Microstructure and mechanical properties of bulk NiTi shape memory alloy fabricated using directed energy deposition

Directed energy deposition (DED) can potentially allow manufacturing of Nitinol (NiTi) parts that are difficult to fabricate using traditional methods. Here, large NiTi square bars were built vertically in the z-direction by Laser Engineered Net Shaping (LENS™), using a pre-alloyed Ni-Ti powder. Laser power, laser scan speed, and powder mass flow rate were optimized using the design of experiments (DOE) methodology. Each bar was cut at the top, middle, and bottom sections for microstructural and mechanical analyses. The middle section was found to be the densest, and with a complex grain structure slightly more equiaxed and finer than in the top and bottom sections. X-ray diffraction detected only the B2 phase in all three sections. A surprising mechanical behavior is reported, for the first time, where the middle section exhibits the lowest Young's and shear moduli (based on an ultrasonic pulse-echo test) and the highest microhardness. Atom probe tomography (APT) allowed to rationalize the anomalous mechanical behavior based on the formation of a fine dispersion of Ni₄Ti₃ nanoprecipitates with the highest volume fraction in the middle section. These microstructural and mechanical heterogeneities have both fundamental and practical implications.