Effect of bimodal heterostructure features on strength-ductility trade-off breaking in Inconel 718 Ni-based superalloy

• A bimodal heterostructure is constructed via selective δ phase precipitation. • Heterogeneous δ phase distribution is achieved by composition control and thermal mechanical processing. • Dynamic strain partitioning and GND accumulation govern the synergy. • CG regions beyond ∼22 μm lose effective strain partitioning. • Critical stress balance governs synergy: moderate CG size and sufficient FG fraction prevent cracking. A bimodal heterostructure with alternating δ phase-rich fine-grained (FG) and δ phase-lean coarse-grained (CG) regions was constructed in Inconel 718 via the selective precipitation of the δ phase, enabled by controlled Nb content and heat treatment. Compared to the homogeneous counterpart, the optimal heterostructure achieves a significant enhancement in yield strength (222 MPa increase) and ultimate tensile strength (213 MPa increase) while maintaining good ductility. Multiscale characterization revealed an effective heterointerface influence range of ∼22 μm. CG regions beyond this effective range exhibits quasi-homogeneous deformation, thereby impeding dynamic strain partitioning and HDI strengthening that arise from the accumulation of geometrically necessary dislocations (GNDs). Notably, a stress equilibrium mechanism is identified. When the coarse grain size is moderate and the proportion of FG regions is sufficiently high, the forward stress generated by dislocation pile-ups can be effectively borne by the FG regions and transformed into beneficial work hardening. Conversely, under the condition of excessively large coarse grains and a low FG regions fraction, the local stress at the heterointerface is likely to exceed the accommodation limit, triggering premature microcrack initiation in FG regions and a loss of the synergy. This work provides a strategy for designing heterostructure via second-phase engineering.