Tailoring the Ti4Ni2OX morphology in NiTi via powder bed fusion-laser beam for excellent shape memory effect

NiTi (Nitinol) produced via powder bed fusion-laser beam (PBF-LB) is often constrained by the uncontrolled non-equilibrium solidification process, which leads to the formation of non-equilibrium solidification microstructures, such as Ti 4 Ni 2 O X , that irreversibly affect the alloy's functional properties. To address that challenge, this study introduces a layer-alternating PBF-LB (ALT-PBF-LB) method that enables systematic modulation of hatch spacing ( hs ) along the build height. ALT-PBF-LB facilitates directional control over the morphology of Ti 4 Ni 2 O X , achieving spherical and sub-ellipsoidal types by changing the formation process. Detailed compositional analysis of the melting pool and grains revealed that both peritectic and eutectic reactions contribute to Ti 4 Ni 2 O X formation and are directly related to hs . Spherical Ti 4 Ni 2 O X forms a coherent interface with the B2 phase, reducing stress concentration and promoting work hardening through an elastic strain field. Conversely, sub-ellipsoidal Ti 4 Ni 2 O X forms an incoherent interface, intensifying local stress fields, facilitating dislocation accumulation, and impeding plastic deformation. The combination of spherical and sub-ellipsoidal Ti 4 Ni 2 O X achieved through ALT-PBF-LB enhances the shape memory effect to 98.8 % with 5.93 % recover strain. This study explores the relationship between the morphological changes of Ti 4 Ni 2 O X induced by hs and the shape memory effect, offering new insights into optimising the shape memory effect of PBF-LB NiTi. • The formation of Ti 4 Ni 2 O X requires an inhomogeneous compositional field. • The morphology variation of Ti 4 Ni 2 O X are attributed to the temperature through changes in hs. • The combination of spherical and sub-ellipsoidal Ti 4 Ni 2 O X effectively enhance shape memory effect.