Csaba Cserháti

Intrinsic diffusion in Ni3Al system

Abstract Interdiffusion experiments were performed between different Ni–Al two-phase alloys. A method is given to estimate the tracer diffusion coefficient of one of the constituents in a binary alloy knowing the tracer diffusion coefficient of the other species. The procedure is based on the so-called Darken-Manning analysis. The tracer diffusion coefficient of Al was determined in this way in Ni3Al: D Al ∗ =5.05×10 −7 −1.117×10 −7 +2.28×10 −6 exp −243±16 KJ / mol RT m 2 s Our results are compared with others available in the literature. The values for the self diffusivity of Al calculated from interdiffusion experiments give consistent result. All these measurements support the theory that diffusion of the minor element occurs through a vacancy mechanism in Ni3Al.

Mechanical loading stimulates chondrogenesis via the PKA/CREB-Sox9 and PP2A pathways in chicken micromass cultures

Biomechanical stimuli play important roles in the formation of articular cartilage during early foetal life, and optimal mechanical load is a crucial regulatory factor of adult chondrocyte metabolism and function. In this study, we undertook to analyse mechanotransduction pathways during in vitro chondrogenesis. Chondroprogenitor cells isolated from limb buds of 4-day-old chicken embryos were cultivated as high density cell cultures for 6 days. Mechanical stimulation was carried out by a self-designed bioreactor that exerted uniaxial intermittent cyclic load transmitted by the culture medium as hydrostatic pressure and fluid shear to differentiating cells. The loading scheme (0.05 Hz, 600 Pa; for 30 min) was applied on culturing days 2 and 3, when final commitment and differentiation of chondroprogenitor cells occurred in this model. The applied mechanical load significantly augmented cartilage matrix production and elevated mRNA expression of several cartilage matrix constituents, including collagen type II and aggrecan core protein, as well as matrix-producing hyaluronan synthases through enhanced expression, phosphorylation and nuclear signals of the main chondrogenic transcription factor Sox9. Along with increased cAMP levels, a significantly enhanced protein kinase A (PKA) activity was also detected and CREB, the archetypal downstream transcription factor of PKA signalling, exhibited elevated phosphorylation levels and stronger nuclear signals in response to mechanical stimuli. All the above effects were diminished by the PKA-inhibitor H89. Inhibition of the PKA-independent cAMP-mediators Epac1 and Epac2 with HJC0197 resulted in enhanced cartilage formation, which was additive to that of the mechanical stimulation, implying that the chondrogenesis-promoting effect of mechanical load was independent of Epac. At the same time, PP2A activity was reduced following mechanical load and treatments with the PP2A-inhibitor okadaic acid were able to mimic the effects of the intervention. Our results indicate that proper mechanical stimuli augment in vitro cartilage formation via promoting both differentiation and matrix production of chondrogenic cells, and the opposing regulation of the PKA/CREB-Sox9 and the PP2A signalling pathways is crucial in this phenomenon.

Diffusion-induced bending of thin sheet couples: Theory and experiments in Ti-Zr system

Numerical and analytical calculations of concentration and stress distributions of thin-sheet diffusion couples have been carried out as well as the time dependence of the Kirkendall shift, xk, and the curvature has also been determined. It is shown that the concentration distribution is not sensitive to the boundary conditions (bent and planar, constrained, samples) and is influenced mainly by the feeding back effects of stresses (described by the stress term in the genealized diffusion potential) only. The stress distributions obviously are different for bent and planar samples and the effect of cutting off, caused by the dislocation glide, is also illustrated. It is found that the Kirkendall shift follows the parabolic law only in high creep rate limit. For intermediate creep rates, as a function of the time, t, a change of the slope of the xk(t) function is expected due to the stress development and relaxation. It is shown that the curvature of samples, caused by the diffusion stresses, is proportional to the annealing time and the difference of the intrinsic diffusion coefficients in a wide range of input parameters. By the example of experiments on Ti-Zr thin-sheet diffusion couples it was illustrated that the theoretical results are in good agreement with the measurements.

High-temperature nitridation of Ni-Cr alloys

The nitriding behavior of nickel-chromium alloys was investigated at 1398 K over the range 1 to 6000 bar of external nitrogen pressure. The morphology of the nitrided zone depends on the concentration of chromium in the initial alloy and the N2 pressure (fugacity) applied upon the system. The transition from CrN to Cr2N precipitation was observed within the reaction zone after nitriding at 100 to 6000 bar of N2 when the chromium content in the initial alloys was 28.0 at. pct or higher. It is shown that the ternary phase π (Cr10Ni7N3) is formed in this system at 1273 K. through a peritectoid reaction between Cr2N and nickel solid solution and becomes unstable above 1373 K. The thermodynamic evaluation of the Ni-Cr-N system was performed and phase equilibria calculated. Evidence for “up hill” diffusion of nitrogen near the reaction front during the internal nitridation of Ni-Cr alloys at 1398 K was found. It was attributed to the relative instability of chromium nitrides and strong Cr-N interaction in the matrix of the Ni-based solid solution within the nitrided zone.

