N. Mattern

ZrNbCuNiAl bulk metallic glass matrix composites containing dendritic bcc phase precipitates

We report on phase formation of a multicomponent Zr66.4Nb6.4Cu10.5Ni8.7Al8 glass-forming alloy upon copper mold casting. A bcc phase embedded in a glassy matrix forms for moldcast bulk samples yielding an in-situ bulk metallic glass matrix composite upon slow cooling from the melt. Upon annealing, the first exothermic transformation of the material is related to precipitation of an icosahedral phase from the glassy matrix. The formation of the bcc phase-containing metallic glass composite is strongly governed by the alloy composition and the actual cooling rate during solidification. Room-temperature compression tests reveal significant yielding and plastic deformation before failure.

Crystallographic and morphological characterization of reactively sputtered Ta, TaN and TaNO thin films

Abstract This paper concentrates on the deposition of Ta, Ta N and Ta N O thin films by r.f. magnetron sputtering in Ar/N 2 /O 2 gas mixtures. The film properties and their suitability as diffusion barriers and protective coatings in silicon devices were characterized using four-point probe measurements, Auger electron spectroscopy, Rutherford backscattering, glancing angle X-ray diffractometry, atomic force microscopy and scanning electron microscopy. With the addition of N 2 to the gas mixture a transition from tetragonal Ta to b.c.c.-Ta(N) was detected, leading to the nanocrystalline metastable b.c.c.-Ta(N) phase with approximately 20 at.% interstitially incorporated nitrogen. Increasing the nitrogen flow above a critical value, an abrupt transition between metal-sputtering to nitride-sputtering mode was observed, resulting in a sharp increase in the N:Ta atomic ratio slightly above the stoichiometric value for the TaN phase, which was found to exhibit f.c.c. structure. With the addition of oxygen at fixed nitrogen flow the films tend to grow in an amorphous state. Due to the lack of short-circuit diffusion paths, the as-deposited amorphous Ta(N,O) films are considered as excellent candidates for ultra-thin diffusion barriers and protection layers in future Cu-metallized ULSI devices.

Structural bulk metallic glasses with different length-scale of constituent phases

Abstract Bulk metallic glass composites containing constituent phases with different length-scales are prepared via an in situ method by copper mold casting homogeneous Zr–Ti–Nb–Cu–Ni–Al melts. The phase formation and the microstructure of the composite materials are investigated by X-ray diffraction, optical, scanning and transmission electron microscopy, and microprobe analysis. The composition of the melt as well as the cooling conditions realized during casting determine the type and the morphology of the phases present in the composite. The mechanical properties of composite materials with quasicrystalline or ductile bcc phase reinforcements are tested in uniaxial compression at room temperature, showing that the deformation is controlled by the type of the constituent phases and their morphology. Ductile phase-containing metallic glass composites demonstrate improved work hardening and ductility compared to monolithic metallic glasses. Similar results are obtained for composites with ductile bcc phase dendrites embedded in a nanocrystalline matrix. The improved ductility of the composites is due to the presence of the ductile second phase, which counteracts catastrophic failure by shear localization.

Enhanced magnetism in Fe-filled carbon nanotubes produced by pyrolysis of ferrocene

By optimization of the synthesis of ferromagnetic-filled carbon nanotube ensembles on Si substrates (catalytic decomposition of ferrocene) and following annealing at 645°C, marked hysteresis loops can be measured by the alternating-gradient method. Unusually high coercivities and strong anisotropies with an easy magnetic axis parallel to the alignment of the nanotubes are observed from the as-grown samples, whereas an enhanced magnetic saturation moment (up to a factor of 2) and a decreased anisotropy are realized after annealing at 645°C. The increase of the magnetic saturation moment of the Fe-filled carbon nanotube ensembles is caused by the entire transformation within the tubes of the γ-Fe and Fe3C phases to ferromagnetic α-Fe and graphite. X-ray diffraction with different glancing incidence shows that the γ-Fe is predominantly at the tips of the nanotubes, while the iron carbide resides closer to the substrate. However, after the annealing process only α-Fe is found. At an annealing temperature of 6...

Short-range order of Zr62−xTixAl10Cu20Ni8 bulk metallic glasses

Abstract The short-range order and crystallization behavior of slowly cooled Zr 62− x Ti x Al 10 Cu 20 Ni 8 bulk metallic glasses have been investigated in terms of the atomic pair correlation function as a function of Ti content x (2≤ x ≤7.5). The structural parameters point to the presence of chemical short-range order in these bulk glasses. An enhanced local excess free volume around the Ti atoms is concluded from density measurements. The first stage of crystallization in Zr 62− x Ti x Al 10 Cu 20 Ni 8 bulk glasses is related to changes in the medium-range order while the first neighborhood is retained. The atomic pair correlation functions of the first crystallization products are similar for all titanium contents. There is no indication of any special atomic arrangement for the particular alloy forming quasicrystals upon heating ( x =3). In case of Zr 54.5 Ti 7.5 Al 10 Cu 20 Ni 8 an ultrafine microstructure consisting of clusters of 2 nm in size is formed as the first step of crystallization.

