Abstract: The strength-ductility trade-off was evaded by deploying a triple-level heterogeneous structure into a CoNiV-based medium-entropy alloy (THS MEA). The innovative hetero-structures comprise chemical short-range ordering (CSRO) at the atomic level, B2 precipitates at the nanoscale level, and heterogeneous grains at the microscale level. The THS MEA exhibits superior mechanical properties, displaying a yield strength from 1.1 GPa to 1.5 GPa alongside a uniform elongation of 18 %-35 %. Compared with its coarse-grained (CG) counterpart, the THS MEA demonstrates the pronounced up-turn phenomenon and enhanced hardening behavior attributed to hetero-deformation-induced (HDI) hardening. The detailed microstructural characterizations reveal that CG MEA primarily accommodates deformation through extensive planar dislocations and Taylor lattices. However, the THS MEA exhibits a more complex deformation profile, characterized by planar and waved dislocations, deformation twins, stacking faults, and Lomer-Cottrell locks. Additionally, the interactions between dislocations and B2 nanoprecipitates play a pivotal role in dislocation entanglements and accumulations. Furthermore, the CSRO within the matrix effectively retards the dislocation motion, contributing to a substantive hardening effect. These findings underscore the potential of a heterogeneous microstructure strategy in enhancing strain hardening for conquering the strength-ductility dilemma.

Key words: Medium-entropy alloy, Hetero-structures, Precipitation hardening, Hetero-deformation-induced hardening, Chemical short-range ordering