Achieving high strength and ductility in equimolar FeMnNi medium entropy alloy by tuning microstructural entropy

Highlights: • Microstructure and micro-texture analyses reveal stress-strain partitioning in multimodal distributions. • Fine grains are mainly stress carriers while coarse grains accommodate strain in a heterogeneous microstructure. • Multimodal grain sizes are characterized by microstructure entropy parameter as opposed to grain size in Hall-Petch behavior. • Engineering equiatomic FeMnNi based on microstructure entropy parameter with twinning to evade strength-ductility trade-off. Microstructural engineering approach was adopted to produce a multimodal recovered-recrystallized grain size distribution, in order to achieve optimum strength and ductility in equimolar FeMnNi medium entropy alloy (MEA). Detailed quantitative microstructure and micro-texture analysis using electron back scatter diffraction reveals the ability of the finer recovered grains to carry more stress and large recrystallized grains to impart ductility by promoting strain hardening. Furthermore, it was shown that the concept of microstructure entropy, a phenomenological parameter based on grain size distribution, provides a better prediction for the yield strength for heterogeneous microstructure compared to average grain size-based Hall-Petch model. Thus, the microstructural entropy model capturing the grain size distribution provides a robust methodology to establish structure – property correlation in heterogeneous microstructure and defect structure of FeMnNi MEA.