A first-principle study of the structural and electronic properties of amorphous Cu-Zr alloys

The atomic and electronic structures of amorphous CuxZr100-x(x=36,46,50,56,64) alloys were simulated using first-principle calculations within a 400-atom supercell.The pair correlation function,coordination numbers,local cluster structures and electronic density of states were analyzed.Reasonable agreements between the theory and the experiments were obtained.The amorphous alloys exhibit different local cluster structures and can all be explained with cluster formulas [cluster](glue)1,3,where the clusters a...     

Invar behaviour in crystalline and amorphous alloys

A review is given of the wide range of ferromagnetic materials which exhibit Invar characteristics. It is proposed to illustrate the thesis that Invar behaviour is very general among such materials and does not necessarily demand the following conditions to be satisfied: (1) presence of iron; (2) incipient antiferromagnetism; (3) heterogeneous alloying; (4) closeness to a martensite transformation.

For most but not all of the materials discussed weak itinerant ferromagnetism accompanies and causes Invar characteristics. An example counter to this behaviour is Fe₃Pt. Here, Pettifor and Roy proposed as the origin of Invar behaviour a more general itinerant electron instability than is characteristic of weak itinerancy.     

Local magnetic moments in crystalline and amorphous alloys

Hyperfine Interactions - Recent57Fe and61Ni Mössbauer spectroscopy studies of Fe and Ni magnetic moments in crystalline and amorphous alloys, as well as X-ray photoelectron spectroscopy...     

Investigation of amorphous and crystalline Sc3Fe alloys

Amorphous Sc₃Fe alloy has been prepared by splat-cooling with levitation melting. The quadrupole splitting distributions for Sc₃Fe were determined from Mössbauer spectra: the zero-field distribution has a Gaussian form. The analysis of Mössbauer spectra taken in applied magnetic field show that both positive and negative signs of principal component (V_z2) of electrical field gradients (EFG) are present. The crystallization process has also been investigated. Heat treatment of the amorphous alloys causes crystallization first into a metastable, and then into the stable phase.     

Diffusion of silicon in Fe-based amorphous and crystalline alloys

The segregation rate of silicon was measured in three different Fe-based amorphous and crystallized alloys with different silicon contents: 3.5, 5 and 9 at%. Analysis of the segregation kinetics yielded the diffusion activation energies E, as well as the frequency factors D₀. A linear dependence was found between In D₀ and E. In general, the D₀ and E values were lower for the amorphous specimens than for the crystalline ones, were independent of silicon content and are explained in terms of an oversaturated concentration of structural defects. In the crystalline specimens, diffusion behaviour was influenced by silicon content near the solubility limit of silicon in iron.     

NMR study of rapidly quenched crystalline and amorphous Fe-B alloys

Amorphous and microcrystalline Fe-B alloys (4–25 at % B) obtained by rapid quenching of the melt were studied using the pulsed nuclear magnetic resonance (NMR) of ¹¹B nuclei at 4.2 K. Alloy samples were prepared from both a natural isotope mixture and a mixture of the ⁵⁶Fe and ¹¹B isotopes. The NMR spectra were measured as a function of the boron content. The maximum hyperfine fields at the ¹¹B nuclei sites are 25–29 kOe and overlap the values of the hyperfine fields at the ¹¹B nuclei sites in the tetragonal and orthorhombic Fe₃B phases and also in the α-Fe phase containing boron as a substitutional impurity. The short-range order and local atomic structure of the amorphous Fe-B alloys were determined. The amorphous alloys are found to consist of microregions (clusters) with a short-range order similar to that in the tetragonal or orthorhombic Fe₃B phase or the α-Fe phase.     

