Elucidating the mechanism for high-temperature heat treatment induced embrittlement of laser-powder-based fusion manufactured NiTi alloy

Abstract

Powder bed fusion-laser beam with metals (PBF-LB/M) can be used to manufacture intricate NiTi components. However, the ductility of NiTi alloys printed by PBF-LB/M is generally ∼20% less than those made via conventional processes. Although many heat treatment methods have been proposed, solving this issue has been proven difficult. An intractable problem is the brittleness of PBF-LB/M-manufactured NiTi after solid solution treatment at 1000°C. By investigating the microstructural and fractography change after heat treatment in the range of 100-1000°C, this study found that this ductile-to-brittle transition stems from abnormal oxygen-containing Ti-rich precipitates being generated in the PBF-LB/M fabricated Ni-rich NiTi. We identified laser processing-induced local oxygen segregation and tiny TiO₂(B) particles at the fusion and grain boundaries. During the heat treatment at temperatures above 700°C, these oxides decompose due to their low thermal stability. After this decomposition, most oxygen diffuses into the matrix, with titanium remaining in local regions. This process enriches titanium in the interfaces, forming a brittle oxygen-rich Ti₂Ni network that is known to hinder the recrystallization process in heat treatment. Furthermore, when subjected to external loading, these precipitates can induce high misfit levels and local distortion, resulting in brittle fractures along the interfaces. Based on these results, we also propose approaches to avoid high-temperature-induced embrittlement in Ni-rich NiTi.