Modulation of Ti2Ni Morphology in Laser Powder Bed Fused Ni-Ti Alloys for Excellent Shape Memory Effect

Abstract

NiTi alloy produced by laser powder bed fusion (LPBF) is often limited by uncontrolled non-equilibrium solidification, which generates non-uniform tissue such as Ti2Ni with irreversible effects on its mechanical and functional properties. To address this challenge, a layer-alternating LPBF (ALT-LPBF) method that programs hs changes along the building height was proposed for the first time. ALT-LPBF enables directional control over the morphology of Ti2Ni (from spherical to sub-ellipsoidal) by influencing inhomogeneous residual stress. Precise testing of the compositional fields within the melting pool and grains revealed that both peritectic and eutectic reactions are involved in Ti2Ni development, contributing to a deeper understanding of the mechanisms governing Ti2Ni formation during ALT-LPBF. Additionally, spherical Ti2Ni forms a coherent interface with B2 matrix, reducing stress concentration and promoting work hardening, through an elastic strain field. In contrast, sub-ellipsoidal Ti2Ni forms an incoherent interface, intensifying the stress field, promoting dislocation accumulation and inhibiting plastic deformation. The combination of spherical and sub-ellipsoidal Ti2Ni induced by ALT-LPBF enhances ultimate tensile strength to 850.6 MPa and shape memory effect 98.8%. This work lays the foundation for achieving ideal shape memory effect through ALT-LPBF and expands biomedical devices and smart materials applications.

Keywords: NiTi alloy, Additive Manufacturing, Shape memory effects, Nanoprecipitate