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

Relative role of enthalpy and entropy in the stabilization of senary FCC Al–Co–Cr–Fe–Ni–Mn high entropy alloys was investigated via a high throughput combinatorial solid-to-solid diffusion couple approach. Many off-equiatomic compositions of FCC AlpCoqCrrFesNitMnu were generated by the diffusing Al and Ni in equiatomic Co20Cr20Fe20Ni20Mn20 alloy, i.e., the Al48Ni52 vs Co20Cr20Fe20Ni20Mn20 diffusion couple, annealed at 900°, 1000°, 1100°, and 1200 °C. Above 1000 °C, the solubility limit of Al in off-equiatomic AlpCoqCrrFesNitMnu alloy was determined to be higher than the solubility limit of Al in equiatomic AlxCoCrFeNiMn alloy. Compositions corresponding to the highest solubility limit of Al in off-equiatomic AlpCoqCrrFesNitMnu alloy exhibited a lower free energy of mixing, i.e., higher thermodynamic stability, than equiatomic AlxCoCrFeNiMn 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.