Extraordinary ductility and strain hardening of Cr26Mn20Fe20Co20Ni14 TWIP high-entropy alloy by cooperative planar slipping and twinning

High-entropy alloys (HEAs), containing at least five major metal elements in equal or near equal atomic ratios, have drawn increasing attention because they open entirely new materials avenues for designing alloys with exceptional properties. In the literature, a well-studied equiatomic, face-centered cubic CrMnFeCoNi HEA reportedly exhibits a yield strength of 410 MPa and a ductility of 57% as well as a deformation mechanism of dislocation slip at room temperature [B. Gludovatz, et al., Science, 345 (2014) 1153-1158]. Some recent works also observed that twinning actually happens more or less in the equiatomic CrMnFeCoNi alloy at room temperature [Z.J. Zhang, et al., Nature Comm. 6 (2015) 10143]. In this study, we prepared a non-equiatomic, face-centered cubic Cr26Mn20Fe20Co20Ni14 HEA with a comparatively low stacking fault energy (SFE) by making an appropriate adjustment of the composition ratio. Our HEA has a yield strength of 180 MPa and a high strain hardening exponent, n, of 0.46 as well as a higher ductility (73%) than those of the CrMnFeCoNi alloy. Investigation of the deformation mechanisms at specific strain levels revealed a clear transition from planar slip dislocations in the initial deformation stage to twinning at high tensile strain. Cooperative planar slipping and twinning resulted from the comparatively low SFE and were responsible to the extraordinary ductility and strain hardening capability. Besides deformation twins, other hardening mechanisms including forest dislocations, sessile Lomer-Cottrell locks, dislocation-stacking fault interactions, sub-grain boundary and phase boundary were revealed.