TEM Characterization of Nitinol Powder for Additive Manufacturing

Additive Manufacturing (AM) gets increasing attention for Nitinol processing for medical devices because it provides the opportunity to circumvent many of the challenges associated with conventional machining of Nitinol as well as the tailoring of patient-specific implants. Frequently used AM techniques for Nitinol are powder-bed technologies such as Selective Laser Melting (SLM) which is a suitable method for creating complex parts. However, functional Nitinol parts are strongly influenced by the powder properties. In particular, the oxygen content is a critical factor when components with medical grade Nitinol are required. Trace amounts of oxygen form an oxide covering the powder surface with a nanometric thickness as well as complex structure and composition due to the high specific surface energy of the powder. The oxide layer is depending on ambient conditions during powder manufacturing as well as powder handling. The present work provides characterization of the oxide layer and the microstructure for a pseudoelastic Ni_50.8Ti_49.2 Nitinol alloy powder for AM by TEM investigations. Furthermore, for an estimation of the powder oxide layer thickness, the calculated oxide layer thickness resulting from the oxygen content and the particle size is compared with the measured oxide layer thickness by an EDX line-scan.