Ultra-high strength and excellent ductility high entropy alloy induced by nano-lamellar precipitates and ultrafine grain structure

Abstract

We describe the development of a non-equiatomic CoCrFeNi2Al0.3Ti0.25 high entropy alloy (HEA). Both high-density nano-lamellar precipitates and ultrafine grain structure are simultaneously introduced by cold rolling then annealing at 600–800 °C for 1 h. The HEA annealing at 700 °C (CR-700) exhibits ultra-high yield strength (σy) of 1900 ± 15 MPa, ultimate tensile stress of 2074 ± 17 MPa and excellent total elongation (εte) of 16.7 ± 1.5. This succeeds in overcoming the strength-ductility trade-off that often hampers the development of high-performance alloys. Compared with the as-cast alloy, σy of the CR-700 HEA increases by 171% and εte decreases by 54%. The ultra-high strength is mainly attributed to the synergistic strengthening effects of both nano-lamellar precipitates and ultrafine grains. The high coherent FCC/L12 interface facilitates the transmission of dislocations, which eliminates the stress concentration during deformation and effectively maintains uniform deformation. Meanwhile, high-density stacking faults and Lomer-Cottrell locks are formed during the deformation process. These promote the multiplication of dislocations and further improves the work-hardening ability. The combination of high-density nano-lamellar precipitates and ultrafine grain structure can potentially be applied to other conventional alloys. This work provides a new strategy for fabricating materials with ultra-high strength and excellent ductility.