H. Teichler

Diffusion in metallic glasses and supercooled melts

Amorphous metallic alloys, also called metallic glasses, are of considerable technological importance.The metastability of these systems, which gives rise to various rearrangement processes at elevatedtemperatures, calls for an understanding of their diffusional behavior. From the fundamental point ofview, these metallic glasses are the paradigm of dense random packing. Since the recent discovery ofbulk metallic glasses it has become possible to measure atomic diffusion in the supercooled liquid stateand to study the dynamics of the liquid-to-glass transition in metallic systems. In the present article theauthors review experimental results and computer simulations on diffusion in metallic glasses andsupercooled melts. They consider in detail the experimental techniques, the temperature dependenceof diffusion, effects of structural relaxation, the atom-size dependence, the pressure dependence, theisotope effect, diffusion under irradiation, and molecular-dynamics simulations. It is shown thatdiffusion in metallic glasses is significantly different from diffusion in crystalline metals and involvesthermally activated, highly collective atomic processes. These processes appear to be closely related tolow-frequency excitations. Similar thermally activated collective processes were also found to mediatediffusion in the supercooled liquid state well above the caloric glass transition temperature. Thisstrongly supports the mode-coupling scenario of the glass transition, which predicts an arrest ofliquidlike flow already at a critical temperature well above the caloric glass transition temperature.

Structural dynamics on the μs scale in molecular-dynamics simulated, deeply undercooled, glass-forming Ni0.5Zr0.5

Abstract Results are reported from molecular dynamics (MD) simulations on the structural dynamics in a Ni0.5Zr0.5 metallic-glass model. Presented are, in particular, the mean squares displacements (MSD) for Ni and Zr atoms over a time window of 0.7 μs at 700 K. They indicate a change of the system between temporarily arrested and dynamical transient phases. The former extend up to 0.3 μs. From the data, diffusion coefficients are estimated and related to those from earlier simulations for higher temperatures. Atomic chain transitions in two-centre configurations are identified as the basic mechanisms of the dynamics. For some cases, the barrier heights of the centres are calculated and the pressure effect deduced. The latter indicates an activation volume of 7×10 −30 m 3 , a value comparable to the experimental activation volume of diffusion in NiZr.

Structural properties of laser deposited metallic alloys and multilayers

Abstract Pulsed laser deposited (PLD) metallic alloys and multilayers are characterized by the formation of amorphous or metastable nanocrystalline phases with high solid solubilities, unusually enlarged lattice spacings in growth direction and intermixed interfaces. The differences to sputtered and evaporated samples are discussed with respect to the high instantaneous deposition rate, which is about 105 times larger than during sputtering or thermal evaporation, and the high kinetic energy of the deposited particles of up to more than 100 eV at high laser fluences inducing atomic mixing, a large number of defects and a high stress in the deposited films.

Bridging the gap between molecular dynamics simulations and phase-field modelling: dynamics of a [NixZr1-x]liquid-Zrcrystal solidification front

Results are presented from phase-field modelling and molecular dynamics simulations concerning the relaxation dynamics in a finite-temperature two-phase crystal–liquid sample subjected to an abrupt temperature drop. Relaxation takes place by propagation of the solidification front under formation of a spatially varying concentration profile in the melt. The molecular dynamics simulations are carried out with an interatomic model appropriate for the NiZr alloy system and provide the thermophysical data required for setting up the phase-field simulations.Regarding the concentration profile and velocity of the solidification front, best agreement between the phase-field model and molecular dynamics simulation is obtained when increasing the apparent diffusion coefficients in the phase-field treatment by a factor of four against their molecular dynamics estimates.

Change of dynamical cooperativity in the glass-transition regime: computer modelling results for Ni0.5Zr0.5

Abstract Results are reported of molecular dynamics simulations for an amorphous Ni0.5Zr0.5 model. Presented are the mean squares displacements of the atoms on the μs-scale at 700, 785 and 900 K, well below the dynamical glass temperature Tc=1120 K. According to the data, the dynamical cooperativity characteristic for dense melts around Tc, that is irreversible motion of individual chains of atoms, becomes ineffective at low temperatures. Here a new type of higher organised dynamical cooperativity seems to take over, characterised by the occurrence of avalanches of mutually triggered transitions of chain of atoms separated by periods of calmness.

Parallel Molecular Dynamics Simulations of Liquid and Amorphous Metals

We report about molecular dynamics simulations on a virtually shared-memory parallel computer (KSR1-32). Applications comprise massive runs of several nanoseconds simulation-time with models using nearest-neighbour pair-interactions in the study of the dynamics close to the glass transition, and modelling of the atomistic structure of amorphous and liquid metals, using more involved interaction-potentials. Satisfactory parallelization, optimized to small sample sizes, may be achieved in both types of simulations.

Limitations of the diffusion approximation for muon motion in metals

It is discussed that applicability of the diffusion approximation depends critically on the magnitude of the phonon-induced incoherency between muon levels in adjacent interstitial sites. By means of two examples it is demonstrated that the temperature below which the diffusion approach breaks down may be different for different physical situations.