Computer simulation of the matrix-inclusion interphase in bulk metallic glass based nanocomposites

Atomistic models for matrix-inclusion systems are generated. Analyses of the systems show that interphase layers of finite thickness appear interlinking the surface of the nanocrystalline inclusion and the embedding amorphous matrix. In a first approximation, the interphase is characterized as an amorphous structure with a density slightly reduced compared to that of the matrix. This result holds for both monatomic hard sphere systems and a Cu(47.5)Zr(47.5)Al(5) alloy simulated by molecular dynamics (MD). The elastic shear and bulk modulus of the interphase are calculated by simulated deformation of the MD systems. Both moduli diminish with decreasing density but the shear modulus is more sensitive against density reduction by one order of magnitude. This result explains recent observations of shear band initiation at the amorphous-crystalline interface during plastic deformation.

Geometry and spectral properties of the protonated homodimer of pyridine in the liquid and solid states. A combined NMR, X-ray diffraction and inelastic neutron scattering study

The structure and spectral signatures of the protonated homodimer of pyridine in its complex with a poorly coordinating anion have been studied in solution in CDF(3)/CDClF(2) down to 120 K and in a single crystal. In both phases, the hydrogen bond is asymmetric. In the solution, the proton is involved in a fast reversible transfer that determines the multiplicity of NMR signals and the sign of the primary H/D isotope effect of --0.95 ppm. The proton resonates at 21.73 ppm that is above any value reported in the past and is indicative of a very short hydrogen bond. By combining X-ray diffraction analysis with model computations, the position of the proton in the crystal has been defined as d(N-H) = 1.123 Å and d(H···N) = 1.532 Å. The same distances have been estimated using a (15)N NMR correlation. The frequency of the protonic out-of-plane bending mode is 822 cm(-1) in agreement with Novak's correlation.

Rapid electron beam assisted patterning of pure cobalt at elevated temperatures via seeded growth

A new method of direct, rapid nano- to micro-scale patterning of high purity cobalt is presented. The method utilizes a combination of electron beam induced deposition (EBID) and seeded growth at elevated temperatures below the temperature of spontaneous thermal decomposition. Dicobalt octacarbonyl Co₂(CO)₈ is used as the precursor and carbon as a seed layer. Seeded deposition is carried out in the substrate temperature range from 55 to 75 °C. Deposition yield is significantly higher than conventional EBID and magnetotransport measurements indicate that resistivity, 22 µΩ cm, and saturation magnetization, 1.55 T, are much closer to the corresponding values for bulk Co than those for standard EBID.

Glass and Quasicrystal Formation in a Zr-based Multicomponent Alloy

The phase formation of a Zr57Ti8Nb2.5Cu13.9Ni11.1Al7.5alloy has been investigated upon cooling from the melt at different quenching rates as well as upon annealing of as-cast specimens. The different samples are characterized by x-ray diffraction, transmission electron microscopy and differential scanning calorimetry. Rapid quenching using the melt spinning technique results in amorphization. Slower cooling as realized upon copper mold casting leads to an icosaheadral quasicrystalline phase coexisting with a small amount of amorphous phase. The primarily formed quasicrystals have a grain size of about 1 νm. Upon annealing, the amorphous phase formed upon melt spinning precipitates quasicrystals in the first step of a series of transformations to the crystalline equilibrium compounds. The quasicrystals formed by annealing do not exceed a size of 5 to 10 nm.

Synthesis And Characterization Of Ball-Milled Nanocrystalline Fcc Metals

Nanocrystalline fee metals (Al, Cu, Ni, Pd, Rh, Ir) have been prepared by ball milling. The development of the microstructure is investigated by x-ray diffraction, differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). The final grain sizes range from 6 to 22 nm and scale with the melting point and the bulk modulus of the elements: metals with higher melting point and bulk modulus have a smaller final grain size. From this a general relation between the deformation mechanism during ball milling and the ultimate grain size achievable by this technique is inferred. With decreasing grain size the lattice strain is enhanced and deformation enthalpies of up to 40 % of the heat of fusion are stored in the material. The contributions of the lattice strain and of die excess enthalpy of the grain boundaries to the stored enthalpies are critically assessed. The kinetics of grain growth are investigated by mermal analysis. The activation energy for grain boundary migration is derived from a modified Kissinger analysis and estimates of the grain boundary enthalpy are given.

Alloy Effects And Extended Solubilities In Binary Mixtures Of Nanometer-Sized Fe-Cu Crystals

Binary mixtures of Fe-Cu powders in the range of 10–95 at.% Fe have been prepared by mechanical alloying and investigated by x-ray diffraction, differential scanning calorimetry (DSC), and transmission electron microscopy (TEM). The final grain sizes of the powders vary between 6 and 20 nm, and depend on the composition of the material. Indications for the formation of single-phase alloys with up to 60 at.% Fe in Cu and 20 at.% Cu in Fe have been found although the Fe-Cu system exhibits only vanishingly small solid solubilities under equilibrium conditions. Between 60 at.% and 80 at.% Fe a two-phase region of fee and bec solid solutions exists. Alloy formation is discussed with respect to the thermodynamic conditions of the material. The influence of the large grain boundary fraction, as well as the role of internal strains and stored enthalpies introduced by ball milling, is critically assessed.

