In-situ neutron diffraction investigation of deformation mechanisms in an additively manufactured medium-entropy alloy with coherent nanoprecipitates

Coherent L1₂ nanoprecipitates are widely used to strengthen face-centered cubic alloys. However, their influence on stress partitioning, dislocation evolution, and deformation stability remains unclear. In this study, a (CoCrNi)₉₄Al₃Ti₃ medium-entropy alloy containing coherent L1₂ nanoprecipitates was fabricated by additive manufacturing. In-situ neutron diffraction reveals that the L1₂ nanoprecipitates progressively carry a larger fraction of the applied load during deformation, while the FCC matrix exhibits no obvious stress relaxation, indicating mechanically coupled deformation and cooperative load sharing. The dispersed nanoprecipitates reduce local stress concentrations and mitigate plastic instability. Their low lattice mismatch (∼0.194 ± 0.001%) minimizes interfacial stress, enabling smooth dislocation transfer across phase boundaries. Furthermore, the L1₂ nanoprecipitates promote dislocation accumulation in both phases, thereby enhancing the work-hardening capability. This work provides mechanistic insights into deformation mediated by coherent nanoprecipitates and offers guidance for tailoring the strength–ductility balance in FCC alloys.