Enhancing the strength-ductility trade-off in a NiCoCr-based medium-entropy alloy with the synergetic effect of ultra fine precipitates, stacking faults, dislocation locks and twins

Abstract

In this work, multi strengthening/deformation mechanisms were introduced into a Ni-rich Ni40Co32Cr20Al3.5Ti3Nb1.5 (at%) medium-entropy alloy. After thermo-mechanical procedures, the microstructure is characterized by ultra fine γ' particles (2–18 nm) and short stacking faults. The alloy has superior mechanical properties with gigapascal yield strength, excellent ductility and strong work hardening ability. The initial high strength is caused by the blocking effect of ultra fine γ' particles, dislocation locks (Lomer-Cottrell locks), solute atoms and grain boundaries to dislocation movement. TEM observations at different strains indicate that the increasing number of stacking faults, Lomer-Cottrell locks and deformation twins are the main reasons for the improvement of work hardening ability during deformation. Twinning and dislocation slip, including perfect dislocations and partial dislocations, are the main deformation mechanisms. Our findings show that the coordination of multi strength/toughing mechanisms may be an effect way to enhance the strength-ductility trade-off.