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Metastability-based high entropy alloy design opens a new strategic path for designing highstrength materials. However, high strength is always coupled with poor damage tolerance under cyclic loading conditions (fatigue). To overcome this drawback, here we present grain-refined Fe 42 Mn 28 Cr 15 Co 10 Si 5 exhibiting significantly high fatigue strength as compared with leading transformation induced plasticity steels upon friction stir processing. The enhanced fatigue behavior is attributed to the metastability-promoted γ → ε transformation that caused local variation in workhardening activity near the crack tip, and subsequent crack branching. Thus, decreased γ phase stability assisted not only in attaining strength but also in making the alloy fatigue-resistant. IMPACT STATEMENT Fatigue resistance of dual-phase TRIP Fe 42 Mn 28 Cr 15 Co 10 Si 5 was evaluated. Metastability-promoted γ → ε transformation improved fatigue life due to local enhancement in work-hardening near the crack tip.

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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.