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In the present work, the mechanical properties of a newly developed metastable 65Fe-15Ni-8Co-8Mn-3Ti-Si (at%) ferrous medium-entropy alloy were investigated at 298 K and 77 K. The short-time annealing followed by aging treatment is employed to gain heterogeneous microstructure containing fine- and coarse-grains decorated with nanometric precipitates. The yield strength of the alloy enhances substantially from 0.9 GPa at 298 K and 1.3 GPa at 77 K in the annealed state, respectively, to 1.1 GPa and 1.5 GPa after the aging treatment, while the total elongation sustains more than 20%. This extraordinary improvement results from the synergistic effect of hetero-deformation-induced strengthening, mechanical nano-twins, and martensitic phase transformation along with precipitation strengthening of the well-distributed nano-scale Fe₂SiTi and Ni₃Ti precipitates during tensile deformation of the aged alloy …