Achieving grain refinement and high superelastic strain during wire-arc directed energy deposition NiTi alloys by addition of La₂O₃

https://doi.org/10.1016/j.addma.2024.104585

Abstract

NiTi alloy components fabricated by wire-arc directed energy deposition (DED) process exhibit pronounced columnar grain structure. Even though grain refinement by in-situ alloying has now become a routine way in powder bed fusion techniques, this is still challenging in wire-arc DED processes since it is difficult to evenly mix other alloying elements with the bare wire. In this study, the addition of La₂O₃ particles during the deposition process was employed to control the microstructure and mechanical properties of NiTi alloy. The results showed that with the addition of 5 wt% La₂O₃ particles, the width of the columnar grains in the NiTi alloy decreased from 144 μm to 91 μm. Significant changes in the distribution and size of precipitates were also achieved in the samples. Superelastic recovery under a high strain condition of 9% was realized in the NiTi-5La₂O₃ sample. La₂O₃ particles not only serve as effective heterogeneous nucleation sites for grain refinement in the deposition process of NiTi alloy, but also influence the morphology and distribution of precipitates. The improvement in superelastic performance is mainly attributed to the reduced stress concentration during deformation enabled by the uniform distribution of fine precipitates. This study provides a potential method for achieving superelasticity in NiTi alloys prepared by wire-arc DED under high strain conditions.