One of the greatest promises of additive manufacturing is enabling new technologies, especially new paradigms in functional complexity.

A big limitation of shape memory alloys (SMAs) like nickel–titanium (NiTi) is that their unique characteristics only work with relatively simple, axially symmetric shapes like tubes and square stock. Conventional NiTi SMAs require cold work to achieve the proper superelastic behavior; while additive manufacturing could enable more complex part geometries, postprocessing requirements like cold work negate AM as a reasonable processing method.

ADAPT collaborated with NASA Glenn Research Center (GRC) and ADAPT member Confluent Medical Technologies to develop 38 wrought NiTiHf SMAs for custom functional performance. These compositions cover a range of functional performances, including high-temperature actuators, biomedical implants and ultra-dent-resistant bearing materials. Funding for the work was provided by NASA GRC, Confluent and the National Science Foundation.

These alloys, which use hafnium as a strengthening precipitate, hold the promise of requiring only heat treatment to attain functional shape memory performance. This opens the door to using AM to fabricate metal parts with shape memory characteristics and geometries that are far more complex than those made with conventional NiTi alloys.

Compared with the cold work required to add strength to NiTi, new NiTiHf alloys reach high strength and superelasticity through the formation of H-phase precipitates without cold working. These alloys can therefore be good candidates for using AM for SMAs.

Our researcher of the month, Dr. Behnam Amin-Ahmadi, has collaborated with NASA and Confluent to apply different chemical compositions and heat treatments to NiTiHf alloys to achieve the desired mechanical properties and martensitic transformation temperatures. The microstructures of these alloys have been investigated in detail using advanced microscopy techniques, such as scanning electron microscopy (SEM), focused ion beam (FIB), electron backscatter diffraction SEM (EBSD-SEM), high-resolution transmission electron microscopy (TEM), scanning TEM (STEM), and STEM with energy-dispersive X-ray (EDX) analysis, among others. These fundamental studies reveal the responsible mechanisms affecting transformation temperature, superelasticity and plastic deformation in these alloys.

The second part of Amin-Ahmadi’s project focuses on optimizing process parameters for AM NiTiHf and characterizing 3D-printed parts in detail along with two students that he is mentoring: ADAPT PhD student John (Charlie) Fuller and undergraduate research assistant Katerine (Katya) Flaska.

Once ideal alloy compositions were identified, two more ADAPT members contributed to the work: ATI Specialty Materials made the compositions in bulk powder (with cost share from ATI and funding from the Colorado Office of Economic Development and International Trade), and Elementum 3D printed test parts for characterization work in the ADAPT lab.

The use of AM with NiTi-X alloys removes the limitations of conventional NiTi SMAs and enables more complex parts, promising a new realm of applications for SMAs not only in the biomedical field but also in aerospace, automotive and wind energy applications.