Superelastic and robust NiTi alloys with hierarchical microstructures by laser powder bed fusion

Nickel-titanium (NiTi) alloys perform large recoverable deformation and can be produced into sophisticated and customized components through additive manufacturing (AM). However, AM NiTi alloys often suffer from poor tensile superelasticity (or pseudoelasticity), strength, and total elongation due to their coarse column grains and undesirable textures. Herein, we showcase novel hierarchical NiTi alloys prepared by laser powder bed fusion. The hierarchical NiTi alloys compromise fish scale micron grains, sub-micron herringbone grains, sub-micron equiaxed grains, and high-density dislocations, predominantly featuring <111> and <110> orientations parallel to both the building and tensile direction. Consequently, they demonstrate a record-high maximum recoverable strain (epsilon(rec)) of 5-6 %, an excellent stable epsilon(rec) of similar to 3.5 % without failure over 120 tensile cycles, an ultrahigh tensile strength from 875 to 1029 MPa, and a large total elongation from 8.7 % to 14.2 %. These remarkable property combinations outperform those of state-of-the-art AM NiTi alloys. This work significantly resolves a longstanding performance dilemma in AM NiTi alloys without needing post-heat treatment, offering a fresh perspective on strengthening AM shape memory alloys through microstructure modification.