Effects of Direct Aging Heat Treatments on the Superelasticity of Nitinol Produced via Laser Powder Bed Fusion

This work investigates the effects of short-time direct aging heat treatments on the mechanical properties and microstructure of additively manufactured Nitinol (NiTi) alloy. Cylindrical samples were produced through laser powder bed fusion (L-PBF), directly aged at different temperatures and compared to the solution annealed and aged conditions. Compression tests were carried out at room temperature both in cyclic mode at constant strain and incremental cyclic mode, to provide a comprehensive analysis on the superelastic features of NiTi after direct aging heat treatments. Furthermore, cyclic compression tests were performed at 37 °C to evaluate the superelastic effect at the body temperature and, therefore, the possibility to use these treatments for biomedical components. The effects of direct aging on the microstructure were characterized using transmission electron microscopy (TEM). High cyclic stability and superelastic recovery up to 10 pct of deformation emerged for the direct aged alloys. The comparable results obtained with and without the solution treatment points out that this step was not necessary in reaching superelasticity, proving the effectiveness of direct aging.