Binary Amorphous Alloys Research Articles

Theoretical studies of the phase stratification process in amorphous binary alloys

Theoretical studies of the phase stratification process in amorphous binary alloys

Atomistic Formation Mechanism of Multiple Shear Bands in Amorphous Metals

At room temperature, plastic deformation of amorphous metals is localized in narrow bands, so called shear bands (SBs), which is a key issue of their brittle/ductile deformations. The extremely disordered structure and the abrupt fracture obtained in the experiments hinder us from making clear the nature and dynamics of SBs. In this study, atomistic modeling to produce the multiple shear bands (MSBs), not single shear band, was proposed, and the evolution process of MSBs including their coalescence and stationary was investigated using molecular dynamics simulations. We prepared a plate of Cu-Zr binary amorphous alloy model by the melt-quench process, and then performed uniaxial tensile and compressive tests under plane stress condition. During the loading, the deformation was suddenly localized in narrow bands shortly after the onset of yielding. The propagation of SBs was accompanied with drastic stress drop and significant local heating caused by the friction of atoms. Also, critical stresses of SB nucleation considerably differed between under tension and compression. This result indicates that Tresca or von Mises criterion, commonly used as a yield condition in the crystalline metals, is not appropriate to describe the yielding of the amorphous metals. The SB angles to the loading axis are observed to be 45∼57° under tension, while 40∼46° under compression. These angles agree well with the fracture angles observed in the experiments with multi-component metallic glasses. It is concluded that the critical stress state of SB nucleation is dependent on not only the shear stress but also the stress normal to the SB, and it can be described by Mohr-Coulomb criterion.

Analysis of Compositionally and Structurally Graded Si:H and Si1−xGex:H Thin Films by Real Time Spectroscopic Ellipsometry

ABSTRACTSilicon-germanium (Si1−xGex:H) thin films have been prepared by plasma enhanced chemical vapor deposition of SiH4 and GeH4 and measured during growth using real time spectroscopic ellipsometry. A two-layer virtual interface analysis has been applied to study the structural evolution of Si:H films prepared in multistep processes utilizing alternating intermediate and low H2-dilution material layers, which have been designed to produce predominately amorphous films with a controlled distribution of microcrystalline particles. The compositional evolution of alloy-graded a-Si1−xGex:H has been studied as well using similar methods. In each case, depth profiles of microcrystalline content, fμc, or Ge content, x, have been extracted. Additionally, real time spectroscopic ellipsometry has been used to monitor post-deposition exposure of a-Si:H, a-Si1−xGex:H, and a-Ge:H films to a hydrogen plasma in situ in order to determine sub-surface amorphous film modification similar to that which would occur when a highly H2-diluted layer is deposited on a layer prepared with lower dilution. These analyses provide guidance for enhanced performance of Si:H based solar cells, through controlled bandgap grading using compositionally graded amorphous binary alloys (a-Si1−xGex:H) or the incorporation of controlled fractions of microcrystallites into bulk amorphous i-layer materials, and by providing a fundamental understanding of the modification of component layers during the deposition of subsequent layers in multilayer stacks.

Heterogeneous magnetism in Fe-doped bulk-amorphous and nanostructured Pd-basedalloys

It has been shown in a previous work (Kapaklis et al 2005 J. Appl. Phys. 98 044319) that whereas the alloycomposition Pd40Cu30Ni10P20 can be easily cast as a bulk metallic glass, a partial replacement of Pd by Fe(Pd35Cu30Ni10Fe5P20) leads to a significant crystalline fraction under the same casting conditions. In order tounderstand the formation and properties of these precipitates, fully amorphous ribbons ofboth alloy compositions were also prepared by conventional melt-spinning. Magnetic datareveal that doping with 5 at.% iron leads to the formation of nanosized crystalline inclusionsin the residual bulk amorphous matrix which are completely absent in the ribbonmelt-spun from the same ingot. In both types of Fe-containing alloys the amorphous matrixcontains giant moments. Their size is of a few hundred (ingot) and a few tens (ribbon) ofBohr magnetons. The iron-free alloy corrected for the Langevin diamagnetism shows Pauliparamagnetism with similar susceptibility values in both the ingot and the ribbon.These values are close to the Pauli susceptibility of a control sample, a melt-spunPd82P18 alloy which is expected to be as homogeneous as the typical binary amorphous alloys. Thepossible origin of different, controversial magnetic behaviors reported in the literature forFe-doped Pd-based bulk and melt-spun alloys is discussed.

