Enhancing thermal stability of laser powder bed fusion fabricated 60NiTi alloy via Nb alloying

60NiTi (Ni55Ti45) alloy has a unique microstructure composed of a B2 NiTi matrix with supersaturated Ni atoms and numerous nanoscale Ni4Ti3 precipitates, resulting in excellent strength, hardness, and superelasticity. However, these characteristics make it challenging to directly form complex components. In addition, significant decrease in Ni saturation degree and coarsening of Ni4Ti3 precipitates lead to performance degradation of 60NiTi alloy during prolonged service. In this study, a 60NiTi-Nb (Ni55Ti40Nb5) alloy was successfully designed and fabricated using Laser Powder Bed Fusion (LPBF), which exhibits complex geometry and superior thermal stability. Particularly, after a heat-treatment at 500 degrees C for 5 h, the size of precipitates Ni4Ti3 in the Ni55Ti40Nb5 alloy increases by just over 10 nm, maintaining a hardness above 620 HV, whereas the Ni55Ti45 alloy experiences a precipitate size increase close to 200 nm, leading to a hardness decrease of 100 HV. The superior thermal stability is primarily attributed to the enrichment of Nb surrounding the Ni4Ti3 precipitates, which reduces the growth rate of the Ni4Ti3 precipitates, and the coexistence of spinodal decomposition and ordering in the matrix, thus enhancing microstructural stability. This study provides valuable insights for the development of 60NiTi complex structural components with high thermal stability, thereby promoting the application of 60NiTi alloys.