  Until recently, it was textbook knowledge that the diffusion coefficients
could not be estimated in a multi-component system following the widely
practised diffusion couple method. The recently proposed constrained diffusion
couple methods need two intersecting diffusion paths in a multi-component space
with very well-controlled compositions of the diffusion couple end members,
which may be tricky depending on the complications of diffusion paths in
certain systems. In this study, we have proposed a method for estimating all
types of diffusion coefficients directly at the Kirkendall marker plane from a
single diffusion couple. The estimation method and design strategy for
producing diffusion couples are discussed in concentrated ternary Ni-Co-Fe,
Fe-rich quaternary Fe-Ni-Co-Cr and Ni-rich Ni-Co-Fe-Cr-Al quinary alloys. As
demonstrated further, one can even estimate the impurity diffusion coefficients
utilizing the composition profiles at the ends of the diffusion couples, which
has been rarely practised in multicomponent systems until now. We have further
demonstrated the importance of estimating the tracer and intrinsic diffusion
coefficients in a concentrated or multi-principal element alloy in which
interdiffusion coefficients can be vague and misleading for understanding the
elements' diffusional interactions and relative mobilities. We have also shown
the importance of considering the vacancy wind effect in concentrated alloys.
The method and design strategy of producing diffusion couples demonstrated in
this study can be suitable for generating a mobility database in Ni-, Co-based
(superalloys) and Fe-based (steel) multicomponent systems with relative ease,
which was considered impossible until recently.
