Studies of "sluggish diffusion" effect in Co-Cr-Fe-Mn-Ni, Co-Cr-Fe-Ni and Co-Fe-Mn-Ni high entropy alloys; determination of tracer diffusivities by combinatorial approach

Highlights: • Diffusion studies in quinary and quaternary high entropy alloys are presented. • Combinatorial approach is used for determination of diffusivities in quinary HEAs. • The determined values are used to simulate diffusion profiles in quaternary systems. • The quality of fit to experimental data is almost the same as for the quinary system. • The change of components number did not influence the diffusion kinetics. 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.