W. Frank

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.

Diffusion of gold in dislocation-free or highly dislocated silicon measured by the spreading-resistance technique

The diffusion of Au in dislocation-free or plastically deformed Si (1011 to 1013 dislocations/m2) was measured with the aid of the spreading-resistance technique. The Au profiles produced indislocation-free Si slices by in-diffusion from both surfaces possess nonerfc-type U shapes as predicted by the so-called kick-out diffusion model. This model is used to calculate the contribution of self-interstitials to the (uncorrelated) Si self-diffusion coefficient,DISD=0.064×exp(−4.80 eV/kT)m2 s−1, from the present and previous data on the diffusivity and solubility of Au in Si in the temperature range 1073–1473 K. Inhighly dislocated Si the diffusion of Au is considerably faster than in dislocation-free Si. From the erfc-type penetration profiles found in this case, effective Au diffusion coefficients were deduced and combined with data on the solubility of Au in Si. ThusCieqDi=0.0064 ×exp(−3.93 eV/kT)m2 s−1 was obtained in the temperature range 1180–1427 K, whereCieq andDi are the solubility and diffusivity of interstitial Au in Si.

A theory of void-lattice formation

Abstract A theory of void-lattice formation in crystals under irradiation is presented that is based on the so-called two-interstitial model and makes use of no additional assumptions. It is shown that the formation of void lattices having the same crystallographic type and orientation as the host crystal lattices, as found by experiment, is intimately related to the existence of a one-dimensionally diffusing metastable excited self-interstitial state (crowdion). The transition from a random assembly of voids to a regular void lattice takes place if the void number density is so high that in their excited state self-interstitials can traverse the mean void distance before returning to their ground state. If the void density is lower and does not increase in the course of irradiation, the random void arrangement reaches a steady state. From the view-point of thermodynamics, void-lattice formation is a non-equilibrium phase transition in an open system. It is an example of self-organization of an ordered structure in a dissipative system in the sense of synergetics.

Intrinsic point defects in hexagonal close-packed metals

Abstract After a review of computer-simulation results and of experiments on intrinsic point defects in hexagonal close-packed metals a comprehensive analysis of the available data is presented. Particular emphasis is put on the detailed interpretation of the observations on X-ray Huang scattering after electron irradiation, on lattice-parameter changes induced by fast-neutron irradiation, on perturbed angular γ-γ correlation in Zn and Cd, on positron annihilation in Zr and Ti, and on self-diffusion in Zr. The main conclusions are that in most hcp metals two distinct self-interstitial configurations exist, which undergo long-range migration in the recovery stages IE and III, that mono-vacancy migration takes place in recovery stage IV, and that self-diffusion occurs predominantly via a vacancy mechanism. In a brief discussion of irradiation growth in Zr and its alloys the technological importance of a comprehensive knowledge on intrinsic point defects and their agglomerates is demonstrated.

Properties of two growth-stimulating proteins isolated from fetal calf serum.

Two proteins (S1, S2) which stimulate cell growth in cultures of embryonic rat fibroblasts, have been isolated from fetal calf serum. S1 is a protein complex consisting of an α2-macroglobulin and insulin, asc ould be shown by immunological methods. For growth-activating activity both components are essential; when added separately to the cultures, they are ineffective. S2 has a molecular weight of about 26 000 D and does not cross-react with S1 in immunological tests. For optimal stimulation of the cultures both factors must be present together in the medium. As demonstrated by autoradiographic studies, there probably exist two different cell populations in the rat fibroblast cultures which either respond to S1 or S2. S1 and S2 strongly influence the metabolism of isolated fat cells. S1 behaves like insulin; S2 stimulates glucose oxidation and lipogenesis, but, in contrast to insulin and non-suppressible insulin-like activity, also activates the adenylate cyclase in these cells.

