Static relaxation of bcc crystal fragments using a Lennard–Jones potential: Transition to hexagonal or cubic close packing

Computer relaxation of fragments of an atomic bcc crystal, using a Lennard–Jones atom–atom interaction potential, yields a variety of close-packed polycrystalline structures, with well-defined grain boundaries between ordered fcc and hcp domains.The strong dependence of the results on shape and size of the fragments, as well as on their orientation and precise position with respect to the bcc matrix, is attributed to the surface structure, which apparently determines in detail what will happen upon relaxation, and suggests that the new phase starts (‘nucleates’) at the surface and proceeds inwards. Although the nature of the relation between surface structure and final result remains obscure, and no trends are apparent, single crystals with fcc or hcp structures can be obtained, with hcp being dominant. Possible effects of non-hydrostatic pressure are discussed in connection with constrained relaxations. The results are affected by simulated ‘thermal’ disorder, however small.Radial distribution functions, and simulated diffraction patterns are presented, the latter in comparison with some experimentally observed traces of small Argon clusters.     

Investigation of the influence of electron irradiation on the properties of cobalt nanotubes

Cobalt nanotubes have been synthesized by electrochemical deposition in pores of ion track membranes. The structures obtained have been studied by scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction analysis, and gas permeability method. The influence of electron irradiation on the cobalt nanotube structure has been investigated. It is shown that an increase in the irradiation dose leads to the transformation of the sample crystal structure. This fact can be explained by the simultaneous reduction of the β-Co metastable phase and relaxation of the microstress formed by the fcc phase in the lattice. The degree of sample texturing along the [100] direction increases under electron irradiation. The dependences of the resistive and magnetic properties of cobalt nanotubes on the irradiation dose have been analyzed.     

Effect of the preparation method on the structural and magnetic properties of the Fe2Nb alloy

In this paper we carried out the structural and magnetic study of the Fe₂Nb stoichiometric alloy by means of x‐rays diffraction and Mössbauer spectrometry. The samples were obtained by two different techniques, arc‐melting and mechanical alloying, and then they were thermally treated. The structural properties of the alloy depend on the method of preparation: by the melting method it was possible to obtain the hcp‐Fe₂Nb ordered structure; while by mechanical alloying besides of the hcp structure also were formed the fcc‐NbO and bcc‐FeNb structures. After heat treatment of the melted alloy, this was submitted to mechanical alloying during 16 and 32 hours. Mechanical alloying allows the formation of bcc‐FeNb grains which behave as superparamagnetic due to their small size and they are surrounded by hcp grains. Before heat treatment of the alloy obtained by mechanical alloying it was also found the existence of superparamagnetic bcc grains and the heat treatment increases the size of these bcc grains and the superparamagnetic‐ferromagnetic transition is induced.     

Ti、V、Ni、Mo对CVD金刚石涂层形核影响

基于密度泛函理论,计算分析了CH₃基团在含有过渡金属元素Ti、V、Ni、Mo的孕镶金刚石颗粒硬质合金基底表面的吸附能、Mulliken电荷分布、电荷密度差和态密度(density of states, DOS)等一系列性质,研究Ti、V、Ni、Mo对孕镶金刚石颗粒硬质合金基底化学气相沉积(chemical vapor deposition, CVD)金刚石涂层形核阶段的影响及其作用机理。计算结果表明:与CH₃基团在WC表面及金刚石表面的吸附相比,Ti、V、Ni、Mo与C原子间有较强的弱相互作用,这使得其对CH₃基团有更强的吸附能力(其中Ti>V>Mo>Ni);吸附能力大小与各原子的价电子结构相关,含有Ti元素的表面对CH₃的吸附最稳定;CH₃基团与Ni原子间更易发生电荷的转移形成共价键,Mo有利于促进CH₃基团的脱氢反应;形核阶段适当添加Ti、V、Ni、Mo这几种过渡金属元素将有利于增加形核密度,改善CVD金刚石膜基界面结合强度。     

Relationship between excess entropy and microstructure of undercooled liquid metals

Molecular dynamics simulations were performed for stable and undercooled liquid metals. The inherent structures formalism was used to explore the potential energy landscape, showing a competition between crystalline (fcc and bcc) and icosahedral (regular and distorted) order. The two-body approximation of the excess entropy is used to study the interplay between the microstructural ordering and the entropy of undercooled liquid metals. A strong correlation between the excess entropy and the relative changes in the crystalline and icosahedral order is obtained, indicating that entropic effects are coupled to the microstructural changes of undercooled liquid metals. Thermal entropy behavior was obtained by using structural information of both inherent structures and atomic configurations.     

