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

Abstract

NiTi shape memory alloys produced via additive manufacturing are suffering low tensile 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 through establishing high-density Ni-rich local chemical inhomogeneity (LCI) entities within B2 matrix. Compared with other documented microstructures such as long-range ordered Ni4Ti3 precipitates, the present Ni-rich LCI entities are unique to 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 endows tensile superelasticity, i.e., tensile ultimate strength of 958.7 MPa, total tensile elongation of 11.2%, superelastic strain exceeding 7%, and superior cyclic stability. The results advance our capabilities in fabricating high-performance superelastic SMAs with complex geometries through additive manufacturing and LCI engineering. Using laser powder bed fusion and a tailored heat treatment, the authors produce NiTi shape memory alloys with improved strength, ductility, and superelasticity enabled by high-density of Ni-rich local chemical inhomogeneity entities.