Defect and Diffusion Forum Vol. 263

Abstract: In this paper, four examples from the literature are introduced in which the presence of high density of twin boundaries could explain an anomalous diffusion behavior. (i) In the case of the grain boundary (GB) self-diffusion in nickel, leakage from the random GBs to considerably high fraction of deformation and/or annealing twins in the samples studied can be a reason for the diverse literature values of activation enthalpy. The diffusion model including participation of two types of GBs is essential for data evaluation. (ii) Segregation of Ge atoms with negligible solubility to twin boundaries and diffusion in both type A and type AB transition regime can clarify the unusual 71Ge diffusion in the Mg alloys. (iii) Anomalous diffusion of gold in polycrystalline silicon presented in the literature was discussed here. Because profiles corresponding to type AB transition regime were detected, transition from type B to type A diffusion regime could be an alternative explanation of the anomaly. (iv) Effective diffusion of the 63Ni tracer in the presence of unidirectional defects in single-crystalline β-tin might be a simple reason for the uncommon features published earlier: low activation enthalpy, high diffusion anisotropy and values of the extrapolated solid-state Ni diffusion coefficients above those in the Sn liquid phase.

Abstract: Grain boundary self-diffusion in both the cast and the cold-rolled Puratronic 4N5 nickel was studied in the temperature range from 600 °C to 1000 °C. The experiments were carried out with the samples pre-annealed at 1100 °C in comparison to the samples pre-annealed at intended individual diffusion temperatures. The relative grain orientation was analyzed on the same samples by means of electron backscatter diffraction (EBSD) and grain boundaries (GBs) were characterized in terms of the coincidence site lattice (CSL) model. Considering the non-linear Arrhenius temperature dependencies obtained for most specimens by using conventional method of profile evaluation in the B-type kinetics and the appearance of two high-diffusivity paths in diffusion profiles measured, a more suitable BB-type and AB-type diffusion models were applied for data evaluation.

Abstract: The study of composition and microstructure of welded joints was performed before and after the diffusion annealing at elevated temperatures for different annealing time. The Kirkendall effect in the Ni/Ni3Al diffusion couples was observed by means of different methods, e.g. using light, scanning electron and atomic force microscopies. The study suitably completes and specifies the morphology features of Kirkendall voids at different evolution stages, i.e. at nucleation, growing and coalescence. Kirkendall voids occurred in the region between the Matano and γ/γ´interface planes. The location of the γ/γ´ interface that moved in the direction of Ni3Al phase during the annealing resulted from the Al concentration profile measured by EDAX. The Matano plane location was determined by means of Boltzmann-Matano’s method using concentration profile data. It was observed that the void size was increasing in the direction from the Matano plane to the γ/γ´ interface. The obtained results were completed by surface topography of Kirkendall voids of slightly etched specimens by atomic force microscopy (AFM).

Abstract: Literature data on (self-)diffusion in transition metal borides are extremely sparse due to the low atomic mobility of the constituents and due to the fact that for B there exist no suitable radioactive tracers and only two stable isotopes with a high natural abundance of 19 % (10B) and 81 % (11B), respectively. The present paper reviews our experiments on the tracer diffusion of transition metals and boron in TiB2, WB2+x, and (TixWyCrz)B2 which were carried out using stable isotopes and secondary ion mass spectrometry (SIMS). For tracer deposition, ion implantation and magnetron sputtering were used. In order to measure boron diffusion, a specially designed experiment was build up where a TiB2 layer was sputtered on an isotope-enriched Ti11B2 bulk ceramic sample. In addition, first results on chemical interdiffusion in the system (TixWyCrz)B2 will be presented. Here, a method based on magnetron sputtered layers and secondary neutral mass spectrometry (SNMS) was used which allows to determine much lower diffusivities (down to 10-19 m2/s) than the conventional EDX line-scan method on cross-sectional samples.

Abstract: The demand for higher efficiency of energy production in fossil-fired power stations needs the increase of working temperatures up to 625 °C. This limitation is given by creep properties of the materials used for steam turbines and the other parts under long high-temperature exploitation. The most widely used materials for this application remain steels. From thermodynamic points of view, the steels can be thermodynamically treated as Fe-Me-N-C based closed systems. Phase diagrams and temperature dependent carbon and nitrogen activities of the steels can be calculated using CALPHAD approach, which is based on semiempirical thermodynamic model of the multicomponent/multiphase system and integral thermodynamic condition of phase equilibrium. The activities of all elements can be obtained mathematically. In this contribution, the CALPHAD method is applied on an investigation of the heterogeneous welds, for bainitic and martensitic base materials and for electrodes and filler weld alloys. Calculated temperature dependences of carbon and nitrogen activity are very important for a design of dissimilar weld joints. This information is essential, for example, for life-time estimations of the heterogeneous welds under high temperature creep.

Abstract: Interaction of lead-free solders with copper substrate represents an important phenomenon in the issue of reliability of solder joints. New experimental data describing phase equilibria in the Cu-In-Sn system after long-time diffusion annealing at the 400 °C/50 hours, 600 °C/310 hours and 600 °C/48 hours will be presented. The composition of solders was: 100 % Sn, 75 % Sn + 25 % In, 50 % Sn + 50 % In, 25 % Sn + 75 % In, 100 % In. The fast quenching method was employed to freeze thermodynamic equilibrium after annealing, followed by metallography, microhardness measurements, SEM (Scanning Electron Microscope) and WDX (Wave Dispersive X-ray) analysis. New phase equilibrium data, together with the data from literature, represent the best existing experimental description of phase equilibria in the system in question. The obtained experimental results of the phase equilibria were compared with the thermodynamic modelling by the CALPHAD (Calculation of Phase Diagrams) method and with other authors.

Abstract: In this paper we give a presentation of the recently developed approaches concerned of the rate of oxidation the Arema steel at high-temperature in first stage of this process. The comparison analysis was performed on the basis of the experimental results of oxidation of cylindrical specimens made of above steel. The experiment was carried out in chamber furnace on series of specimen with dimension Φ = 20 , l=30 mm. The specimens were oxidized at 1000 °C for 10 - 2280 minutes in air and then quenched in silica sand, afterwards were measured and weighed both with and without of scale. The analysis reveals that for longer oxidation time than one hour influence of linear dependence on parabolic growth of scale can be neglected.