Formation of quasicrystals and metallic glasses in relation to icosahedral clusters

It has been widely accepted that quasicrystals and at least some metallic glasses are built up with icosahedral clusters. Information about the cluster structures can be obtained from crystalline counterparts. In this paper, we will describe the formation rules of bulk metallic glasses, originally developed for quasicrystals, by combining cluster structures with phase diagram features. We will introduce the e/a-constant and e/a-variant criteria for ternary systems, and e/a-constant and atomic size constant criteria for quaternary systems. We will show how the glass forming composition optimization is realized by applying these rules in the Zr–Al–Ni and Zr–Al–Ni–Cu systems. In both systems the optimized glass-forming composition is related to a common binary icosahedral cluster Zr₉Ni₄ derived from the fcc Zr₂Ni phase. A novel route to reach amorphous forming composition is also attempted by mixing Al-based quasicrystal-forming compositions with Zr.     

低碳铁素体钢中纳米富Cu相由9R向fcc结构的转变

低碳铁素体钢样品经880 ℃加热0.5 h水淬,660 ℃调质处理10 h,然后在400 ℃等温时效11000 h后,利用高分辨透射电镜(HRTEM)和能谱仪(EDS),分析低碳铁素体钢中析出的纳米富Cu相不同晶体结构的特征及联系.观察到长轴约9.5 nm,短轴约6.6 nm的短棒状9R结构的纳米富Cu相,并且均为单斜9R以及呈互为孪晶结构,说明Cu早期析出时优先转变成具有互为孪生关系的单斜9R结构,同时还观察到孪生结构的fcc结构的Cu,以及具有莫尔条纹的非孪晶fcc结构的Cu.表明9R结构除了退孪晶化转变成非孪晶fcc结构外,还可以直接转变成具有孪生关系的fcc结构的纳米富Cu相.     

On microcrack nucleation at twin vertices and boundaries in bcc and fcc metals

Nucleation of microcracks at the vertex and boundary of a decelerated twin is studied for a number of bcc and fcc metals. A twin and its boundary are represented as stepped pileups of twinning dislocations located in the neighboring glide planes. The formation of microcracks through the merging of head dislocations by the force and thermally-activated mechanisms is analyzed. Analytical expressions are obtained that describe the conditions necessary for microcrack nucleation at the vertices of stepped dislocation pileups. It is established that with an increase in the shear-modulus value, the critical parameters of microcrack nucleation by the two above mechanisms become closer in all the metals considered.     

Investigation of the structure of thin rhenium films

The phase composition of thin rhenium films as depending on deposition conditions has been studied. In films deposited at a pressure of 10⁻⁵ torr impurity phases with simple cubic and hexagonal lattices were found. In films evaporated at a pressure of 10⁻⁸ torr only an hep phase equilibrium in bulk samples is formed. It has been concluded that in thin rhenium films no transitions to a fcc phase occurs. In the present work the dependence of thin film phase composition was investigated as a function of film thickness (∼5–500 Å), deposition rate (∼0.1–10 Å/sec) and substrate temperatur (∼ 20–450°C). The films were deposited on NaCl single crystals at a pressure of 10⁻⁵ torr (oil diffusion pump evacuation) an 10⁻⁸ torr (“Orbitron”-type pump evacuation). The measurement of film thickness was conducted by the optical interference method.     

