Abstract
High-entropy alloys have four core effects: high entropy, sluggish diffusion, severe lattice distortion, and cocktail. The aim of this study is to demonstrate high entropy and sluggish diffusion effects in a quantitative way. High entropy effect has been found to enhance the formation of solid solutions whereas large atomic size difference and large negative mixing enthalpy between unlike atom pairs have been found to enhance the formation of ordered phases. In order to gain more understanding of such an order-disorder competition, this study proposes two parameters to analyze the correlation between alloy compositions and phase types. One is the modified thermodynamic parameter ε which represents the competition between mixing entropy and mixing enthalpy, while the other is the topological parameter δ which represents the atomic size difference. From the analyses of these two parameters of published high-entropy alloys, the well-defined criteria for the formation of random solid solutions and ordered phases are obtained. When δ is small and ε is large (i.e. the effect of mixing entropy dominates over that of mixing enthalpy), alloys tend to form random solid solutions. On the other hands, when ε < 1.1 (i.e. the effect of mixing enthalpy dominates over that of mixing entropy), alloys tend to form ordered phases. Furthermore, when δ is too high to retain simple structures alloys will form intermetallic phases with more complex structures. These criteria could provide a useful guideline for alloy design of high-entropy alloys. Beside, this study directly confirms the sluggish diffusion phenomenon by the measurement of diffusion parameters for the Co-Cr-Fe-Mn-Ni alloys using a quasi-binary diffusion couple method. Comparing the diffusion parameters of the five component elements measured in the present HEAs with that in the reference FCC metals, it can be found that the diffusion coefficients decrease with the number of constituent elements in the matrix, whereas the normalized activation energies Q/Tm increase with the number of constituent elements. These tendencies are certainly the direct evidences of the sluggish diffusion effect in HEAs. The mechanism behind such effect has also been proposed. The fluctuation of lattice potential energy (LPE) was calculated using quasichemical model. The larger LPE fluctuation in the whole-solute matrix of HEAs provides abundant sites with lower potential energy, which become the traps of atoms and cause higher normalized activation energies and lower diffusion rate.