Aa Alexander Kodentsov

The diffusion couple technique in phase diagram determination

The diffusion couple technique is a valuable experimental approach in studying phase relations in multicomponent systems. The use of different modifications of the method is illustrated by examples of phase diagram determination in various ternary systems. It is also shown that a number of error sources may appear when multiphase diffusion experiments are employed for constructing isothermal cross-sections. The difficulties connected with the concentration measurement at the interfaces and problems associated with the formation of a quasi-equilibrated diffusion zone are discussed. It is demonstrated that the efficiency of the diffusion couple technique is very high. However, in order to increase the reliability of the information obtained about a ternary diagram, a combination of the diffusion methods with an investigation of selected alloys is desirable.

Intrinsic diffusion and Kirkendall effect in Ni-Pd and Fe-Pd Solid solutions

Abstract Intrinsic diffusion and the Kirkendall effect in the Ni–Pd (at 900–1200°C) and Fe–Pd (at 1100°C) solid solution systems were investigated. The diffusion couple technique including incremental and “multi-foil” couples was employed. A theoretical analysis of the Kirkendall effect, which manifests itself by migration of inert markers inside the interdiffusion zone, was performed for a binary solid solution system. It was demonstrated that depending upon the relative mobilities of the components in different parts of the interaction zone of such binary diffusion couples, the appearance of two or more “Kirkendall” planes as marked by inert particles can be expected. This phenomenon, which indeed was predicted and found in the multiphase Ni/Ti diffusion couple, was not observed in the experiments on the single-phase Ni–Pd and Fe–Pd systems. The diffusion process in these binary systems exhibiting a minimum in the liquidus curve was found to show special features with respect to the concentration dependence of the diffusion coefficients.

Intermetallic growth and Kirkendall effect manifestations in Cu/Sn and Au/Sn diffusion couples

Abstract The Kirkendall-effect induced migration of inert markers during a diffusion-controlled growth of intermetallic compounds in the Cu/Sn and Au/Sn couples at 215 and 180°C were studied. It was shown that the behaviour of markers in the multiphase reaction zones can be rationalized in terms of the Kirkendall velocity construction. Observations on the microstructural features of the product intermetallic layers and the role of the Kirkendall effect in the morphogenesis of the interdiffusion systems are discussed. It was demonstrated that the velocity of markers in a product layer, the appearance of the Kirkendall plane(s), their location(s) and the morphological evolution of the reaction products can also be explained from a purely chemical point of view considering the diffusion-controlled interactions at the interphase interfaces. A representation of the reaction scheme and kinetics of intermetallic growth in each of the two systems is given.

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.

Formation of Co–Si intermetallics in bulk diffusion couples. Part I. Growth kinetics and mobilities of species in the silicide phases

Abstract Diffusion couples, in which one single-phased layer of Co-silicide is growing from its saturated adjacent phases, were employed to study diffusion properties of the Co–Si intermetallics over the temperature range 914–1217°C. The position of the Kirkendall marker plane inside the reaction zones revealed that in this temperature interval Co is by far the fastest diffusing element in the Co 2 Si-intermetallic, the intrinsic diffusivities of the components are practically equal in the cobalt disilicide and that Si is virtually the only mobile species in the monosilicide CoSi. The concept of integrated diffusion coefficient is used to describe the growth kinetics of the intermetallic compounds. The integrated diffusion coefficient in an intermetallic is related to the tracer diffusivities of the components and the thermodynamic stability of the phases involved in the interaction. The tracer diffusion coefficients of the elements in the CoSi 2 - and CoSi-phases were obtained at various temperatures in the temperature range studied. The results are consistent with the customary Arrhenius relationship.

Phase equilibria in the Cu-Fe-Ti system at 1123 K

The isothermal cross-section through the ternary phase diagram Cu-Fe-Ti at 1123 K was constructed by means of a special diffusion technique developed earlier. The results have been verified on essential points by the investigation of equilibrated alloys. Five ternary intermetallic compounds designated as T~ (Ti33Cu67_xFex; 1<x<2.5), Tz (Ti40Cu~_xFex; 5<x< 17), T3 (Ti43Cus7__xFeA 21 <x < 24), ~-(Ti37Cu63_xFeA 6 <x < 7) and z~ (Ti45Cu55_xFeA 4 <x < 5) exist as equilibrium phases in this system at 1123 K. Nearly 38 at.% Cu can be dissolved in cubic TiFe. This leads to an expansion of the lattice. It was found that at 1123 K the Cu-based solid solution is in equilibrium with intermetallic compounds of the binary Ti-Fe system and with the terna=y phases T2 and T3.

The brazing of Si3N4 with Ni-Cr-Si alloys

The use of commercially available and laboratory made Ni-Si-Cr brazes for wetting and bonding Si3N4 has been explored. The microstructure and chemistry of brazed joints has been characterized by techniques such as optical microscopy, scanning electron microscopy, electron probe micro-analysis and X-ray diffraction. Si3N4 was wetted well by all the braze alloys at 1150–1200 °C, but the joint quality obtained was poor because of the formation of brittle phases and solid solutions.

Pattern formation in Pt-SiC diffusion couples

Abstract The morphology of reaction zones between platinum metal and silicon carbide ceramic at 973 and 1023 K is considered in detail. A periodic pattern of carbon bands embedded in Pt7Si3 is observed at both temperatures. The growth of platinum silicides at 973 and 1023 K is compared. Cross-sections of the Pt-Si-C phase diagram at those temperatures are presented. The periodic layered morphology is explained via a ‘repeated splitting’ mechanism.

Periodic pattern formation in solid state reactions related to the Kirkendall effect

Abstract The periodic layered morphology for solid state reactions is shown to occur in various ternary and higher order material systems including metal/metal and metal/ceramic diffusion couples. The experimental results on different systems where the periodic pattern formation has been observed are systematized and early explanations for this peculiar phenomenon are discussed. It is shown that a special class of structures periodic in time and in space can be considered as a manifestation of the Kirkendall effect accompanying reactive phase formation in the solid state.

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.