High Entropy and Sluggish Diffusion “Core” Effects in Senary FCC Al–Co–Cr–Fe–Ni–Mn Alloys

Abstract

alloy exhibited a lower free energy of mixing, i.e., higher thermodynamic stability, than equiatomic Al x CoCrFeNiMn compositions, at 1100 °C and above. Therefore, the role of enthalpy was estimated to be significant in achieving higher thermodynamic stability in off-equiatomic alloys, since they always have lower entropy of mixing than their equiatomic counterparts. The magnitude of interdiffusion coefficients of individual elements in Al−Co−Cr−Fe−Ni−Mn alloys were compared to the interdiffusion coefficients in relevant quinary, quaternary, and ternary solvent-based alloys. Interdiffusion coefficients were not necessarily lower in FCC Al−Co−Cr−Fe−Ni−Mn alloys; therefore no sluggish diffusion was observed in FCC HEA, but diffusion of individual elements in BCC Al−Co−Cr−Fe−Ni−Mn alloy followed the sluggish diffusion hypothesis except for Ni. All compositions in the FCC Al−Co−Cr−Fe−Ni−Mn alloy were observed to comply with existing empirical single phase formation rules in high entropy alloys.

Investigation of sluggish diffusion in FCC Al0.25CoCrFeNi high-entropy alloy

“…whereD eff i is the average effective interdiffusion coefficient of component i for the composition range, C i (x 1 ) to C i (x 2 ). The detailed procedure for the determination ofD eff i and limitations of using the Boltzmann-Matano method extended to a multi-component system to determine the main and cross interdiffusion coefficients, specifically for HEAs, have been documented previously by Mehta and Sohn [15,16]. Figure 1(a-d Tracer diffusion coefficient can be measured using the analytical formalism proposed by Belova et al [26]: where a is a constant and the value of G 2 can be determined using:…”

“…spike profile) overlaid on each other, and the corresponding Gaussian fitted profile obtained after mathematical subtracting the spike profile from the interdiffusion profile at 900°C, 950°C and 1000°C. [20], CoCrFeNiMn [20], AlCoCrFeNi [15], and AlCoCrFeNiMn [16] FCC alloys. alloys.…”

“…Other experimental investigations [10][11][12][13] also suggested that sluggish diffusion kinetics may not be true in CoCrFeNi and CoCrFeNiMn HEAs. Few interdiffusion investigations [14] supported the sluggish diffusion effects, while others [15][16][17][18][19] suggest that the sluggish diffusion effect cannot be generalized for all HEAs.…”

“…Direct experimental measurement of tracer diffusion coefficients was previously performed in CoCrFeNi and CoCrFeNiMn alloys [9] and experimental measurement of interdiffusion coefficient was performed in CoFeMnNi [17], CoCrMnNi [17], CoCrFeNi [20], CoCrFeNiMn [20], AlCoCrFeNi [15], AlCoCrFeNiMn [16], CoCr-CuFeNi [21], AlCoCrFeTiNi [22] alloys. Few other investigations had either numerically estimated the tracer diffusion coefficients using interdiffusion data, for example in Al-Co-Cr-Fe-Ni [14,19], Co-Fe-Mn-Ni [17], Co-Cr-Mn-Ni [17], Co-Cr-Cu-Fe-Ni [21], Al-Co-Cr-Fe-Ti-Ni [22] alloy, or suggested that intrinsic diffusion coefficient is similar to tracer diffusion coefficient based on ideal solution behavior in CoCrFeNiMn 0.5 alloy [6].…”

Measurement of Interdiffusion and Tracer Diffusion Coefficients in FCC Co-Cr-Fe-Ni Multi-Principal Element Alloy

“…Thermodynamically, HEAs would have a high configurational entropy of mixing, and therefore form in a simple solid solution instead of complex low entropy, enthalpy-driven intermetallic phases. [1] However, various researchers [2][3][4] had previously cast doubt on this theory of entropic stabilizations and suggested that the role of enthalpy of mixing cannot be neglected in the stability of a single phase. It has also been observed that prolonged annealing of HEAs (e.g., CoCrFeNiMn) can result in decomposition at intermediate temperatures (* 500-900 °C).…”