Hierarchical precipitates, sequential deformation-induced phase transformation, and enhanced back stress strengthening of the micro-alloyed high entropy alloy

We report the annealing time-dependent microstructures and deformation mechanisms of the novel face-centered cubic Fe49.5Mn30Co10Cr10C0.2Ti0.1V0.1Mo0.1 HEA. Three types of precipitates, σ-phase, Cr-rich MC-type carbides, and nano-scale (Ti, V, Mo)C, are present after cold-rolling and annealing at 600 °C. Such hierarchical precipitates could lead to sluggish recrystallization and grain growth upon annealing. The partially recrystallized microstructures and hierarchical precipitates could lead to a high yield strength even for prolonged annealing conditions. Deformation mechanisms change with annealing time. The materials annealed for short times (< 2 h) are deformed by dislocation glide, deformation twinning, and deformation-induced ε phase. A longer annealing time (> 10 h) triggers a multi-variant ε phase, reverse transformation from ε to γ, and the multi-step sequential transformation, γ → ε → reverse transformed γ from ε → ε transformed from the reverse transformed γ. Further, materials annealed for longer times shows a higher contribution of back stress strengthening, which could be attributed to the increase in γ/ε and γ/σ interfaces. The activation of various deformation mechanisms and high back stress strengthening could lead to a superior strain hardening capacity and strength-ductility combination (YS: 699 MPa, UTS: 1041 MPa, TE: 45%) of the material annealed for 10 h. The present work provides the novel microstructure design solution of the metastable high entropy alloys with exceptional mechanical properties, utilizing hierarchical precipitates, sequential deformation-induced phase transformation, and enhanced back stress strengthening.