Shear Banding Analyses of Amorphous Alloys with Crystalline Particles

Plastic deformability of the binary (copper and zirconium) amorphous alloy with embedded nanosized crystals under uniaxial tension and compression is analyzed using molecular dynamics simulations. The number and the size of the nanocrystals are taken as the study parameters. The number of nanocrystals affects the distribution of defects, that is, shear bands nucleation and thus changes the stress-strain curve, whereas the size of the nanocrystals does not significantly influence the response. As already reported in the experimental works, coalescent voids are found under tension in the shear bands or at the interface between crystalline and amorphous phases. This suggests that much attention should be paid to the interface strength around the particles.

Crystal Growth in Amorphous Binary Alloys of Zr-Ni System

A crystallization process in an amorphous state under isothermal condition is examined for binary alloys ZrNi and ZrNi2 by differential thermal analysis (DTA). Time dependence of DTA curves is measured at several constant temperatures just below crystallization temperature. The fraction of crystallized volume in amorphous state and its time evolution during isothermal annealing are measured. These data are analyzed by the Johnson-Mehl–Avrami formula. The Avrami exponent is 2.4±0.1 for ZrNi and 3~4 depending on the set temperature for ZrNi2. The activation energy for crystallization of amorphous ZrNi and ZrNi2 was estimated by plots of lnt1/2 vs. 1/T.

Binary Ni–P bulk amorphous glass prepared by electrodeposition method

Compact and uniform bulk Ni–P binary amorphous alloys (BAAs) with thickness of up to 1 mm were prepared using electrodeposition method. This is the first finding that amorphous alloy can be obtained up to ‘bulk’ size not by rapid solidification but by electrochemical method. By improving the electrodeposition techniques, selecting proper plating solution, electrodeposition is probably a new method for preparing bulk amorphous alloys. The formation mechanism of the bulk amorphous was discussed within the concept of ‘disorder solid’.

Local structure characterization in quasicrystal-forming Zr80Pt20 binary amorphous alloy

The local structure of melt-spun Zr80Pt20 alloy was investigated in the amorphous and icosahedral quasicrystal (QC)-formed states by x-ray diffraction and extended x-ray absorption fine structure measurements. While the local environment around the Zr atom in the amorphous state is considerably different from that in the QC-formed state, it remains during the quasicrystallization around the Pt atom. It is suggested that the stable icosahedral local structure is mainly formed in the center of the Pt atom in the amorphous state.

Superconducting State Parameters of CuCZr100−CBinary Amorphous Alloys by Pseudopotential Approach

We present the screening dependence theoretical investigations of thesuperconducting state parameters, i.e. electron–phonon coupling strengthλ, Coulomb pseudopotential μ*, transition temperature Tc,isotope effect exponent α, and effective interaction strengthN0V of some CuCZr100−C (C = 25, 30, 33, 35, 40,45, 50, 55, 57, 60 at.%) binary amorphous alloys by employing the well-knownempty core model potential of Ashcroft. Five screeningfunctions proposed by Hartree, Taylor, Ichimaru-Utsumi, Farid etal. and Sarkar et al. are used to study the screening influenceon the aforesaid superconducting properties. The transition temperatureTc obtained from the Ichimaru-Utsumi screening function isfound to be in excellent agreement with available theoretical orexperimental data. Also, the present results are found in qualitativeagreement with the other earlier reported data, which confirms thesuperconducting phase in the amorphous alloys.

Compression Behaviour of Bulk Metallic Glasses and BinaryAmorphous Alloy

The compression properties ofZr41Ti14Cu12.5Ni10Be22.5,Zr44.4Nb7Cu13.5Ni10.8Be24.3 bulk metallicglasses and Ni77P23 binary amorphous alloy areinvestigated at room temperature up to 24 GPa, 39 GPa and 30.5 GPa,respectively, using in-situ high pressure energy dispersive x-raydiffraction with a synchrotron radiation source. The pressure–volumerelationship of Ni77P23 amorphous alloy isconsistent well with the second order Birch–Murnaghan (B-M) equationwithin the experimental pressure range. However, under higher pressure,the experimental data of Zr-based specimens deviate from the B-Mequation. Compare to the binary amorphous alloy, less excess freevolume existing in the bulk metallic glass and multi-component atomicconfiguration results in a two-stage relationship betweencompressibility and pressure.

