Tomoyasu Aihara

Amorphization of intermetallic compounds by hydrogen absorption

Abstract The crystallographic structures of AxB1−x (A = a hydride-forming metal, B = non-hydride-forming metal) intermetallic compounds before and after hydrogen absorption have been identified using X-ray diffraction, to know the types of amorphizing intermetallic compounds. Amorphous alloys are formed by hydrogenation of the intermetallic compounds with the C15, C23, B82, D019 and L12 structures and having relatively low thermal stabilities. The compositions of these compounds are expressed as AB2, A2B and A3B.

Dispersion of collective excitations of amorphous and liquid Zr67Ni33 alloys

Molecular dynamics simulations have been performed to study the collective excitation of Zr67Ni33 amorphous and liquid alloys with a pair functional potential. The spectrum and the correlation function of the particle current are calculated for longitudinal and transverse modes. The dispersion relation is estimated from the spectrum. It is related to the peak position of the static structure factor and is strongly dependent on temperature at large wavenumbers. The collective excitation is evident for the Zr-Zr correlation but not for the Ni-Ni correlation. The motion of the Zr-Zr correlation is non-localized but that of the Ni-Ni correlation is localized. The sound velocity and the Debye temperature are calculated from the dispersion relation. The calculated power spectrum agrees with the previously reported experimental result. The power spectrum of the Zr atom of the amorphous state is similar to that of the crystal state. On the other hand, the power spectrum of Ni shows an obvious difference between the amorphous and crystal states.

Molecular dynamics simulation for binary amorphous Zr-Ni alloys

Abstract Molecular dynamics (MD) simulations of binary amorphous Zr-Ni alloys were performed in a computational model experiment. The composition dependence of the structural and dynamical properties of Zr x Ni 100− x ( x = 0, 17, 33, 50, 67, 83, 100) alloys was analyzed. The static structure was evaluated by the structure factors of the alloys. The longitudinal and transverse collective dynamics, i.e., phonon dispersion relations, were estimated by using the spectrum of the particle current. The composition dependences of these dynamical properties were estimated.

Oxidation behavior of sputter-deposited Cu-Ta alloys in air

Abstract Surface region of a series of Cu-Ta alloys prepared by sputter-deposition were characterized by XRD, XPS and AES. Just after preparation, Ta was enriched in the topmost region as an oxide film while Cu was enriched under the surface film. The surface film thickness on alloys increased by a 5 h air-exposure. The surface film thickness after 5 h exposure to air plotted against alloy composition had a large maximum at 22 at.% Ta. The surface films consisted of (Cu, Ta)O x (OH) y · n H 2 O type compound where Cu and Ta were enriched in low and high Ta alloys, respectively, after 5 h exposure although just Ta was enriched in the surface film on as-prepared alloys. Enrichment of Cu in the metallic state under the surface film was observed on all the alloys. When the oxidation proceeds on low Ta alloys, enriched metallic Cu under the surface film diffuses through the surface film to the top surface to form copper oxide film. On high Ta alloys, however, the Ta-rich surface oxide film on high Ta alloys is protective enough to prevent diffusion of Cu through the surface film. The composition dependence of the oxidation behavior of Cu-Ta alloys in air is largely related to the protectiveness of the initial oxide film.

Atomistic computer simulation for liquid-glass transition in ZrNi alloy

Abstract The liquid-glass transition for ZrNi alloy is investigated using molecular dynamics simulation and the empirical many-body potential described by the embedded atom method. The structure, distributions of the atomistic local site energy and the mean square force are investigated. The amorphous structure is compared with the crystal structure at room temperature. The potential, kinetic and total site energy distributions exhibit Maxwell-Boltzmann-type profiles. The liquid-glass transition is clearly detected for the atomistic site energy, and equivalently for each specimen. The results are discussed from the viewpoint of non-equilibrium. The mean square force exhibits an exponential dependence on the temperature.

