Tailoring microstructure in functionally graded NiTi alloys using in-situ alloying directed energy deposition

NiTi alloys fabricated via additive manufacturing (AM) often suffer from coarse grains, brittle intermetallic phase accumulation, and limited control over phase transformation behavior, resulting in compromised performance and impeded functional applications. To address this challenge, a generalisable strategy for intermetallic modulation and functional gradient design has been proposed and validated through directed energy deposition (DED). By employing multiple deposition modes (Mixed NiTi, Graded Ti/Ni, and Graded Ti/NiTi), tailored microstructure gradients were achieved. This approach enabled spatial control over the formation of key intermetallics, resulting in simultaneous enhancement of martensitic transformation behavior and mechanical performance (nanohardness, compressive strength). A coupled simulation-experimental analysis revealed universal mechanisms of temperature evolution and solute transport in melt pools, which underlie intermetallic development during AM. The findings contribute a broadly applicable methodology for designing gradient architectures in metallic systems, offering new avenues for tailoring functional and structural performance.