Diffusion mechanisms in amorphous alloys

Abstract A critical analysis of experimental data shows that self-diffusion in relaxed amorphous alloys is controlled by mechanisms involving collective thermal activations. Whereas in FeZr alloys both 59Fe and 95Zr diffuse via direct mechanisms that change with the alloy composition (e.g. in the densely packed Fe24Zr76 the large 95Zr tracers move collectively with many neighbouring atoms), further experiments are needed to decide whether 59Fe in metal-metalloid alloys also diffuses directly or indirectly via mobile spread-out quasi-vacancies present at thermal equilibrium. The tracer diffusivity decrease occurring in as-quenched amorphous alloys reflects that the initial contribution of a quasi-vacancy mechanism slows down because of the elimination of excess free volume during relaxation.

An entropy barrier against vacancy-interstitial recombination in silicon

Abstract It is suggested that the delayed recombination of vacancies and self- interstitials found in silicon at high temperatures is a consequence of the extended nature of these defects. The phenomenon is described in terms of an entropy barrier against vacancy-interstitial recombination.

Self-diffusion of 71Ge and 31Si in Si–Ge alloys

Abstract The tracer self-diffusaon coefficients DT of implanted 71Ge and 31Si in both relaxed monocrystalline SiyGe1-y epilayers and specimens made from bulk SiyGe1-y have been measured as functions of temperature T (653°C ≤ T ≤ 1263°C) and composition y (0 ≤ y ≤ 1) by means of radiotracer techniques, in which serial sectioning was done by ion-beam sputtering. For all compositions, the T dependencies of DT for 71Ge and 31Si are of Arrhenius type. The y dependencies of the pre-exponential factors of the self-diffusaon coefficients and of the diffusaon enthalpies show a break at y a 0.65. This is interpreted in terms of a transition from interstitialcy-(y ≥ 0.65) to vacancy-(y ≤ 0.65) mediated self-diffusion.

Study of point defect mobilities in zirconium during electron irradiation in a high-voltage electron microscope

Abstract A high-voltage electron microscope (HVEM) was used to investigate the nature of intrinsic point defects in α-Zr by direct observation of dislocation climb and cavity growth or shrinkage. The material used was Marz grade Zr that had been pre-irradiated with neutrons at about 740 K in the Doureay Fast Reactor. Dislocation loops of vacancy character that had been produced during the neutron irradiation were studied by further irradiation with electrons in the HVEM. Growth of the loops was observed at temperatures as low as 230 K indicating that, under the conditions of the experiment, some vacancy-type defects were mobile in the temperature regime 230–300 K. The nature of these defects is unknown. One possibility is that these defects are not intrinsic in nature, but may be vacancy-Fe complexes. In addition to the climb of dislocation loops, c-component network dislocations and cavities were also studied. Basal-plane climb of the network dislocations was observed at 573 K, but was not readily apparent at 320 K. This suggests that preferred climb planes (and possibly loop habit planes) are sensitive to temperature. Cavities that were already in the foil after neutron irradiation, or were induced by electron irradiation, grew along the c-axis and shrank along a-directions during electron irradiation. This radiation-induced shape change of the cavities strongly suggests the existence of a diffusional anisotropy difference between interstitials and vacancies in α-Zr.

Unified analysis of diffusion and relaxation processes in amorphous metallic alloys

Abstract Self-diffusion data on amorphous metallic alloys determined in long-time radio-tracer experiments and activation-enthalpy spectra deduced from short-time structural relaxation studies on such materials are reviewed and analyzed in terms of current random transition rate models. It is shown that the seeming discrepancy between the self-diffusion enthalpies and the comparatively small enthalpies obtained from measurements of the magnetic after-effect arises from the existence of activation-enthalpy spectra. Whereas the short-time experiments reveal the small-activation-enthalpy parts of these spectra, the long-time self-diffusion experiments are controlled by the larger activation enthalpies. Assuming that the activation-enthalpy distributions are Gaussian, their characteristic parameters have been determined by comparing the two types of measurements. It is shown that the Arrhenius-type diffusion coefficients found by experiment are compatible with half-widths of the activation-enthalpy spectra of...