As-cast quasicrystalline phase in a Zr-based multicomponent bulk alloy

An icosahedral quasicrystalline phase is obtained directly from the melt by copper mold casting of a Zr57Ti8Nb2.5Cu13.9Ni11.1Al7.5 alloy. On the other hand, rapid quenching of the alloy leads to an amorphous phase. Upon annealing, the amorphous structure precipitates quasicrystals in the first stage of crystallization. The microstructure of the quasicrystalline state is quite different for the two preparation routes, which is correlated with the asymmetry of the nucleation and growth rate upon cooling or heating. The quasicrystals formed upon slow cooling from the melt have a size of about 1 μm. In contrast, the quasicrystals formed by annealing do not exceed a size of 5–10 nm.

Structural behavior of Pd40Cu30Ni10P20 bulk metallic glass below and above the glass transition

The thermal behavior of the structure of Pd40Cu30Ni10P20 bulk metallic glass has been investigated in situ through the glass transition by means of high-temperature x-ray synchrotron diffraction. The dependence of the x-ray structure factor S(q) of the Pd40Cu30Ni10P20 glass on temperature follows the Debye theory up to the glass transition with a Debye temperature θ=296 K. Above the glass transition temperature Tg, the temperature dependence of S(q) is altered, pointing to a continuous development of structural changes in the liquid with temperature. The atomic pair correlation functions g(r) indicate changes in short-range-order parameters of the first and the second neighborhood with temperature. The temperature dependence of structural parameters is different in glass and in supercooled liquid, with a continuous behavior through the glass transition. The nearest-neighbor distance decreases with temperature, changing the slope at Tg. The interatomic distances of higher coordination shells expand analogo...

The influence of Co on the structure and magnetic properties of nanocrystalline FeSiBCuNb and FeZrBCu-based alloys

Abstract The magnetic and structural properties of as-quenched and crystallized Fe 73.5 Si 15.5 B 7 Cu 1 Nb 3 and Fe 86 Zr 7 B 6 Cu 1 alloys with Co partially substituted for Fe were investigated. Nanocrystallization takes place only in bcc structures. With increasing Co content the coercivity increases. In the FeCoZrBCu system a maximum B s of 1.67 T at about 35 at% Co is obtained. With the Co content in both systems, λ s passes a maximum in the nanocrystalline state.

Structure and thermal stability of graded Ta–TaN diffusion barriers between Cu and SiO2

Abstract Sputter deposited Ta and TaN single layers of 10 nm thickness as well as graded TaN/Ta and Ta/TaN/Ta layer stacks that act as diffusion barriers for Cu metallization were investigated after annealing at temperatures between T an =300 and 700 °C. By means of glancing angle X-ray diffraction, glow discharge optical emission spectroscopy and transmission electron microscopy, results of microstructure and phase characterization were correlated with diffusion phenomena. For the pure Ta barrier, Ta diffusion through the Cu cap layer to the sample surface is observed at T an =500 °C, and the transformation of initially grown metastable β-Ta into the equilibrium α-Ta phase occurs at T an =600 °C. In contrast, a fcc TaN layer remains stable at least up to T an =700 °C. In the case of the graded layer stacks, first signs of N diffusion out of the TaN film into the adjacent Ta layers are observed after annealing at T an =300 °C, and formation of hexagonal Ta 2 N starts at T an =500 °C. Whereas in the course of thermal treatments for the threefold graded Ta/TaN/Ta barrier all TaN reacts with Ta to form Ta 2 N, some fcc TaN remains in the twofold graded TaN/Ta barrier.

Enhancement of plastic deformability in Fe-Ni-Nb-B bulk glassy alloys by controlling the Ni-to-Fe concentration ratio

The influence of partial replacement of Fe by Ni in (Fe1−xNix)71Nb6B23 (0≤x≤0.5) bulk glassy alloys on the enhancement of the plasticity has been investigated. The increase of the Ni-to-Fe concentration ratio effectively improves the compressive plasticity. The (Fe0.5Ni0.5)71Nb6B23 alloy exhibits an enhanced plastic strain of ∼4.2%, together with high strength and distinct strain hardeninglike characteristics, when compared with the Ni-free Fe71Nb6B23 glass. The improved mechanical properties are ascribed to the control of the intrinsic elastic properties, followed by favorably tuning the interatomic interaction.