A Mystery of "Sluggish Diffusion" in High-Entropy Alloys: The Truth or a Myth?

Abstract

High entropy alloys (HEAs) are considered as a novel class of materials with a large number of components (five and more) available in equiatomic or nearly equatomic proportions.One of the characteristic properties of HEAs was believed to be so-called 'sluggish' diffusion that should be crucial for intended high-temperature technological applications. The faith on this myth instead of rigorous experimental analysis played such a dominant role that the first set of data on interdiffusion, in fact based on an improper analysis, were cited in hundreds of articles to state the presence of sluggishness of diffusion rates in high entropy alloys.In this review, the recent data on atomic diffusion in HEAs are presented and critically discussed. The discussion is focused on tracer diffusion which is already measured dominantly for polycrystalline, but in some cases for single crystalline high-entropy alloys. The radiotracer technique provided a unique access to the diffusion rates of elements in the alloys and in fortunate cases these transport quantities could be measured in a single experiment, as in the case of Co, Cr, Fe and Mn diffusion in the CoCrFeMnNi HEA.Alternatively, a rigorous analysis of the interdiffuson experiments, which provide the diffusion rates of chemical species, too, becomes more and more sophisticated for three and more elements in an alloy and it is challenging to derive physically sound quantities from a general multicomponent diffusion experiment. Most promising in this case is the diffusion couple technique, especially the so-called pseudo-binary approach. This approach is analyzed with a focus on the applicability and the possible errors induced if up-hill diffusion appears.In the overview, it is shown that atomic diffusion in HEAs cannot a priori be considered as sluggish and both atomic interactions as well as correlation effects are responsible for the observed trends. Even if estimated on the same homologous scale, the diffusion retardation induced by a 'high entropy' in FCC crystals is not simply proportional to the number of alloying components and it is shown to be similar to that induced by, e.g., the L1 2 ordering in a binary system. Furthermore, the importance of cross-correlations in diffusion of different species in HEAs is highlighted. * Since there is no consensus about the way to list the principal elements in HEAs (alphabetic order, periodic table order, atomic percentage, etc), we will use the simple alphabetic order, as e.g. CoCrFeMnNi for the Cantor system.

State-of-the-Art Diffusion Studies in the High Entropy Alloys

“…--Review article -- [51] FCC-face-centered cubic, BCC-body-centered cubic, CALPHAD-CALculation of PHAse Diagrams.…”

Concentration-dependent atomic mobilities in FCC CoCrFeMnNi high-entropy alloys

“…2a reveals that we are dealing with a non-ideal pseudo-binary couple (in terms of Ref. 31 ) since the Ni and Mn concentrations in the nominally Co 25 CrFeMnNi 15 alloy deviate by less than 0.4 at.% from their nominal values. A remarkable feature is the appearance of up-hill diffusion in the concentration profiles of the nominally equiatomic constituents Cr, Fe and Mn.…”

“…In fact, this assumption was found to be correct in the framework of the random alloy model 30 . However, the basic principles of the analysis by Tsai et al 20 were seriously questioned recently 31 . The direct radiotracer measurements, being focused on measuring the bulk and short-circuit diffusion rates of the constituting elements in absence of any chemical interaction due to low diffusant concentrations, are preferable but typically limited to single compositions of the given alloy system.…”

Solving the issues of multicomponent diffusion in an equiatomic NiCoFeCr medium entropy alloy

“…In a non-ideal diffusion couple, the components which are kept with the same composition in both the end-members may develop a hump or uphill diffusion. In such a situation, this method cannot be utilized unless the non-ideality is minor [10,11,19]. Tsai et al utilized this method (although named it differently as the quasi-binary approach) [9] even in the presence of major non-ideality without showing the profiles except one ideal profile [20] and also without utilizing a proper step leading to the estimation of the data without any physical significance [11].…”