Microstructural origin of the superior strength–ductility synergy of γ′-strengthened high-entropy alloy with heterogeneous grain structure and discontinuous precipitation configuration

The pursuit of a superior combination of strength and ductility in metallic materials for structural applications has long been a challenge. Recently, L12 (γ′) precipitation-strengthened high-entropy alloys (HEAs) that undergo direct aging from cold-rolled states have exhibited exceptional strength-ductility synergy. However, the origins of this synergetic effect in direct aging are still a subject of controversy. Herein, we aim to unravel the fundamental factors contributing to the improved mechanical properties of directly aged HEAs. Two distinct aging processes, namely cold-rolling and direct aging (CA), and recrystallization and aging (RA), are employed on the Cr18Fe18Co28Ni28Al4Ti4 alloy. The RA process induces continuous precipitation (CP) behavior, while the discontinuous precipitation (DP) is confined near the grain boundaries. On the contrary, the CA process leads to DP-dominant reaction accompanied by simultaneous recrystallization. Additionally, the inherent heterogeneity of the cold-rolled alloy results in a heterogeneous structure in the CA alloys. The CA alloy aged at 600 °C exhibits significantly increased strength and ductility compared to the cold-rolled state, with a remarkable yield strength of 1649 MPa, tensile strength of 1741 MPa, and total elongation of 19.5%. This ultra-high strength is attributed to the precipitation strengthening and heterogeneous deformation-induced (HDI) strengthening resulting from the heterogeneous grain structure. Furthermore, the fine grain and ultra-fine grain regions promote additional HDI strengthening during plastic deformation, leading to sustained strain hardening. The findings of this study establish a correlation between microstructural features and mechanical behavior in directly aged γ′ precipitation-strengthened HEAs, providing valuable insights for the design of such alloys.