Minimum Grain Size in Nanocrystalline Metal Powders

Nanocrystalline metal powders can be synthesized by mechanical attrition in a highenergy ball mill. A general relation determining the grain size of these materials is inferred. The ultimate grain size of nanocrystalline metals (typically 6 − 22 nm) is governed by the competition between the severe plastic deformation introduced during ball milling and the recovery behavior of the material. The lower bound grain size achievable by mechanical attrition is given by the minimum distance between two dislocations in a pile-up within a grain for all pure metals. Foar binary alloys the ultimate grain size depends on the composition of the material. Varying the composition changes the grain size reversibly. This can be explained by introducing solid solution hardening effects in the general relation for the lower bound grain size in pure metals. Thus, the proposed model for the ultimate grain size achievable by ball milling seems to be. applicable to all metals and alloys subjected to heavy mechanical deformation. However, reversible grain size changes are not restricted to mechanical attrition, but have also been observed for nanocrystalline Pd-H solid solutions produced by hydriding at constant pressure. Solid solutions prepared at different compositions, i.e. samples with different compositions, exhibit different grain sizes. Cycling between different temperatures/compositions changes the grain size reversibly. This cannot be explained by a model based on plastic deformation as in the case of ball-milled metal powders. The results are compared with data for ball-milled powders and samples prepared by inert gas condensation. The grain size changes are discussed with respect to the compositional changes and the grain boundary energy of the material.

Relaxation and Grain Growth Behavior of Nanocrystalline Iron

Nanocrystalline Fe has been prepared by inert gas condensation and ball milling. The kinetics of relaxation and grain growth are investigated by differential scanning calorimetry. The development of the microstructure is monitored by x-ray powder diffraction and transmission electron microscopy. Emphasis is placed on the differences observed for samples prepared by the two different techniques. We find that the kinetics of relaxation and grain growth are very sensitive to the sample preparation method. Samples with the same initial average grain size, as determined by the peak broadening in x-ray diffraction, show very different recovery behavior. The differences are discussed in terms of the estimated grain boundary energies and the initial grain size distribution obtained by the two preparation techniques.

Investigations on the Electrochemical Behavior of Zr-Al-Cu-Ni Bulk Metallic Glass

The passivation behavior of bulk glassy Zr55Al10Cu30Ni5 alloy samples in weakly alkaline sulphate solution (pH = 8) is investigated in comparison to the arc-melted crystalline alloy, to the main alloying component zirconium and to aluminium. Results of potentiodynamic and potentiostatic polarization measurements reveal the formation of a stable passivating surface film on the glassy alloy grown by a high-field mechanism. Auger electron spectroscopic investigations of anodized sample surfaces show that the films formed on the glassy Zr-Al-Cu-Ni alloys exhibit a composition gradient in cross-sectional direction. In 0.001M NaCl electrolytes bulk glassy Zr-Al- Cu-Ni samples are susceptible to pitting corrosion, which is due to the existence of crystalline inclusions.

On the atomic structure of Zr60Cu20Fe20 metallic glass

The structure of Zr(60)Cu(20)Fe(20) metallic glass has been studied with high-energy x-ray diffraction, neutron diffraction and extended x-ray absorption spectroscopy and modelled with the reverse Monte Carlo simulation technique. It is found that Cu and Fe atoms prefer Zr as a nearest neighbour. The mean interatomic distance between Cu/Fe and Zr atoms in the glass is remarkably shorter than the sum of the respective atomic radii. The coordination numbers for Cu/Fe-Cu/Fe pairs are very close to each other, suggesting a regular distribution of Cu and Fe atoms in the Zr(60)Cu(20)Fe(20) metallic glass.

Transformation-mediated ductility in CuZr-based bulk metallic glasses

Bulk metallic glasses (BMGs) generally fail in a brittle manner under uniaxial, quasistatic loading at room temperature. The lack of plastic strain is a consequence of shear softening, a phenomenon that originates from shear-induced dilation that causes plastic strain to be highly localized in shear bands. So far, significant tensile ductility has been reported only for microscopic samples of around 100 nm (ref. 4) as well as for high strain rates, and so far no mechanisms are known, which could lead to work hardening and ductility in quasistatic tension in macroscopic BMG samples. In the present work we developed CuZr-based BMGs, which polymorphically precipitate nanocrystals during tensile deformation and subsequently these nanocrystals undergo twinning. The formation of such structural heterogeneities hampers shear band generation and results in macroscopically detectable plastic strain and work hardening. The precipitation of nanocrystals and their subsequent twinning can be understood in terms of a deformation-induced softening of the instantaneous shear modulus. This unique deformation mechanism is believed to be not just limited to CuZr-based BMGs but also to promote ductility in other BMGs.