Additively manufactured dual-area NiTi alloys with quasi-linear superelasticity and cyclic stability via grain boundary modification

Highly nickel-rich NiTi alloys with ultrahigh strength and cyclic stability are essential for elastocaloric refrigeration and energy conversion industries. However, the inherent trade-off between strength and superelastic strain often complicates efforts to enhance synergistically. Here, an in-situ alloying dual-area Ti-55Ni (at.%) alloy, characterized by non-uniformly distributed Ni-rich precipitates, is fabricated using laser powder bed fusion (LPBF) with pre-mixed feedstock. The alloy, exhibiting a modulus of 27 GPa, demonstrates quasi-linear superelasticity with an impressive elastic strain exceeding 9% (under 1600 MPa) and exceptional cyclic stability over 2 × 10⁵ cycles (under 1200 MPa) in the building direction. This remarkable mechanical performance is attributed to the specialized heterogeneous microstructure, as revealed through multi-scale microscopic analysis. During in-situ cyclic heat treatment of LPBF, the high-modulus Ni₃Ti and Ni₃Ti₂ phases, as a non-transforming area, mainly separate from the grain boundaries, while the primary and secondary Ni₄Ti₃ phases with low modulus precipitate within the columnar grain interiors. Given the unique distribution characteristic, the transition of superelasticity from plateau-type to quasi-linear one and the contribution of the high-modulus grain boundaries to quasi-linear superelastic behavior are elucidated through finite element simulations. This work is anticipated to offer a practical and feasible approach for designing and fabricating novel superelastic materials.