Interdiffusion, Solubility Limit, and Role of Entropy in FCC Al-Co-Cr-Fe-Ni Alloys
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s (HEAs) were proposed to exhibit four ‘‘core’’ effects: high entropy, sluggish diffusion, lattice distortion, and cocktail effect.[1] Except for the cocktail effect, all other core effects may not be significant as were first proposed.[2] The mechanism of stabilization of single phase, i.e., the high entropy effect, initially hypothesized that a large number of constituent elements in an equal amount would increase the entropy of mixing, which would lower the overall Gibb’s free energy of mixing, particularly at high temperatures. This is true as the entropic contribution is typically higher than the enthalpy contribution toward the overall thermodynamic stability of HEAs, i.e., | TDS| > |DH|, at high temperature. Therefore, high entropy
ABHISHEK MEHTA and YONGHO SOHN are with the Department of Materials Science and Engineering, Advanced Materials Processing and Analysis Center, University of Central Florida, Orlando, FL 32816. Contact e-mail: [email protected] Manuscript submitted October 10, 2019. Article published online April 19, 2020 3142—VOLUME 51A, JUNE 2020
phases, e.g., random/disordered solid solution phases, would be stabilized in comparison to low entropy phases, e.g., intermetallic phases. The theory on entropic stabilization of phases due to high configurational entropy, however, falls short in regard to explaining the multiple phases observed in various experimental, near-equiatomic alloys, e.g., AlCoCrFeNi,[3] AlCoCrFeNiMn,[4] and CoCrFeNiMo.[5] Intuitively, a simple replacement of an element in an HEA by another element would not ensure the formation of single-phase solid solution, e.g., replacing Mn with either Al or Mo in single-phase, equiatomic CoCrFeNiMn alloy. Therefore, entropy of mixing alone may not always result in lowering of the Gibbs free energy.[6] Otto et al.[7] also suggested that an increase in configurational entropy may not stabilize the single phase in all alloys, since this effect may not be sufficient to overcome the driving forces that favor the formation of secondary phases. The diffusion in HEAs was hypothesized to be sluggish.[8] Various studies[8–15] have been carried out to determine the tracer diffusion coefficients. There has been no common consensus on the sluggish diffusion hypothesis: Some studies reported that diffusion is
METALLURGICAL AND MATERIALS TRANSACTIONS A
indeed sluggish in HEAs, while others did not. In potential engineering applications where diffusion may occur under the concentration gradients, interdiffusion coefficients that combine the influence of both the thermodynamics and kinetics may be more relevant. Limited studies[12,16,17] have examined the interdiffusion in HEAs; however, no relevant comparison was made to elucidate the possible ‘‘sluggish diffusion’’ in HEAs. In this study, two core effects, i.e., high entropy and sluggish diffusion, were examined in Al-Co-Cr-Fe-Ni alloys using solid-to-solid diffusion couple investigation as it can generate many local equilibrium compositions. The b-Al48Ni52 vs Co25Cr25Fe25Ni25 diffusion couple was annealed at 900 C, 1000 C
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