In situ nanoscale heterostructure engineering for additive manufacturing of dynamic adaptive alloys

Achieving precision and balanced properties in additive manufacturing (AM) of metallic alloys remains challenging due to rapid melting-solidification cycles, leading to unpredictable microstructures and inconsistent mechanical performance. Conventional AM suffers from high costs, limited adaptability, and brittle secondary phases, compromising load-bearing capacity and structural precision, particularly in alloys like nickel-titanium (NiTi) that are sensitive to high-energy inputs. To overcome these limitations, we present an in situ nanoscale heterostructure engineering (INSHE) strategy that addresses the trade-offs in AM for achieving enhanced mechanical and functional properties. INSHE leverages high-energy laser powder bed fusion (L-PBF) to create uniformly distributed multi-nanoprecipitate assemblies with lattice-matched interfaces, achieving precise modulation of non-equilibrium microstructures from the bottom up. By controlling the inclusion of multi-precipitate seeds and ultrafine heterostructures, INSHE maintains thermodynamic stability under L-PBF's extreme conditions, achieving consistent distribution across the build direction. When applied to a NiTi shape memory alloy, INSHE formulates boron carbide (B₄C) additively to form a nanoengineered B2-TiB₂-TiC phase heterostructure that enhances both strength and adaptability beyond conventional alloys. INSHE-based shape memory alloys (SMAs) demonstrate an ultrahigh yield strength of up to 2.15 GPa with a delayed plasticity of 12%, more than double the hardness of pristine SMAs (4.94 GPa), and a 66% increase in the hardness-to-modulus ratio (H/E_r), which promotes wear resistance via lubricating oxide film formation. INSHE-engineered SMA mechanical metamaterials (SMMMs) demonstrate superior strain recovery and durability under cyclic and gradient loads over conventional SMAs. These SMMMs achieve specific energy absorption (SEA) up to 25 J g^-1, surpassing the typical 1-20 J g^-1 range of composites and other micro-/nanolattice metals, demonstrating exceptional resilience. The activation of corresponding variant pairs (CVPs)-twin-related martensitic variants-enhances cyclic phase transformations, prevents stabilized martensite retention, and enables efficient energy absorption. INSHE's nanoengineered heterostructure within the B2 phase promotes multiple synergistic deformation mechanisms that resist shear-induced amorphization and prevent crystalline fragmentation. This results in significantly improved damping capacity, linear elastic behavior across transformation temperatures, and nearly ideal shape recovery, achieving superior actuation efficiency over multicomponent and commercial Snoek-type high-damping alloys. Micro-CT imaging confirms INSHE's superior internal quality and precision in complex geometries, addressing powder adhesion and surface roughness through enhanced powder absorption and thermochemical tuning. INSHE also advances printability by refining powder feedstock design and laser processing parameters. This transformative strategy paves the way for the application of alloys with tailored multifunctional properties in topological architectures, unlocking new possibilities for dynamic adaptive materials with superior structural and functional performance.