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Some long-standing problems concerning the diffusion in open systems are resolved in the context of Darken and Wagner methods with Nernst–Planck flux. Explicitly, the known results on the reactive and interdiffusion processes are significantly extended as follows: (1) the diffusion fluxes are defined in the volume fixed reference frame that is rigorously defined, (2) the Darken method is extended and allows analyzing the nonideal alloys showing different molar volumes, (3) the boundary conditions take into account both the interdiffusion and reactive diffusion, and (4) the flux constraints are used to couple processes occurring at different time scales. We demonstrate solutions of the interdiffusion in the Fe-Ni-Cu alloy and heterogeneous reaction between the multicomponent alloy and oxidant. The interdiffusion fluxes can be computed starting from the Fick’s constitutive diffusion flux formula to the combined Darken–Onsager approach. This method allows modeling pure interdiffusion in multicomponent systems as well as a wide class of the reaction-diffusion problems associated with solid-state chemistry, corrosion, surface treatment, etc. [ABSTRACT FROM AUTHOR]

Abstract: The duration of the selective oxidation period is a key factor determining the life time of the corrosion resistant alloys. Diffusion processes during the selective oxidation of an alloy containing a noble metal (Pt, Au) and a oxidizable metal (Ni, Cu or Zn) were theoretically analyzed by C. Wagner. He had shown the analytical method to calculate the oxidation rate (growth of the scale) as a function of initial concentration of the reacting metal. Wagner simplified the problem of alloy oxidation assuming the constant concentration at the alloy∣oxide interface. In this work we extend the Wagner model by introducing (i) the time dependence of the reacting metal concentration at the alloy∣oxide interface and the instantaneous rate constant, (ii) the composition dependent diffusivities of the alloy components and (iii) the finite geometry of the oxidized alloy. The model can be applied to predict the life time, i.e., the critical consumption of the reacting element and the evolution of the components concentration in alloy and at its boundaries. We present the comparison of Wagner’s results and our extended analysis. [Copyright &y& Elsevier]

The development of the high entropy alloys (HEAs) is amongst the most important topics in the field of materials science during the last two decades. The concept of multicomponent, near-equimolar systems has been already applied to the number of other systems, including oxides, carbides, diborides, silicides, and it can be expected that other groups of materials will follow. One of the main driving forces for the development of HEAs is the so-called "four core effects": high entropy effects, severe lattice distortion, cocktail effect, and sluggish diffusion effect. Their existence and extent has been a subject of heated discussion. Probably the least studied of them is the sluggish diffusion effect, which is of the, especially, high importance from the point of view of the most possible applications of HEAs—as high-temperature materials. Its alleged existence carries a promise of obtaining materials with superior mechanical properties, higher creep resistance, and less susceptibility to high-temperature corrosion. In the current review, the state-of-the-art of diffusion studies in HEAs was presented, as well as the resulting conclusions concerning the existence of the sluggish diffusion effect. Based on the literature analysis, it can be stated that there is no experimental evidence, which would support the existence of the sluggish diffusion in HEAs on the level of tracer and self-diffusivities. Nevertheless, it can be pointed out that our current state of knowledge on the diffusion in HEAs is still far from complete; therefore, further directions of studies are proposed. [ABSTRACT FROM AUTHOR]

A new method, which allows determination of intrinsic diffusivities values on the Kirkendall plane in multicomponent systems, is presented. The approach is based on the zero contribution of drift flux to the total flux value of i -th component, when calculated relatively to the Kirkendall plane position. The method allows precise determination of intrinsic diffusivities without limitations of Boltzmann-Matano based methods e.g. in situations when uphill diffusion occurs. The approach is validated on a number of theoretical and experimental systems. The results indicate that the method have a very high potential in application for multicomponent systems, including high entropy alloys. [ABSTRACT FROM AUTHOR]

Experimental and theoretical studies of diffusion in quinary Co-Cr-Fe-Mn-Ni and quaternary Co-Cr-Fe-Ni and Co-Fe-Mn-Ni FCC-structured high entropy alloys were performed. The diffusion couples, with thorium dioxide markers placed at initial joint positions, were annealed at temperature of 1350 K for 72 or 73 h. The concentration profiles obtained from the quinary system were used to determine tracer diffusivities of all components, by using a combinatorial approach, i.e. the Darken method with thermodynamic description provided by Miedema's scheme combined with the optimization method. The results showed good qualitative agreement with the tracer data from radiotracer experiments. The calculated thermodynamic factor, ranged from tens to hundreds of %, show importance of mixing enthalpy on interdiffusion kinetics. The values determined for 5-component system were then used a priori to simulate the concentration profiles for 4-component ones, showing very good agreement with the experimental data. The results indicate that change of components number did not influence the diffusion kinetics in the investigated systems and do not support existence of the sluggish diffusion effect. [ABSTRACT FROM AUTHOR]