An excellent combination of strength and ductility via hierarchical precipitation structures in Co-free medium-entropy alloys

A Co-free non-equiatomic Ni_2·5CrFeAl_0·25Ti_0.25 medium-entropy alloy (MEA) with an excellent strength-ductility synergy was fabricated, which shows a multiphase structure composed of face-centered cubic (FCC), L1₂ (ordered FCC), and Cr-rich body-centered cubic (BCC) phase by thermomechanical processing. Specifically, the aged sample displays the outstanding yield tensile strength (YTS, ∼1188 MPa), ultimate tensile strength (UTS, ∼1560 MPa) and work-hardening rate (WHR, ∼4.5 GPa) values as well as an acceptable plasticity of ∼16.6%. Theoretical calculations suggest that precipitation strengthening significantly contributes to achieving the fascinating tensile strength among various strengthening contributors. Further analyses reveal that multiple nanoscale stacking-fault (SF) networks are activated during plastic deformation in the aged alloy. Accordingly, the dual effects consisting of the hierarchical precipitation structure and SF networks lead to the combination of excellent tensile strength and strain-hardening capacity.