Metastability-engineering strategy in CoCrFeMnNi-based medium- and high-entropy alloys: Unraveling the interplay with recrystallization, grain growth, and mechanical properties

Abstract

Novel face-centered cubic (FCC) phase CoCrFeMnNi-based medium- and high-entropy alloys (M/HEAs) with the following nominal compositions Co15Cr15Fe50Mn10Ni10 (Co15Cr15), Co20Cr20Fe40Mn10Ni10 (Co20Cr20), and Co25Cr25Fe30Mn10Ni10 (Co25Cr25) in at.%, were designed via metastability-engineering strategy to trigger different deformation mechanisms, such as twinning-induced plasticity (TWIP) and/or transformation-induced plasticity (TRIP). Both mechanisms are governed by the stacking fault energy (SFE), which depends on composition. Fully recrystallized samples with different grain sizes ranging from 2.7 to 102.5 µm were obtained. Tensile tests were conducted at room temperature (298 K), and Hall-Petch relationships were established. The annealed and deformed samples were characterized by a combination of electron backscatter diffraction (EBSD), high-energy synchrotron X-ray diffraction (HE-SXRD), and transmission electron microscopy (TEM) to correlate deformation microstructures with phase stability. It was revealed that grain refinement was more effective in the Co25Cr25 alloy, given by the high Hall-Petch coefficients (kHP = 516 MPa.µm1/2 and σ0 = 198 MPa). For a grain size of 2.7 µm, the product of yield strength (∼500 MPa) and uniform elongation (∼45 %) in the Co25Cr25 alloy reaches its maximum (∼23 GPa%), achieving the optimal strength-ductility synergy. Due to the decrease in the effective SFE (from 26.6 to 3.5 mJ m-2), a transition in the dominant deformation behavior occurred from TWIP (Co15Cr15) to TWIP/TRIP (Co20Cr20) and finally to TRIP (Co25Cr25). The calculations further showed that kHP and the total dislocation density exhibit an inverse relationship with the effective SFE. Such findings highlight the potential of compositional tuning for developing high-performance M/HEAs with designed deformation mechanisms.