Transmission electron microscopy analysis of melt pool and cellular dendritic boundaries of laser processed Ti-Nb SMA

Due to the metastable nature, titanium-niobium beta-type shape memory alloys (SMAS) can be tailored via thermo-mechanical processing to have an elastic modulus ranging from 30 GPa to 110 GPa. Laser-based additive manufacturing techniques can further utilize such novel properties to parts requiring a locally varying elastic modulus. Based on our hypothesis, the apparent variation in modulus of Ti-Nb SMAS is related to a martensitic phase transformation with low triggering stress caused by microstructural defects. Therefore, elemental micro-segregations, phase transformations, defect generation, and internal stress states from laser processing can drastically impact the mechanical performance. In this study, we utilized the laser powder fusion bed (L-PFB) technique to perform line scans on a Ti-25 at% Nb alloy. We extracted transmission electron microscopy (TEM) samples across the melt pool boundary to capture both the melted and unmelted region. We observed martensite in the unmelted region but not in the melted region. The microstructure of the latter consists of Nano-cellular dendrites with an unusually large elastic strain field, sandwiched between nearly strain-free inter-dendrite boundaries. Precipitates are detected in dendrites but not inter-dendrite boundaries. This highly heterogeneous microstructure may be responsible for the unusual mechanical properties observed.