Potentiodynamic polarization studies on amorphous Zr46.75Ti8.25Cu7.5Ni10Be27.5, Zr65Cu17.5Ni10Al7.5, Zr67Ni33 and Ti60Ni40 in aqueous HNO3 solutions

Potentiodynamic polarization studies were carried out on virgin specimens of Zr-based bulk amorphous alloys Zr46.75Ti8.25Cu7.5Ni10Be27.5 and Zr65Cu17.5Ni10Al7.5, and conventional-type binary amorphous alloys Zr67Ni33 and Ti60Ni40 in solutions of 0.2M, 0.5M and 1.0M HNO3 at room temperature. The values of the corrosion current density (Icorr) for the bulk amorphous alloy Zr46.75Ti8.25Cu7.5Ni10Be27.5 were found to be comparable with those of Zr65Cu17.5Ni10Al7.5 in 0.2M and 0.5M HNO3, but the value of Icorr for the former was almost three times more than that of the latter in 1.0M HNO3. In the case of conventional binary amorphous alloys, Ti60Ni40 showed lower value of Icorr as compared to Zr67Ni33 in 0.5M and 1.0M HNO3 and a comparable value of Icorr in 0.2M HNO3. In general, the binary Ti60Ni40 displayed the best corrosion resistance among all the alloys in all the cases and the corrosion current density (Icorr) for all the alloys was found to increase with the increasing concentration of nitric acid. It is noticed that the bulk amorphous alloys do not possess superior corrosion resistance as compared to conventional binary amorphous alloys in aqueous HNO3 solutions. The observed differences in their corrosion behavior are attributed to different alloy constituents and composition of the alloys investigated.

Phonon Dispersion in Amorphous Ni-Alloys

The well-known model potential is used to investigate the longitudinal and transverse phonon dispersion curves for six Ni-based binary amorphous alloys, viz. Ni31Dy69, Ni33Y67, Ni36Zr64, Ni50Zr50, Ni60Nb40, and Ni81B19. The thermodynamic and elastic properties are also computed from the elastic limits of the phonon dispersion curves. The theoretical approach given by Hubbard–Beeby is used in the present study to compute the phonon dispersion curves. Five local field correction functions proposed by Hartree, Taylor, Ichimaru–Utsumi, Farid et al. and Sarkar et al. are employed to see the effect of exchange and correlation in the aforesaid properties.

Kinetics of crystallization of titanium based binary and ternary amorphous alloys

Metallic glasses are kinetically metastable materials. These amorphous materials can be transformed into a crystalline state by both isothermal and isochronal methods. The study of this transformation, and hence the thermal stability of metallic glasses, are important from an application view-point. In the present work, the non-isothermal crystallization kinetics of two titanium-based amorphous alloys namely, Cu 50Ti 50 and Ti 50Ni 30Cu 20, are reported. The activation energies for crystallization, E c for both the systems have been evaluated using different non-isothermal methods viz. derived through Kissinger, Augis and Bennet and Ozawa. The values of E c obtained using these methods are consistent for both the metallic glasses and it is found that E c for the ternary metallic glass is considerably higher than the binary metallic glass. The increase in the activation energy on the substitution of Ni in the Cu–Ti metallic glass suggests the increase in the thermal stability.

Ti-based amorphous alloys with a wide supercooled liquid region

Ti-Cu-Ni-Co quaternary amorphous alloys produce by melt spinning were found to have a wide supercooled liquid region before crystallization, though no glass transition was observed in Ti-Cu binary amorphous alloys. The largest temperature interval of the supercooled liquid region (Δ T x) is as large as 90 K for Ti 50Cu 25Ni 20Co 5.There is a tendency for Δ T x to increase with an increase in storage modulus and with a decrease in loss modulus. It is therefore presumed that the increase in Δ T x for the multicomponent amorphous alloy is due to the suppression of crystallization for the supercooled liquid resulting from the increase in viscosity.

Computational Relationship of Deformation Behavior and Materials Strength of Amorphous Alloys to Short-Ranged Local Structures

CuxZr1-x (x=0.30~0.85) binary amorphous alloy models were constructed using molecular dynamics simulations. In order to characterize the short-ranged local structures and the free volume of the whole cell, Voronoi polyhedral analyses were performed for their initial states and also for the deformed states. Both geometrical and physical factors are intimately related to each other and exhibit a large influence on mechanical properties. The elastic properties of shear modulus and bulk modulus tend to increase as the free volume decreases. The Poisson’s ratio, which is related to the glass fragility, increases as the free volume. However, the relation among three elastic properties obtained from the continuous mechanics of the isotropic body does not hold. Also, the maximum stress in the finite deformation drastically decreases with increasing free volume content.

