Enhanced strength-ductility synergy in laser directed energy deposited IN718 superalloys through heterogeneous deformation nanostructures

Laser directed energy deposition (LDED) shows great promise for repairing superalloy components of aeroengines but often results in coarse microstructures, porosity, and tensile residual stresses. Herein, post-process ultrasonic impact treatment (UIT) is adopted to effectively regulate the surface microstructure and residual stresses in LDED-fabricated IN718 superalloys, enhancing the strength-ductility synergy. The UIT process optimization is achieved through a systematic investigation of the effect of output powers on surface roughness, porosity, deformation microstructure, microhardness distribution, residual stress profile, and tensile behavior. Particularly, a finite element model for simulating residual stress field induced by ultrasonic impact is established, showcasing excellent agreement with experimental measurements. UIT-induced substantial dislocation and twinning activities result in depth-dependent heterogeneous deformation nanostructures, including alternating nano-grains and nano-laminated composite structures on the top surface (<8 μm), dense nanotwins (∼30 μm depth), and substantial dislocation tangles and pile-ups (∼150 μm depth). Compared to untreated samples, the yield strength of the samples treated with optimal UIT parameters increased by ∼40%, with negligible ductility loss. The synergistic strengthening mechanisms are mainly attributed to the work hardening and boundary strengthening. To decouple these effects, a quantitative framework that correlates with depth-dependent dislocation populations and grain/nanotwin sizes is proposed, demonstrating good consistency with experimental measurements. The preserved ductility stems from a macroscopic deformation delocalization strategy facilitated by the hetero-deformation induced stress, compressive residual stress, and reduced porosity, together with the near-surface heterogeneous nanostructures enabling deformation accommodation at the micro-scale. This work elucidates the enhanced strength-ductility synergy through surface heterogeneous nanostructures and provides practical guidance for the additive manufacturing of high-performance materials.