Evolution of dislocation structures in a crystal under deformation

The system of equations describing the dislocation kinetics which provides plastic deformation in fcc and bcc metal crystals over a wide range of temperatures and stresses has been studied. The general types of solution and the two-parameter diagram of solution bifurcation are obtained as functions of stresses and temperatures. The diagrams of various types of solution are compared with the diagram of experimentally observed dislocation structures.     

衬底对沉积碳纳米管薄膜的影响

利用微波等离子体化学气相沉积(MPCVD)方法,分别在不锈钢衬底上和刻线的镍膜上直接沉积了碳纳米管膜。通过SEM、拉曼光谱和XRD表征,讨论了不同衬底对碳纳米管膜生长的影响。结果表明:在一定条件下,可在镍膜上沉积垂直于衬底的高度取向的碳纳米管,但在不锈钢衬底上却长出取向无序的碳纳米管膜,这说明衬底对碳纳米管的取向生长起着关键作用。     

Low-temperature synthesis of C60 thin films by ionized cluster beam deposition technique

C₆₀ films on Si(1 1 1) have been grown at low substrate temperature of 100°C by ionized cluster beam deposition technique. Fourier transform infrared (FTIR) transmission spectroscopy, Raman measurements and X-ray diffraction are employed to investigate the structure of deposited films. The results show that the acceleration voltage plays an important role in the growth of the films. As the acceleration voltage is moderate (100 V), a pristine C₆₀ film with face-centered-cubic (fcc) crystal structure can be grown. Further increase of the acceleration voltage leads to the formation of amorphous carbon (a-C) in the grown films. When the acceleration voltage is increased to 600 V, the deposited film has a complete amorphous carbon (a-C) structure.     

The free energy of hard dimer solids revisited

Free energies of a few crystalline structures of hard, homonuclear dimers have been calculated using the Einstein-crystal method for various sample sizes and extrapolated to the thermodynamic limit. In contrast to the two-dimensional case, large free energy differences between some structures have been found. This can explain the large difference in free energy between a typical aperiodic solid structure representing the dimers’ thermodynamically stable solid phase, known as the degenerate crystal (DC), and some crystalline structures of the dimers studied earlier. The obtained results indicate that structures with orthogonally directed neighboring dimers have much higher free energy than those with parallel neighboring dimers. It is also shown that, in the thermodynamic limit, the free energy difference per particle between structures built of hexagonal layers of parallel dimers differing only in arrangement of atoms at close packing in the fcc and hcp lattice is the same as that between hard sphere fcc and hcp crystals (within experimental error).     

On the curious structural phases in Al-deficient (Al62Cu23Cr15) and Al-rich (Al68Cu17Cr15) quasicrystalline alloys

In the present work investigations on the effect of stoichiometric variations on the Occurrence and stabilization of quasicrystalline (qc) and related phases have been carried out. Based on the explorations of several Al-deficient and Al-rich versions of the ideal nominal composition i.e. Al₆₅Cu₂₀Cr₁₅, it has been found that the alloy compositions corresponding to Al₆₂Cu₂₃Cr₁₅ (Al-deficient) and Al₆₈Cu₁₇Cr₁₅ (Al-rich) exhibit several structural subtleties. The Al-deficient alloy, has been found to exhibit crystalline bcc and fcc phases with a = 8.90 Å and a = 17.98 Å respectively. In addition to these phases, a new crystalline bcc variant (a = 15.42 Å) originating from the bcc (a = 8.90 Å) phase has been found. Also a curious superstructure of the fcc (a = 17.98 Å) has been observed. The Al-rich alloy typified by Al₆₈Cu₁₇Cr₁₅, on the other hand, does not exhibit any structural variants, instead it shows nearly pure i-phase.     

Generating ordered Si nanocrystals via atomic force microscopy

We describe two successful routes for generating ordered arrays of Si nanocrystals by using atomic force microscopy (AFM) and amorphous silicon thin films (200–400 nm) on Ti/Ni coated glass substrates. First, we show that field-enhanced metal-induced solid phase crystallization at room temperature can be miniaturized to achieve highly spatially localized (below 100 nm) current-induced crystallization of the amorphous silicon films using a sharp tip in AFM. In the second route, resistive nano-pits are formed at controlled positions in the amorphous silicon thin films by adjusting (lowering and/or stabilizing) the exposure currents in the AFM process. Such templated substrates are further used to induce localized growth of Si nanocrystals in plasma-enhanced chemical vapor deposition process. In both cases the crystalline phase is identified in situ as features of enhanced current in current-sensing AFM maps.     