Selective hydrogenation of unsaturated nitriles to unsaturated amines over amorphous CoB and NiB alloys doped with chromium

Amorphous binary (CoB, NiB) and ternary (CoBCr, NiBCr) alloys prepared by chemical reduction with NaBH 4 were characterized by atomic absorption spectroscopy (AAS), nitrogen physisorption (BET), X-ray diffraction (XRD), temperature programmed desorption (TPD), temperature programmed reduction (TPR), hydrogen chemisorption, scanning electron microscopy (SEM) coupled with energy dispersive X-ray analysis (EDX) and X-ray photoelectron spectroscopy (XPS) and the influence of their properties on catalytic performance in the hydrogenation of unsaturated nitriles to unsaturated amines was studied. The hydrogenation over NiB(Cr) catalysts lead to preferential hydrogenation of the C C bond, whereas the hydrogenation over CoB(Cr) catalysts occurred preferentially to the unsaturated amine. Addition of chromium dramatically increased the activity of both NiB and CoB catalysts, due to its effect on the binding energy of adsorbed hydrogen and the adsorption strength of the nitrile functional group. Amorphous cobalt borides treated with sodium hydroxide can hydrogenate unsaturated nitriles to primary unsaturated amines with medium-to-high selectivity without the presence of ammonia.

Pair correlation functions and structural building schemes in amorphous alloys

Using molecular dynamics simulations of a model binary amorphous alloy, the short-to-medium range pair correlations observed in the pair distribution functions are attributed to well-defined connection schemes of local structural motifs. The peak splitting features, particularly those in the solute-solute correlation function, are interpreted as due to the ordering of quasi-equivalent clusters in several specific sharing arrangements.

Correlation between local structure and stability of supercooled liquid state in Zr-based metallic glasses

The correlation between the local structure and stability of supercooled liquid state is investigated in the Zr 70(Ni, Cu) 30 binary and Zr 70Al 10(Ni, Cu) 20 (numbers indicate at.%) ternary metallic glasses. The Zr 70Ni 30 binary amorphous alloy with a low stability of supercooled liquid state has a tetragonal Zr 2Ni-like local structure around Ni atom. Meanwhile, the Zr 70Cu 30 binary metallic glass has a different local structure of tetragonal Zr 2Cu, where we suggest the icosahedral local structure by the quasicrystallization behavior in addition of a very small amount of noble metals. The effect of Al addition on the local structure in the Zr–Ni alloy is also examined. We have investigated that the dominant local structure changes in the icosahedral-like structure from the tetragonal Zr 2Ni-like local structure by the Al substitution with Ni accompanying with the significant stabilization of supercooled liquid state. It is concluded that the formation of icosahedral local structure contributes to the enhancement of stability of supercooled liquid state in the Zr-based alloys.

Binary Magnesium-Based Amorphous Alloy Precursors in the Synthesis of Methyl Isobutyl Ketone

Results of our recent studies on the use of M-Mg(O) (M = Cu, Pd or Ni) amorphous alloy powders as catalyst precursors in the one-step conversion of acetone to methyl isobutyl ketone (MIBK) in the presence of hydrogen are summarized. Samples prepared by mechanical alloying and mechanically induced self-sustaining reaction (MSR) produce catalyst materials with varying catalytic properties. Precursor materials, in most cases, could be transformed by controlled oxidation into catalysts of enhanced activities and selectivities.

Electric field gradient in transition metal amorphous alloys

The mean atomic configurations in Hf- and Zr-based amorphous alloys prepared by different routes were studied using the perturbed angular correlations (PAC) technique. For all the as-prepared amorphous samples, the PAC spectra are well described by a wide distribution of frequencies revealing atomic configurations around the probes compatible with those in a dense random packing of ions (DRP). The measured electric field gradient strength V zz seems to be independent of the preparation method and of the addition of second phase nanoparticles (CaO and ZrC) for the same early transition metal content. The V zz values observed in Hf based alloys are systematically higher than that in Zr-based alloys of similar composition and show a greater variation with composition. These behaviors are discussed in terms of the different contributions to the EFG, its dependence with the density of states at the Fermi level and in comparison with similar trends observed in binary amorphous alloys. Also, the Ta probe role on the measured V zz in Hf and Zr systems was analyzed.