Anomalous Evolution of Strength and Microstructure of High‐Entropy Alloy CoCrFeNiMn after High‐Pressure Torsion at 300 and 77 K

Abstract

Ultrafine and nanocrystalline states of equiatomic face‐centered cubic (fcc) high‐entropy alloy (HEA) CoCrFeNiMn (“Cantor” alloy) are achieved by high‐pressure torsion (HPT) at 300 K (room temperature, RT) and 77 K (cryo). Although the hardness after RT‐HPT reaches exceptionally high values, those from cryo‐HPT are distinctly lower, at least when the torsional strain lies beyond γ = 25. The values are stable even during long‐time storage at ambient temperature. A similar paradoxal result is reflected by torque data measured in situ during HPT processing. The reasons for this paradox are attributed to the enhanced hydrostatic pressure, cryogenic temperature, and especially large shear strains achieved by the cryo‐HPT. At these conditions, selected area electron diffraction (SAD) patterns indicate that a partial local phase change from fcc to hexagonal close‐packed (hcp) structure occurs, which results in a highly heterogeneous structure. This heterogeneity is accompanied by both an increase in average grain size and especially a strong decrease in average dislocation density, which is estimated to mainly cause the paradox low strength.

Microstructure, Texture, and Strength Development during High-Pressure Torsion of CrMnFeCoNi High-Entropy Alloy

“…The spotty TEM diffraction pattern for HPT at LNT in comparison to that at RT [4] indicates that with decreasing temperature deformation becomes more heterogeneous. It should be mentioned that despite of differences in the production of the starting HEA material resulting in different grain sizes and probably slightly different textures, and differences in HPT parameters (sample size, hydrostatic pressure, rotation speed), the steady-state crystallite sizes and dislocation and twin densities as well as microhardness values reported agree quite well with those of the present study, both for HPT at RT [23,24] and LNT [23]. An exception are the results given in [25], where evidently the steady-state has not been reached.…”

“…Comparing these values with σ max it is concluded that the yield stress of the nanocrystalline Cantor alloy at RT is predominantly determined by dislocation-dislocation interaction. A similar conclusion has been drawn by Podolskiy et al [23] and Heczel et al [24]. Moreover, Podolskiy et al [23] argued that the smaller strength at cryogenic HPT processing can be understood as an indirect consequence of the shear-induced FCC to HCP martensitic phase transformation.…”

“…In addition, based on TEM and texture investigations the microstructure is quite inhomogeneous. The large scatter in local nanohardness measured by Podolskiy et al [23] supports this statement. Moreover, the material consists of a certain volume fraction of HCP phase after HPT at LNT.…”

“…A plausible reason would be recovery during "self-annealing", as was found for Cr 26 Mn 20 Fe 20 Co 20 Ni 14 [33] in combination with a lowering of the microhardness. However, the same effect has not been observed for the Cantor alloy [23]. Therefore, it has been suggested, that the shear produced by the deformation-induced HCP phase formation leads to a reduction in strain and, thus, to a reduction in dislocation density in the FCC phase.…”

“…However, the inhomogeneity of the microstructure may be just a low temperature effect. For more speculations about the strength anomaly and special experiments to prove its existence under in-situ HPT conditions, the reader is referred to the recent work of Podolskiy et al [23].…”