Local chemical inhomogeneity enables superior strength-ductility-superelasticity synergy in additively manufactured NiTi shape memory alloys

NiTi shape memory alloys produced via additive manufacturing are suffering lowtensile strength, low total elongation,and unstable superelasticity, thus failing to meet the requirements of practical applications. Here, we report an strategy to substantially and synergistically improve the strength, ductility,and superelasticity of NiTi produced by laser powder bed fusion throughestablishinghigh-density Ni-rich local chemical inhomogeneity (LCI) entities within B2 matrix. Compared with other documented microstructures such as long-range ordered Ni4Ti3 precipitates, the presentNi-rich LCI entities are uniqueto increase the resistance against dislocation slip, facilitate stress-induced martensitic transformation, and most importantly, relieve local stress concentration around micro-pore defects and entity interfaces. This specialized microstructure endowstensile superelasticity, i.e., tensile ultimate strength of 991 MPa, total tensile elongation of 11.3%, superelastic strain exceeding 7%, and superior cyclic stability. The results advance our capabilities in fabricatinghigh-performance superelastic SMAswith complex geometries through additive manufacturing and LCI engineering.