The local magnetic states in spin-ordered amorphous and crystalline alloys

The review of experimental investigations of local magnetic states is given for three types of spin ordered materials: the cubic Laves phases with C15 structure, multicomponent heterophase alloys of Nd-R-Fe-M-B type and Heusler alloys. To obtain information on local magnetic states several kinds of experimental techniques were used: measurements of nuclear specific heat at T<1K. Mössbauer effect, nuclear spin-echo. The obtained results are treated in terms of several theoretical models and some essential parameters of itinerant and localized electron spectra are estimated and discussed.     

Distribution of dipole fields in disordered crystalline and amorphous ferromagnetic alloys

By means of computer simulation we have calculated the distribution functions of dipole fields in disordered crystalline and amorphous ferromagnetic alloys A_1-xB_x. It is shown that for all cubic lattice sites in simple cubic, body-centred cubic and face-centred cubic materials as well as for amorphous materials the envelopes of the distribution functions may be obtained in a satisfactory approximation by considering only the nearest contributing atoms. Whereas in crystalline materials we have a complicated structure of the distribution functions for arbitrary values ofx, we obtain simple Gaussian distributions for the case of amorphous materials, using Heimendahl's model of the amorphous structure. The influence of isotropic and anisotropic short-range order is discussed in detail.     

Icosahedral order in Cu-Zr amorphous alloys studied by means of X-ray absorption fine structure and molecular dynamics simulations

We have used the X-ray absorption fine structure method and molecular dynamics (MD) simulations to characterize atomic order in Cu-Zr metallic glasses (MGs). The microstructure of these MGs is described in terms of interconnected icosahedral-like clusters (superclusters) which are basic building units reproducing the stoichiometry of the system. The equilibrium MD configurations are used as an input for ab initio calculations of the extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectra. The theoretical EXAFS and XANES spectra are compared with those measured for rapidly quenched glassy Cu-Zr alloys. We demonstrate that the experimental results are well reproduced by EXAFS modeling of the population of the superclusters derived from the MD configuration. The average local structural motif can be approximated by Cu-centered icosahedral-like cluster satisfying the condition of maximal local packing efficiency and approximating the system stoichiometry. The simulated XANES exhibits good agreement with experiment, indicating that the atomic order of the MD configuration is consistent with that of the real alloy structure over distances of about 1?nm.     

Structural mechanisms of plastic deformation of amorphous alloys containing crystalline nanoparticles

Patterns of plastic deformation of amorphous nanocrystalline composites, caused by the local action of an indenter on a thin electron microscopy foil, have been experimentally investigated for the first time in structural analysis. Classification of the observed types of interaction of shear bands with crystalline nanoparticles is performed. This classification is in good agreement with the theoretically predicted interaction mechanisms.     

Balancing strength and plasticity of dual-phase amorphous/crystalline nanostructured Mg alloys

The dual-phase amorphous/crystalline nanostructured model proves to be an effective method to improve the plasticity of Mg alloys. The purpose of this paper is to explore an approach to improving the ductility and strength of Mg alloys at the same time. Here, the effect of amorphous phase strength, crystalline phase strength, and amorphous boundary(AB) spacing on the mechanical properties of dual-phase Mg alloys(DPMAs) under tensile loading are investigated by the molecular dynamics simulation method. The results confirm that the strength of DPMA can be significantly improved while its excellent plasticity is maintained by adjusting the strength of the amorphous phase or crystalline phase and optimizing the AB spacing. For the DPMA, when the amorphous phase(or crystalline phase) is strengthened to enhance its strength, the AB spacing should be increased(or reduced) to obtain superior plasticity at the same time. The results also indicate that the DPMA containing high strength amorphous phase exhibits three different deformation modes during plastic deformation with the increase of AB spacing. The research results will present a theoretical basis and early guidance for designing and developing the high-performance dual-phase hexagonal close-packed nanostructured metals.     