Computational study of structural change through the glass transition in an amorphous and liquid ZrNi alloy

Amorphous alloys are experimentally or industrially produced by rapid quenching (RQ) from the melt. If a liquid alloy is rapidly cooled at a rate on the order of 10[sup 6]Ks[sup [minus]1], it enters the supercooled liquid regime and its viscosity increases. Finally, the system reaches a state of frozen random structure, which is called the amorphous state. In the attempt to control the properties of amorphous alloys, it is important to understand their structural changes through the glass transition. By a laboratory experiment, however, it is usually difficult to obtain information about the glass transition and supercooled state of an amorphous alloy because of competitive crystallization. Molecular dynamics (MD) simulation, a numerical experiment to solve the N-body problem of Newtonian mechanics, has been performed to investigate the structure of solid and liquid. As the MD simulation can be carried out on the order of picoseconds, one can detect the glass transition without crystallization during RQ. Thus, the authors performed the MD simulation for the production of an amorphous Zr-Ni alloy by RQ and detected static structure and thermodynamic changes through the glass transition. Both properties are related with interatomic potentials.

Structural study of amorphous GdM2 (M = Fe, Co, Ni) alloys by the anomalous X-ray scattering (AXS) method

Abstract Anomalous X-ray scattering (AXS) intensities have been measured at the absorption edges of Fe K and Gd LIII for amorphous GdM2 (M = Fe, Co, Ni) alloys prepared by sputtering. The possible atomic arrangements in the near-neighbor region, such as interatomic distances and coordination numbers of amorphous GdM2 alloys, were estimated from least-squares analysis so as to reproduce the environmental interference functions as well as the ordinary interference function. The coordination numbers for the Gd-Gd pair increase in the order a-GdNi2, GdCo2 to GdFe2, whereas those for the M-Gd pair vary in the reverse order. Such a variation is also seen in hydrogen-induced amorphous GdM2Hx (M = Fe, Co, Ni) alloys.

Molecular dynamics study for diffusion process in amorphous and supercooled liquid zr67ni33 alloys

Abstract The amorphous and liquid Zr 33 alloy is investigated using molecular dynamics simulation. The detail of atom motion in the amorphous and liquid alloys are analyzed by the graphics of atom trajectories. In the amorphous state, the major motion is the thermal vibration. The cooperative jump motion and the slow translational motion are observed as minor motion. The microscopic energy distribution and the amplitude of thermal vibration are analyzed. The microscopic dynamics are discussed with the site energy distribution.

Molecular Dynamics Simulation for Deformation Dynamics of Ni/Ni3Al Composite with FGM or NFGM Type Interface

The concept of functionally graded material (FGM) is extended to nanometer level and applied to a Ni/Ni3A1 (γ/γ′) coherent interface. Molecular dynamics simulations are performed for the elastic and plastic deformation of the Ni/Ni3A1 FGM crystal and the Ni/Ni3A1 nanocomposite with FGM or NFGM type interface. Simulations are performed at 298 K using Finnis—Sinclair-type potentials under a constant uniaxial tensile strain condition. The progress of the dislocation during plastic deformation was analyzed from the viewpoint of the change in atomic configuration. In the Ni/Ni3A1 FGM crystal, there is no slowing of dislocation propagation around the interface. The plastically deformed region of the Ni/Ni3Al nanocomposite with the FGM type interface is larger than that with the NFGM type interface. The Young’s modulus of the FGM type interface model is smaller than that of the NFGM type interface model. The FGM type coherent interface model shows good plastic deformability in comparison with the NFGM type coherent interface model for the Ni/Ni3Al crystal and nanocomposite.

Molecular Dynamics Simulation of Tensile Deformation in Amorphous Zr67Ni33Alloy

Molecular dynamics simulation is performed to study the deformation and fracture processes in a Zr-Ni amorphous alloy. A Finnis-Sinclair type pair functional potential is used. Uni axial strain is applied for a nano-size rod with constant strain rate at 77 K. Tensile processes are traced by checking the stress-strain curve, energy change in potential and by examining snapshots of atom movement The discrete atomic model reproduces the elastic continuum behavior. The plastic deformation occurs by the viscous flow mechanism, and no work hardening is observed. The distribution of atomic level stress is analyzed.