Defect and Diffusion Forum Vols. 309-310

Abstract: General description of the interplay between the Kirkendall shift (as a special way of relaxation) and diffusion induced driving forces in diffusion intermixing of binary systems is given. It is shown that, if the Kirkendall shift is negligible, a steady state Nernts-Planck regime is established with diffusion coefficient close to the slower diffusivity, independently of the type of the diffusion induced field and also independently whether this is a single field or a combination of different fields (e.g. stress field and extra chemical potential of non-equilibrium vacancies). Deviations from parabolic kinetics are expected only before or after this steady state stage. Using the results of our previous paper, on development and relaxation of diffusion induced stresses, it is illustrated that the setting of time of the Nernst-Planck regime is very short: intermixing on the scale of few tenths of nanometer is enough to reach it. It is also illustrated that this stage is realized even in the case of asymmetric interdiffusion (in one side of the diffusion zone the diffusion is orders of magnitude higher than in the other), when the stress distribution has a more complex form (having a sharp peak at the interface). Surprisingly the steady state is longer than it would be expected from the relaxation time of Newtonian flow: This is so because the composition profile is not static but changes fast in the timescale of the stress relaxation, and thus the stress re-develops continuously.

Abstract: The effect and the ways of removal of the segregation of alloying elements within the dendrite cell in single-crystal nickel-base superalloys are discussed. The peculiar features of Re and Ru behavior are underlined.

Abstract: Problems of reactive diffusion at a solid phase - melt contact were studied theoretically. The main intention was to calculate the time course of the solid phase dissolving in the case of planar dissolving. In our work we give heed especially to the dominating process, which is the solid metal A dissolving in solder melt B. During the dissolving, melt B saturates with metal A, and the process is influenced by convections which are characteristic for a given experimental configuration. A theoretical description of the kinetics of solid phase dissolving in the melt will be presented for the case of planar dissolving. The aim is to derive a relation for the interphase boundary movement (t) depending on time and a time course of growth of the element A concentration in the melt B. There are difficulties in accurate determination of the interphase boundary movement after heating of specimens for certain time intervals. It should be performed experimentally, since intermetallic phases are formed in original metal A both via diffusion and upon cooling and some phases segregate upon cooling of the solidifying melt. The main intention was to study experimentally the copper dissolving in melts of various solder alloys and the related reactive diffusion. We used pure Sn and Sn-Cu, Sn-Ag-Cu, Sn-Sb, Sn-Zn alloys as solder materials. Experiments aimed at the study of a Cu plate dissolving in the solder melt were carried out at various selected temperatures and times. The problems of reactive diffusion were studied both theoretically and experimentally and the problems that have to be solved preferably were emphasized. Concentration profiles of elements and thickness of layers of phases can be determined with SEM and X-ray microanalyses (WDX, EDX) of specimens after their diffusion heating.

Abstract: In order to study evolution of the concentration profiles and markers velocities in various time and space scales quasi-one-dimensional model of interdiffusion in a polycrystalline alloy is proposed considering non-equilibrium vacancies and the Kirkendall effect. Sinks/sources of vacancies are assumed to be concentrated mainly at the moving grain boundaries. This model simultaneously takes into account the movement of grains and changes of grain sizes.

Abstract: Recently) the point contact reactions between silicon nanowires (covered by natural oxide) and nanowires or nanodots of metals (nickel, cobalt, platinum) were discovered and studied. These reactions have at least three remarkable characteristics: (1) the reaction product phase is quite different from thin film or bulk reactions (for example, in Ni-Si reaction the appearing phase is Ni1Si1 or Ni1Si2, depending on the orientation of Si, instead of common Ni2Si phase); (2) Phase is formed not in the contact zone but, instead, near the wire tip or between two point contacts; (3) Subsequent phase growth of silicide inside silicon nanowire is a “stop-and-go” process consisting of waiting time before formation of critical island of each new atomic layer and then the fast filling of this new layer (mechanism suggested in 30-s for ideal crystals) Models of surface diffusion along and penetration through silicon oxide are presented. Nucleation models need more place and will be discussed in subsequent parts

Abstract: Our recent modelling works and corresponding numerical simulations realized to describe the UO₂ oxidation processes confirm the theory showing that an applied mechanical strain can strongly affect the local oxygen diffusion in a stressed solid. This result allows us to assume that stress field, previously applied at the surface of a metallic sample on several microns, will delay the degradation during its oxidation. Considering this hypothesis, we implemented a FEM simulation code developed in our laboratory to numerically investigate some different stress fields applied on a sample sub-surface, that might significantly modify the volume diffusion of oxygen during the oxidation process. The results of our simulations are presented and discussed from the perspective to study the consequences of a surface mechanical treatment on the durability of a metallic material.

Abstract: The surface diffusion layers formed in preliminary deformed (350 %) -Fe after nitriding at 853 K in ammonia medium were studied by means of metallography, electron microscopy, microhardness test and X-ray powder diffractometry. The preliminary plastic deformation (PPD) effects non-monotonously on the structure, microhardness and thickness of nitride - and -phases layers formed in -Fe. The narrow intervals of deformations of 3-8 % and 20-30 % were found in which the accelerated formation of nitride - and -phases occurs.

Abstract: The unique capabilities of atom probe tomography (APT) to characterize internal interfaces and layer chemistry with sub-nanometer scale resolution in three dimensions have been recently opened up to materials with poor electrical conductivity by the use of ultrafast laser pulses. The progress in sample preparation (focused ion beam) as well as in instrument performance enable now the analysis of relatively large volumes with typical diameters of 100 to 200 nm and depths of several hundred nm (this corresponds to an increase by several order of magnitude compared to the former instrument) of site specific samples. In this work, APT is used to study the effects of Pt on the formation and stability of Ni silicides. The precise location of this alloy element has been determined at the nanometer scale: In particular, APT allows us to quantify the amount of Pt in the grain boundaries (GB) of Ni2Si for about 100 different grain boundaries and thus to better characterize the GB diffusion and segregation.