Anomalous work hardening behavior of Fe40Mn40Cr10Co10 high entropy alloy single crystals deformed by twinning and slip

The orientation dependence of tensile deformation in Fe40Mn40Co10Cr10 high entropy alloy (HEA) was investigated in [111], [001] and [123] oriented single crystals. Transmission electron microscopy investigations revealed three major mechanisms controlling the deformation stages, depending on the orientation: (i) deformation twinning, (ii) planar slip and (iii) dislocation wall/network formation. While twinning and planar slip were strongly orientation dependent, dislocation walls were observed in all orientations. Twinning was the dominant deformation mode in [111] crystals, while only multi-slip was observed in [001]. Both twins and planar slip were activated in [123] crystals. [111] crystals exhibited the highest strain hardening coefficients and ultimate tensile strength due to the strong twin-twin and twin-slip interactions where twin boundaries reduce the mean free path of dislocations, leading to dynamic Hall-Petch hardening. The decent ductility levels (similar to 45%) were attained in [111] due to nanoscale internal twins and tertiary twin system forming at the later stages of deformation and suppressing necking. In contrast, no twins or stacking faults were observed in [001] crystals, which is consistent with the Copley-Kear effect. [123] crystals had outstanding tensile ductility (similar to 65%), due to the activation of planar slip and twinning. Overall, in this off-stoichiometric HEA, we have determined the stacking faculty energy and critical resolved shear stresses for both twinning and slip, and demonstrated the formation of high dislocation density walls and wavy slip in [001], while the hardening stages of [123] and [111] are primarily governed by planar slip and twinning, which can be rationalized by the Copley-Kear effect. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.