van Mjh Mark Dal

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.

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.

Formation of Co–Si intermetallics in bulk diffusion couples. Part II. Manifestations of the Kirkendall effect accompanying reactive diffusion

Abstract The Kirkendall effect-induced migration of inert markers during a diffusion-controlled growth of Co–Si intermetallics was studied at 1100°C. It is demonstrated experimentally that more than one Kirkendall marker plane can appear within the newly formed Co-silicide layers. It is also shown that, under specific conditions, a marker plane cannot develop at all in any of the product phases. The marker behaviour is discussed in terms of the velocity of the Kirkendall frame of reference relative to the laboratory-fixed (Matano) frame. A phenomenological approach is presented to predict the number and positions of the Kirkendall marker planes developing in a multiphase diffusion zone.

Diffusion studies and re-examination of the Kirkendall effect in the Au-Ni system

Abstract Diffusion studies were performed in the Au–Ni system over the temperature range 750–900°C. Interdiffusion coefficients were obtained by evaluating the concentration profiles of the annealed Au/Ni couples with the Sauer–Freise analysis. Intrinsic diffusivities D Au and D Ni in various Au–Ni alloys, experimentally determined at 900°C by means of multi-foil couples, were compared with those predicted from the model of Darken–Manning with the aid of the thermodynamic and tracer diffusion data. The experimental values of D Ni were close to those predicted by the model, while the intrinsic diffusivities of gold in the Au-rich alloys appeared to be considerably higher than the computed values. The experimental results are consistent with the relatively high Kirkendall-marker velocity within the zone of interdiffusion observed in the Au/Ni couples.

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.

Microstructology of solid-state reactions

The use of a combined thermodynamic and diffusion kinetic approach in predicting the product morphology developed during solid-state displacement reactions is exemplified by the interactions in the GaSb/Co and SiC/Me (where Me=Cr,Pt,Co) systems. The influence of mechanical stresses induced during the interaction on the formation of microstructures is demonstrated. It was shown that the manifestation of the effects accompanying reactive-phase formation in inorganic solids (like the Kirkendall effect and the net volume change during internal precipitation) causes the generation and relaxation of the stresses. This significantly contributes to the morphological evolution of the reaction zone. Different micro- and macrodefects (vacancies, dislocations, pores, cracks, etc.) that can be generated inside the diffusion zone can also affect (or even control) the course of the reaction and determine the topological arrangement of the product phases within the resulting microstructure.

The Kirkendall Effect in Multiphase Interdiffusion

The Kirkendall effect mediated migration of inert inclusions during multiphase diffusion can be rationalized using a Kirkendall velocity construction. It is demonstrated that the Kirkendall plane locations inside the diffusion zone can be identified by grain morphology changes within the microstructure of the reaction products. The latter is the result of different nucleation processes at both sides of the Kirkendall plane.

Deformation Phenomena Accompanying Internal Precipitation in Solids

Abstract. A volume increase associated with the internal nitriding of Ni–5 at.% Cr alloy at relatively low homologous temperatures and high nitrogen fugacities results in a stress gradient between the alloy surface and the precipitation front. Stress relief occurred mainly by transport of nickel (solvent) to the stress-free metal surface mediated by pipe diffusion-controlled creep. This mechanism is prevailing because of a high dislocation density developing within the parent Ni-matrix upon the precipitation of semi-coherent nitride particles. The CrN-precipitates was found to be topotactically oriented with respect to the matrix