Achieving ultrahigh strength and ductility in high-entropy alloys via dual precipitation

The strength-ductility trade-off has been a longstanding dilemma in metallic materials. Here we report an innovative approach to achieve a high strength-ductility synergy via dual precipitation of sheared and bypassed precipitates. (Ni₂Co₂FeCr)_96–xAl₄Nb_x (at.%) alloys strengthened by nanoscale L1₂ particles and Laves precipitates were selected as a model for this study, and their precipitate microstructures and mechanical properties were thoroughly investigated. The dual-precipitation-strengthened alloys exhibit a yield strength of more than 1400 MPa, an ultimate tensile strength of over 1800 MPa, and a uniform elongation of 18%, thus achieving a high strength-ductility synergy. Our analysis reveals that the nanoscale L1₂ precipitates contribute to the strength via the particle shearing mechanism, whereas the Laves phase provides the strengthening through the Orowan bypass mechanism. The study of deformation microstructures shows that the L1₂ precipitates are sheared by stacking faults, which facilitates long-range dislocation gliding through the matrix. As a result, deformation induces the formation of hierarchical stacking fault networks and immobile Lomer–Cottrell locks, which effectively enhance the work hardening capability and plastic stability, thereby resulting in a high ductility at high strength levels. Dislocations are piled-up against the interface between the Laves precipitates and matrix, which increases the work hardening capability at the early stages of plastic deformation but causes stress concentrations. The dual precipitation strategy may be useful for many other alloys for achieving superior mechanical properties for technological applications.