Tailoring the transformation behavior and functionalities of NiTi alloy achieved by L-PBF in-situ alloying using NiTi and Ni powder mixture

It is known that tailoring the transformation behavior and functionalities of NiTi alloys is extremely difficult through conventional metallurgical methods, since they are too sensitive to the microstructural and compositional changes, especially the Ni content. Laser powder bed fusion (L-PBF) is a promising metallurgical approach to address this challenge through in-situ alloying using powder mixture as feedstock. In this work, pre-alloyed Ni_50.6Ti_49.4 (at%) and elemental Ni powders were used to in-situ alloy the NiTi alloys with different Ni content. The results indicate that a wide L-PBF process window exists to produce dense and defect-free Ni_50.6Ti_49.4 + xNi (x = 0 at%, 0.15 at%, 0.30 at%) alloys. Ni is homogeneously distributed in the matrix, and no Ni-rich phase or segregation of Ni is detected in the as-built samples. The combination of adding Ni and varying laser scanning speed provides a feasible approach to tailoring the transformation temperatures over a wide range (e.g., austenite transformation finish temperature, A_f, from 278 to 336 K). The plateau stress ranging from 172 to 556 MPa could be obtained at room temperature. Moreover, a transition from shape memory to superelastic recovery is achieved in the Ni_50.6Ti_49.4 + 0.15Ni alloy by solely regulating the laser scanning speed. The results of this work indicate that L-PBF in-situ alloying is a promising method to tailor precisely the Ni content, and thus the transformation behavior and functionalities of NiTi alloys.