Orientierung der α-Martensitkristalle beim K.-S.-Zusammenhang

During the quenching of steel the fcc γ-phase transforms without diffusion to bcc α-martensite. Accurate relations of orientation exist between the two different lattices. At the KURDJUMOW -SACHS relation 24 α-crystals with different positions can originate from one γ-crystal because of the cubic symmetry. These 24 K.-S.-variants as well as the change of directions during the γ → α transformation have been calculated by means of matrices.     

Study of the Reactivity of Ni Nanotubes in Media with Different рН

Nickel nanotubes have been synthesized by electrochemical deposition, and their reactivity in aggressive media has been investigated. Kinetic curves of the variation in the atomic ratio between Ni and O in the nanotube crystal structure as a function of the medium acidity are built.     

Metallic glass formation in the NiBGe system

Metallic glasses may be formed at transition metal/metalloid ratios substantially removed from the usual 80/20 atom fraction for binary systems such as NiB and CoB. The present investigation is of the NiBGe ternary systems in which the Ge is used as a substitutional probe atom for B in a subsequent EXAFS study. Ribbon samples have been fabricated for a wide range of compositions with metalloid content up to 45 at.%, the glass-forming region of which has been determined by bend ductility, XRD, DSC, SEM, and TEM measurements. Samples found to have relatively low thermal stability also tend to crystallize during ion-milling sample preparation for TEM, or during electron irradiation during TEM observation. Some of the ternary ribbon samples are ductile but are partially crystalline on XRD and TEM investigation. Other samples are brittle but show no crystallites, even after careful TEM study. Most ribbon samples made were fully glassy, as determined by XRD and TEM. The dual glass-forming composition range found in the NiB binary system is alternatively interpreted as a single continuous glass-forming regime, interrupted by an Ni₃B intermetallic region which is of such character as to make metallic glass formation very difficult by melt-spinning.     

Molecular dynamics simulation for cooling rate dependence of solidification microstructures of silver

A molecular dynamics simulation study has been performed to investigate the solid microstructures of Ag at room temperature resulted from rapid solidification with six cooling rates by using Quantum Sutton-Chen (QSC) many body potential. Several structural analysis methods have been employed to characterize and analyze the atomic configurations in the system. The results reveal that as the cooling rate γ ? 1.0 × 10¹³ K/s, an amorphous structure can be obtained; as the cooling rates in the range of γ = 5.0 × 10¹² ∼ 1.38 × 10¹¹ K/s, the coexistence structures of the meta-stable hcp with stable fcc configurations can be formed with phase separating or layering structures. Moreover, by using the 3D graphics technology, the arrangement of atoms in the system reveals that the phase separating occurs before the layering structures can be formed following the cooling rate decreasing. And an interesting layering structures have been found, in which the central atoms of fcc and hcp basic clusters are arranged by alternate close-packed-layers with a new repeating sequence ABCB.     

Einfluß der Stapelfehlerenergie auf den kristallographischen Umgitterungsmechanismus der γ/α-Umwandlung in hochlegierten Stählen

The mechanism of lattice change in the phase transformation fcc γ → bcc α depends on the stacking fault energy (sfe). In the case of large sfe a translation of (111)γ planes by Shockley half-dislocations of b = 1/6 [211]γ type is not possible, and the direct lattice change γ → α happens by two homogeneous shearings by 19°28′ or 10°32′, respectively. In the case of an sfe below 15 — 20 ergs/cm² this gliding occurs, and the ϵ-phase forms. Dependent on the thermodynamic stability the ϵ → α transformation can take place by a further shearing. The transformation γ → α preferably is caused by external elastic stresses, the γ → ϵ → α transformation likewise, but also by plastic deformation on the base of slidable half-dislocations.