Nanoindentation of three metastable dual-phase high entropy alloys (HEAs) was performed to obtain their inherent elastoplastic deformation responses. Excellent combination of hardness and elastic modulus in as-cast condition confirmed that, their inherently higher strength compared to other HEAs reported in literature, can be attributed to alloy chemistry induced phase stability. Further, hardness of 8.28 GPa combined with modulus of 221.8 GPa was obtained in Fe-Mn-Co-Cr-Si-Cu HEA by annealing the as-cast material, which is the best hardness-modulus combination obtained to date in HEAs from nanoindentation. On the other hand, although Fe-Mn-Co-Cr-Si HEA showed lower hardness and modulus than Fe-Mn-Co-Cr-Si-Al and Fe-Mn-Co-Cr-Si-Cu HEAs, the former alloy exhibited the highest strain rate sensitivity, as determined from tests performed at five different strain rates. The three alloys also had subtle differences in incipient plasticity and elastoplastic behavior, while retaining similar levels of hardness; and nanoindentation response showed microstructural dependence in friction stir processed, annealed and tensile-deformed specimens. Thus, the study highlighted that while higher strength was achieved by designing a class of HEAs with similar composition, any of the individual alloys can be tuned to obtain enhanced properties.
Strain hardening in metallic materials delays catastrophic failure at stresses beyond the yield strength by the formation of obstacles to dislocation motion during plastic deformation. Conventional measurement of the instantaneous strain hardening rate originates from load–displacement data acquired during uniaxial mechanical testing, rather than the evolution of obstacles. In order to resolve hardening from the perspective of the very obstacles that cause strengthening, we used an in situ neutron diffraction experimental approach to determine the strain hardening rate based upon real-time measurement of stacking fault interspacing during plastic deformation. Results provide clear evidence of the evolving contribution of obstacles during plastic deformation. The collapse of interspacing between multiple obstacle types enabled immense strain hardening in a Fe38.5Mn20Cr15Co20Si5Cu1.5 high entropy alloy leading to a true tensile strength of ∼1.7 GPa along with elongation of ∼35% at room temperature.
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