Enhanced deformation-induced FCC-HCP transformation of a metastable high-entropy alloy induced by a smaller grain size

This study reports the grain-size-dependent deformation-induced face-centered cubic (FCC)–hexagonal close-packed (HCP) transformation behavior of Fe49.5Mn30Co10Cr10C0.2Ti0.1V0.1Mo0.1 (at. %) high-entropy alloy in the medium grain size range (10–20 μm), their mechanical properties, and deformation mechanisms. The materials annealed at 900 °C for 1 h (A900-1) and 10 h (A900-10) showed a fully FCC, recrystallized microstructure with a larger grain size in the latter owing to grain growth during annealing. The A900-1 sample exhibited a larger strength–ductility balance and a higher strain hardening rate than the A900-10 sample. The A900-1 sample, with a larger number of grain boundary nucleation sites of the deformation-induced HCP phase, exhibited a higher HCP transformation rate than the A900-10 sample in the early stage of deformation below 10 % strain, whereas both materials showed similar HCP transformation rates at strain levels higher than 30 %. The emission of deformation-induced HCP plates in both neighboring grains from the same grain boundary (A900-1) and the activation of the deformation-induced HCP phase in a single grain and only dislocations in the neighboring grains (A900-10) suggest the importance of grain boundary stress in the observed phenomena. The observed additional mechanisms other than dislocation slip and deformation-induced HCP phase, that is, reverse FCC-HCP transformation, HCP {101‾2} twinning, and kink banding, could contribute to deformation accommodation and stress relaxation, thereby leading to the excellent ductility of the investigated materials.