Design, production and advanced characterization of CoCrFeMnNi-based medium- and high-entropy alloys: understanding the interplay with composition, microstructure, and mechanical properties

Abstract

Worldwide, a great research effort is being employed in the development of Medium/high entropy alloys (M/HEAs). Among the M/HEAs produced to date, great emphasis is given to the equiatomic Co20Cr20Fe20Mn20Ni20 (at.%) alloy due to its good combination of strength and ductility. Studies in this field are increasingly being expanded to non-equiatomic compositions, and there is great opportunity in the development of M/HEAs with excellent mechanical properties. Therefore, based on thermodynamics calculations (CALPHAD method) novel non-equiatomic CoCrFeMnNi M/HEAs were designed aiming at obtaining optimized mechanical properties, tailoring deformation behaviors (TRIP-TWIP) via constituent concentrations. A total of 3 non-equiatomic M/HEAs (Co15Cr15Fe50Mn10Ni10, Co20Cr20Fe40Mn10Ni10, and Co25Cr25Fe30Mn10Ni10) were produced; the alloys were then annealed, thermomechanically processed, characterized, and mechanically tested by tensile tests. The annealed and deformed samples were characterized by a combination of electron backscattered diffraction (EBSD), high-energy synchrotron X-ray diffraction (HE-SXRD), and transmission electron microscopy (TEM). It was revealed that increased Co and Cr reduced stacking fault energy (SFE), increasing recrystallization rate and suppressing grain growth rate. Grain refinement was more effective in the Co25Cr25 alloy, where the optimal strength-toughness occurred. Due to the decrease in SFE, a transition in the dominant deformation behavior occurred from TWIP (Co15Cr15) to TWIP/TRIP (Co20Cr20) and finally to TRIP (Co25Cr25), with the TRIP being more pronounced in Co25Cr25. This work sheds light on the development of novel FCC M/HEAs from the CoCrFeMnNi system, by identifying alloys that exhibit an optimal strength-ductility balance.