TEM Characterization of Nitinol Powder for Additive Manufacturing

Additive Manufacturing (AM) gets increasing attention for Nitinol processing because it provides the opportunity to circumvent many of the challenges associated with conventional machining of Nitinol as well as tailoring patient-specific anatomical shape of implants. The properties of the AM manufactured parts are strongly influenced by powder properties, particularly oxygen content is a crucial factor when components with medical grade Nitinol are required.
The present work provides characterization of the microstructure and the oxide surface layer by TEM investigations for a pseudoelastic Ni50,8Ti49,2 powder processed by EIGA with an average particle size of 33 µm. FIB-lamella were prepared from two separate particles. Subsequently, the microstructure was characterized by TEM and diffraction patterns as well as EDX mappings and line scans. The evaluation of the results exhibits that the microstructure of the powder particle consists mostly of a single large grain with no inclusions and with a high dislocation density. Furthermore, two layers can be identified on the particle surface, a titanium oxide layer, and a nickel rich layer underneath the titanium oxide layer. The oxide layer thickness resulting from the line scan was compared with the oxygen layer thickness calculated from the powder oxygen content and the particle size. It was shown that the oxygen content of the powder is completely contained within the titanium oxide on the particle surface.