Distribution of magnetic dipole fields in amorphous ferromagnetic and speromagnetic alloys and in crystalline and amorphous spin glasses

For binary alloys with only one sort of magnetic atoms we have calculated, by means of computer simulations, the distribution of magnetic dipole fields at regular lattice sites and suitably defined interstitial sites for the magnetic ground state of amorphous ferromagnetic and speromagnetic alloys and crystalline and amorphous spin glasses. Thereby we have considered spatially random arrangements of atoms and have neglected all magnetic correlations in the case of speromagnets and spin glasses. We have derived symmetric and isotropic distribution functions, which are fairly well described by Gaussian distributions for large concentrations of magnetic atoms only. The distribution functions are characterized by their second moments, which are important for the discussion of NMR and μ⁺ SR relaxation rates in these materials.     

Width of the ferromagnetic resonance line in highly dispersed powders of crystalline and amorphous Co-P alloys

The resonance characteristics (inhomogeneous FMR linewidth ΔH) in highly dispersed (d=0.1–3 μm) powders of crystalline and amorphous Co-P alloys are investigated as a function of the composition, particle size, and atomic structure. It is established that ΔH for powders of amorphous Co-P alloys is two to three times larger than ΔH for crystalline Co-P powders. According to the investigations performed, this is caused by thermodynamically stimulated segregation of nonmagnetic Co₂P inclusions, apparently an effective relaxation channel, in the amorphous state of Co-P powders. Fiz. Tverd. Tela (St. Petersburg) 41, 464–467 (March 1999)     

Stability of amorphous alloys

The thermal stability of a number of Ti-base and Zr-base amorphous alloys was studied by means of differential scanning calorimetry. The crystallization temperatures of these amorphous alloys and those reported in the literature for a variety of other amorphous alloys (actinide-base alloys an refractory metal-base alloys) were analysed in terms of the corresponding formation enthalpies. It is shown that there is no correlation between the crystallization temperatures and the formation enthalpies. On the other hand the crystallization temperatures were found to scale more or less linearly to the corresponding formation enthalpies of a hole the size of the smaller type of atom in the binary amorphous alloys.     

Laser hydriding of crystalline and amorphous silicon

Crystalline and ion-beam-amorphized silicon samples were irradiated with a pulsed nanosecond excimer laser in a pure hydrogen atmosphere. The hydrogen concentration was determined via the ¹H(¹⁵N,)¹²C nuclear reaction. In the case of crystalline silicon, hydrogen incorporation into the sample surface was found to be well above the hydrogen solubility limit at thermodynamic equilibrium. The hydrogen depth profiles perfectly matched the damage profiles measured via Rutherford backscattering channeling spectroscopy. For the pre-amorphized silicon samples, the laser treatment resulted in epitaxial recrystallization of the amorphous top layer, but the hydrogen uptake was found to be negligible in that case. The experimental data were compared with the results of thermodynamic simulations of the laser–gas–material interaction. PACS 52.38.Mf; 61.72.Tt; 64.70.Dv     

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     

Hydrogen in amorphous and crystalline ytterbium films

Ytterbium vapor condensation on a liquid-helium cooled substrate in a hydrogen atmosphere is used to obtain Yb-H films containing up to 55 at.% hydrogen. Various thermodynamic and kinetic parameters of the transition of these films from the amorphous to the crystalline state (a→c transition) are investigated along with the electrical conductivity of these states. It is shown that the investigated properties of Yb-H films containing up to 40 at.% hydrogen are essentially indistinguishable from those of pure Yb films in the temperature interval 4.2–293 K. Increasing the hydrogen concentration to 55 at.% leads to an insignificant increase in the electrical resistivity, the kinetic temperature, and the activation energy of the a→c transition, and also to a decrease of the propagation speed of self-maintaining avalanche (explosive) crystallization. Reasons for the observed influence of hydrogen on the properties of Yb-H films are analyzed. The examined low-temperature Yb-H condensates can be characterized as a “frozen” solid solution of hydrogen in ytterbium in the temperature interval 4.2–293 K. Storing such films at room temperature leads to the formation of ionic ytterbium dihydride YbH₂. Fiz. Tverd. Tela (St. Petersburg) 41, 177–182 (February 1999)