性能.这些结果丰富了非晶材料的内涵,有助于深化对非晶物理基础科学问题的理解,并推动新型铀合金材料的发展,为这种材料的潜在工程应用奠定了基础.     

Zr基大块非晶中添加元素对非晶形成能力及耐蚀性的影响

运用实空间递归方法研究了添加元素Nb,Ta,Y,La对Zr基非晶合金的非晶形成能力和耐腐蚀性能的影响.用计算机编程构造了Zr基非晶中初始晶化相Zr₂Ni的原子结构模型,用Zr₂Ni中的二十面体原子团簇模拟非晶中的二十面体团簇.计算了替代二十面体中心或顶角位置原子前后Ni,Zr及合金元素的局域态密度、团簇中心Ni与近邻Zr原子及Ni与替代元素Nb,Ta,Y,La间的键级积分,还计算了合金元素替代前后团簇的费米能级.局域态密度计算结果表明：合金元素Cu占据二十面体团 关键词： 电子结构 Zr基大块非晶 非晶形成能力 耐蚀性     

Decagonal quasicrystal plate with elliptic holes subjected to out-of-plane bending moments

In the present paper, we consider only the ideal elastic behavior, neglecting the dissipation associated with the atomic rearrangements. Under these conditions, the decagonal quasicrystal plate bending problems have been discussed. The Stroh-like formalism for the bending theory of decagonal quasicrystal plate is developed. The analytical solutions for problems of decagonal quasicrystal plate with elliptic hole subjected to out-of-plane bending moments are obtained directly by using the forms. The resultant bending moments around the hole boundaries are also given explicitly. When the phonon–phason coupling is absent, the results reduce to the corresponding solutions for the isotropic elastic plates.     

一维短沟槽复合准晶结构减阻效应及模拟分析

本文测试了人工构建的一维短沟槽复合准晶结构对流体的减阻性能,并与一维短沟槽复合周期结构和一维沟槽周期结构的流阻进行了对比.实验结果表明,一维短沟槽复合准晶结构的减阻效果优于一维短沟槽复合周期结构,其中一维短沟槽十二重复合准晶结构的减阻效果最佳,同时与一维沟槽周期结构具有同样的减阻效果.在机理分析方面,建立了二维光栅的夫琅禾费衍射波模型,对通过一维短沟槽复合准晶结构的波谱特征进行模拟分析. 频谱分析表明,经过二维准周期光栅的相干波强度谱具有谱带结构特征,抑制了大角度方向上的强峰形成.这一结果与流体流过一维短沟槽复合准晶结构相对应,展向上的准周期结构在激活边界层微扰动的同时,也使得二次涡分布比较均匀,从而抑制了展向强扰动的形成,所以能够有效减小流阻.     

Characteristic chemical shifts of quasicrystalline Zn–Mg–Zr alloys studied by EELS and SXES

Chemical shifts of the constituent atoms of primitive icosahedral quasicrystal (P-QC), face-centred icosahedral quasicrystal (F-QC) and 1/1-approximant (1/1-AP) of F-QC Zn–Mg–Zr alloys were investigated for the first time using high energy-resolution electron energy-loss spectroscopy (EELS) and soft-X-ray emission spectroscopy (SXES). Among Zn M-shell and Mg L-shell excitation EELS spectra of P-QC, F-QC and 1/1-AP alloys, only the quasicrystalline alloys showed a chemical shift towards the larger binding energy side. In Zn-L and Zr-L emission SXES spectra, the P-QC and F-QC alloys showed a chemical shift towards larger binding energy side. The magnitudes of the shifts in the Zn-L emission spectra of the quasicrystalline alloys were almost the same as for ZnO. These results strongly suggest a decrease in valence charge in quasicrystalline states. Therefore, it should be concluded that bonding in quasicrystalline states involves a characteristic increase in covalency compared with bonding in corresponding approximant and standard metal crystals.     

Nb添加对Cu-Zr非晶合金玻璃转变和晶化动力学的影响

采用铜模吸铸法制备出直径为2 mm的Cu_50.3Zr_49.7-xNb_x(x=0,2)大块非晶合金，利用示差扫描量热分析(DSC)研究了2at%Nb元素添加对Cu-Zr非晶合金玻璃转变动力学和晶化动力学的影响，发现含Nb合金具有较低的脆性指数，和较高的晶化激活能.这表明微量Nb的添加提高了该二元Cu基非晶合金过冷金属液相的热稳定性，从而有利于其非晶合金的形成. 关键词： Cu-Zr非晶合金 Nb添加 玻璃转变动力学 晶化动力学