Formation of β-Ti phase during L-PBF processing of martensitic NiTi

Abstract

Shape memory activations in martensitic NiTi are highly influenced by the processing technique used. Laser Powder Bed Fusion (L-PBF) is a metal additive manufacturing method being investigated for NiTi processing. The rapid transient thermal effects involved in L-PBF processing are found to cause microstructural anomalies which can result in unfavourable mechanical and functional properties. In this paper, anomalous hard protrusions observed in L-PBF processed NiTi microstructure were investigated in detail. The spatial distribution and cause of these protrusions were analysed. Studies were conducted using EDX, BSE, EBSD, TEM, DSC, AFM and nanoindentation, to identify the compositions, crystal structures, thermal characteristics and nanomechanical properties of these protrusions. The protrusions were identified to be hard β – Ti phase.