High Entropy and Sluggish Diffusion "Core" Effects in Senary FCC Al-Co-Cr-Fe-Ni-Mn Alloys

Area of Science:

• Materials Science
• Metallurgical Engineering
• Thermodynamics

Background:

• High entropy alloys (HEAs) are typically stabilized by configurational entropy.
• Understanding the interplay of enthalpy and entropy is crucial for designing advanced HEAs.
• Senary (six-element) alloys offer complex phase behavior and potential for unique properties.

Purpose of the Study:

• To investigate the relative contributions of enthalpy and entropy to the stabilization of senary FCC Al-Co-Cr-Fe-Ni-Mn HEAs.
• To explore the thermodynamic stability of off-equiatomic versus equiatomic compositions.
• To analyze diffusion behavior and phase formation in these complex alloys.

Main Methods:

• Utilized a high-throughput combinatorial solid-to-solid diffusion couple approach.
• Generated numerous off-equiatomic compositions by diffusing Al and Ni into an equiatomic Co$_{20}$Cr$_{20}$Fe$_{20}$Ni$_{20}$Mn$_{20}$ alloy.
• Annealed diffusion couples at temperatures ranging from 900 °C to 1200 °C.

Main Results:

• Above 1000 °C, off-equiatomic alloys showed higher Al solubility and lower free energy of mixing, indicating greater thermodynamic stability.
• Enthalpy was identified as a significant factor in achieving this enhanced stability in off-equiatomic alloys.
• Sluggish diffusion was not observed in FCC Al-Co-Cr-Fe-Ni-Mn alloys, but was present in BCC counterparts (except for Ni).

Conclusions:

• Enthalpy plays a critical role in the stabilization of senary FCC Al-Co-Cr-Fe-Ni-Mn high entropy alloys, particularly in off-equiatomic compositions.
• The findings suggest that entropy alone may not be the sole driver for stabilization in all HEA systems.
• All FCC compositions adhered to established empirical single-phase formation rules for high entropy alloys.