Achieving excellent strength-ductility synergy in twinned NiCoCr medium-entropy alloy via Al/Ta co-doping

Alloying is an effective strategy to tailor microstructure and mechanical properties of metallic materials to overcome the strength-ductility trade-off dilemma. In this work, we combined a novel alloy design principle, i.e. harvesting pronounced solid solution hardening (SSH) based on the misfit volumes engineering, and simultaneously, architecting the ductile matrix based on the valence electron concentrations (VEC) criterion, to fulfill an excellent strength-ductility synergy for the newly emerging high/medium-entropy alloys (HEAs/MEAs). Based on this strategy, Al/Ta co-doping within NiCoCr MEA leads to an efficient synthetic approach, that is minor Al/Ta co-doping not only renders significantly enhanced strength with notable SSH effect and ultrahigh strain-hardening capability, but also sharply refines grains and induces abnormal twinning behaviors of (NiCoCr)₉₂Al₆Ta₂ MEA. Compared with the partially twinned NiCoCr MEA, the yield strength (σ_y) and ultimate tensile strength (σ_UTS) of fully twinned Al/Ta-containing MEA were increased by ∼102 % to ∼600 MPa and ∼35 % to ∼1000 MPa, respectively, along with good ductility beyond 50 %. Different from the NiCoCr MEA with deformation twins (DTs)/stacking faults (SFs) dominated plasticity, the extraordinary strain-hardening capability of the solute-hardened (NiCoCr)₉₂Al₆Ta₂ MEA, deactivated deformation twinning, originates from the high density of dislocation walls, micro-bands and abundance of SFs. The abnormal twinning behaviors, i.e., prevalence of annealing twins (ATs) but absence of DTs in (NiCoCr)₉₂Al₆Ta₂ MEA, are explained in terms of the relaxation of grain boundaries (for ATs) and the twinning mechanism transition (for DTs), respectively.