The effect of carbon on the microstructures, mechanical properties, and deformation mechanisms of thermo-mechanically treated Fe40.4Ni11.3Mn34.8Al7.5Cr6 high entropy alloys

Abstract

The effects of cold rolling followed by annealing on the mechanical properties and dislocation substructure evolution of undoped and 1.1 at. % carbon-doped Fe 40.4 Ni 11.3 Mn 34.8 Al 7.5 Cr 6 high entropy alloys (HEAs) have been investigated. X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atom probe tomography (APT) were employed to characterize the microstructures. The as-cast HEAs were coarse-grained and single phase f.c.c., whereas the thermo-mechanical treatment caused recrystallization (to fine grain sizes) and precipitation (a B2 phase for the undoped HEA; and a B2 phase, and M 23 C 6 and M 7 C 3 carbides for the C-doped HEA). Carbon, which was found to have segregated to the grain boundaries using APT, retarded recrystallization. The reduction in grain size resulted in a sharp increase in strength, while the precipitation, which produced only a small increase in strength, probably accounted for the small decrease in ductility for both undoped and C-doped HEAs. For both undoped and C-doped HEAs, the smaller grain-sized material initially exhibited higher strain hardening than the coarse-grained material but showed a 3 much lower strain hardening at large tensile strains. Wavy slip in the undoped HEAs and planar slip in C-doped HEAs were found at the early stages of deformation irrespective of grain size. At higher strains, dislocation cell structures formed in the 19 µm grain-sized undoped HEA, while microbands formed in the 23 µm grain-sized C-doped HEA. In contrast, localized dislocation clusters were found in both HEAs at the finest grain sizes (5 µm). The inhibition of grain subdivision by the grain boundaries and precipitates lead to the transformation from regular dislocation configurations consisting of dislocationcells and microbands to irregular dislocation configurations consisting of localized dislocation clusters, which further account for the decrease in ductility. Investigation of the formation mechanism and strain hardening of dislocation cells and microbands could benefit future structural material design.

Recrystallized microstructures and mechanical properties of a C-containing CoCrFeNiMn-type high-entropy alloy

“…3-8). Recrystallization of the fcc solid solution after annealing in the examined temperature range can be readily anticipated taking into account available information on alloys with similar compositions [20,38,42,[51][52][53]. However, some features of the carbide particles precipitation required additional consideration.…”

“…Meanwhile, interstitial alloying with elements like carbon can efficiently increase the strength of 3d transition metal fcc HEAs without considerable loss in ductility; mostly due to a strong solid-solution strengthening effect [9,[35][36][37][38][39][40][41][42][43][44][45]. In addition, carbon can change the contributions of operating deformation mechanisms due to stackingfault energy (SFE) changing.…”

“…Much smaller attention has been paid so far to precipitation hardening of HEAs by fine carbide particles. For example, Cr-rich M 23 C 6 /M 7 C 3 carbides can be found in Co-Cr-Fe-Ni-Mn system HEAs in as-cast, heat treated, and thermo-mechanically processed conditions [9,35,42]; yet in most cases the reported carbide particles were coarse and did not result in noticeable strengthening [36,42,46].…”

“…The program alloy was produced by the self-propagating high-temperature synthesis method (SHS) [14,32,47]. Annealing of the cold-rolled alloy was carried in a temperature range of 973-1373 K, which, according to previous results [20,22,35,36,38,42,48], could lead to recrystallization of the fcc matrix and/or precipitation of carbide particles. Detailed microstructural investigations and tensile testing of the annealed alloy were performed, and links between the microstructure parameters and mechanical properties of the alloy were established.…”

Impact of interstitial C on phase stability and stacking-fault energy of the CrMnFeCoNi high-entropy alloy

“…Another promising direction to further improve the mechanical properties of HEAs is interstitial alloying, e.g., with C. The impact of interstitial C atoms in HEAs has been investigated in several previous experimental studies [10,16,18,[20][21][22][23][24][25][26][27]. In equiatomic CrMnFeCoNi, the addition of C atoms is known to increase the yield and the ultimate strengths [18,20,24] while also impacting ductility.…”