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

Abstract

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 Al_<i>p</i>Co_<i>q</i>Cr_<i>r</i>Fe_<i>s</i>Ni_<i>t</i>Mn_<i>u</i> 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 Al_<i>p</i>Co_<i>q</i>Cr_<i>r</i>Fe_<i>s</i>Ni_<i>t</i>Mn_<i>u</i> alloy was determined to be higher than the solubility limit of Al in equiatomic Al_<i>x</i>CoCrFeNiMn alloy. Compositions corresponding to the highest solubility limit of Al in off-equiatomic Al_<i>p</i>Co_<i>q</i>Cr_<i>r</i>Fe_<i>s</i>Ni_<i>t</i>Mn_<i>u</i> alloy exhibited a lower free energy of mixing, i.e., higher thermodynamic stability, than equiatomic Al_<i>x</i>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.