Investigation of β-Ti phase formations during PBF-LB processing of Ti-rich martensitic NiTi

The shape memory activations in martensitic NiTi are highly influenced by the processing technique. The rapid transient thermal effects involved in PBF-LB processing are found to cause microstructural anomalies including the formation of undesired stable phases. These phases can cause adverse effects on the mechanical and functional properties of NiTi SMAs. In this work, anomalous hard inclusions observed in PBF-LB processed NiTi microstructures were investigated. The spatial distribution, properties, causes, and effects of these inclusions on shape memory properties (including cyclic stability) were analyzed in detail. Investigations were conducted using in-situ IR data, DSC, EDX, BSE, EBSD, S/TEM, AFM and nanoindentation, to identify process anomalies, thermal characteristics, compositions, crystal structures, submicron features and nanomechanical properties of these inclusions. These inclusions were identified to be hard β – Ti phase exerting effects on functional stability, a novel finding in the metal additive manufacturing route of NiTi.