Production of hollow hemisphere shells by pure Kirkendall porosity formation in Au/Ag system

Interdiffusion has been studied in Ag/Au hemispherical core-shell structures on sapphire substrate. In isothermal heat treatments first a relatively fast growth of nanovoids was observed, which was followed by a slower shrinkage process. The void formation is interpreted by pure Kirkendall-porosity formation since Ag-50%Au solid solution has been formed in the shell. In contrary, in all previous publications on hollow nanoshell formation a chemical reaction took place and the shell consisted of the reaction product (i.e., of sulphide or oxide). Furthermore, in these cases the shrinkage was observed at temperatures higher than the formation temperature.

Linear growth kinetics of Nanometric silicides in Co/amorphous-Si and Co/CoSi/amorphous-Si thin films

Evolution of the reaction zone on the nanoscale has been studied in bi- and multilayered Co/a-Si as well as in trilayered Co/a-CoSi/a-Si and Co/CoSi/a-Si thin film diffusion couples. The kinetics of the phase boundary movement during solid state reaction has been followed with special interest of the initial stage of the diffusion, i.e. effects happening on the nanoscale (short time, short distance). The interfacial reactions have been investigated in situ by synchrotron radiation. The formed phases were also characterized by transmission electron microscopy and resistance measurements. The effect of phase nucleation and shift of phase boundaries have been separated in order to determine the “pure” growth kinetics of the crystalline CoSi and Co2Si product phases at the very early stages. Deviations have been found from the traditional diffusion controlled parabolic phase growth. Computer simulations based on a kinetic mean field model illustrated that the diffusion asymmetry (large difference in diffusion...

Electron beam-induced mass transport in As?Se thin films: compositional dependence and glass network topological effects

Electron beam (e-beam)-induced changes of surface profile morphology in AscSe1−c (0.2 <c< 0.5) thin films are investigated as a function of the film composition. It is shown that the extent and value of local surface alterations follow the composition-related changes of glass parameters such as softening temperature and glass network connectivity. The giant e-beam-induced surface relief changes detected in the films As0.2Se0.8 are connected with lateral mass transport, which increases drastically near rigidity transition, i.e. at a coordination number r ∼ 2.2 of the glass structures when the rigidity starts to percolate through the structure. The model of the process, which reflects the compositional dependence of the stimulated mass transport, is presented. (Some figures may appear in colour only in the online journal)

Tracer diffusion of 63Ni in Ni3(Al,Ge) ternary intermetallic compound

Abstract Radio-tracer diffusion measurements of 63Ni have been performed in Ni75AlxGe25−x ternary intermetallic compounds at various temperatures. The tracer diffusivity was found to depend exponentially on the Ge content of the alloy. These compounds are ordered with the L12 structure, where the Ni atom diffusion proceeds mainly via Ni sublattice site jumps. The change of the diffusivity can be attributed to the change in vacancy concentration on the Ni sublattice as well as to the composition dependence of the saddle-point energy of the diffusion jumps.

On the relation between interdiffusion and tracer diffusion coefficients in ternary solid solutions

This article is meant as a warning message for those scientists who are working in the area of ternary diffusion. This thorough experimental study has shown the large errors that arise when the four interdiffusion coefficients are calculated from crossing diffusion paths. Even larger is the scatter in values of the tracer diffusion coefficients derived from these interdiffusion coefficient. The practical value of these coefficients is probably low. The effects of theoretical improvements, like taking into account the vacancy wind, cannot be verified because of this large scatter in the experimental values.

On the Spatial Stability and Bifurcation of the Kirkendall Plane during Solid-State Interdiffusion

In a diffusion-controlled interaction, the Kirkendall plane, identified by inert particles placed at the initial interface between the reactants, need not be unique. The Kirkendall plane can microstructurally (spatially) be stable as well as unstable, and can, under predictable circumstances, bifurcate and even trifurcate. The movement of the Kirkendall markers during the interaction can be rationalized using the classical diffusion theory in terms of the Kirkendall velocity construction. The position of a Kirkendall plane is revealed in the reaction zone not only by the presence of inert markers, but also by a different crystal morphology developed on either side of the plane. The role of the Kirkendall plane in the morphogenesis of multiphase interdiffusion systems can be elucidated using equations of the interfacial reactions occurring in the diffusion zone. The appearance of one or more Kirkendall planes, characterized by morphology changes in the reaction layers is related to different nucleation sites of the product grains. The presence or absence of a Kirkendall plane in certain product phases provides insight into the initial stages of the reactive diffusion. Besides, the sometimes observed spatial (and temporal) patterns in a diffusion zone can be interpreted (and globally predicted) as a Kirkendall-effect mediated phenomenon. These conclusions will alter some previous notions about the diffusional growth of reaction layers and will influence the educational treatment in textbooks. It also will have strong technological implications, e.g., in the field of composite materials, thin-film electronic devices, etc.