Microstructure and mechanical behavior of oxide dispersion strengthened NiTi-20Hf alloy by Additive Manufacturing

Additive manufacturing (AM) is gaining high popularity as an advanced manufacturing technique for building complex shapes in a layer-by-layer fashion. The process design parameters during AM fabrication play a very significant role in the structural quality and resulting mechanical properties of the near net shape manufactured product. NiTi-Hf High-Temperature Shape Memory Alloys (HTSMAs) have been increasing in popularity as a high temperature actuator material and new forms are being explored using alternative manufacturing routes. In the past, NiTiHf alloys have been strengthened by solid solution and/or precipitate strengthening, but the latter is possible only if the alloys are Ni-rich. Thus, to accommodate slight variations in compositions, attention was given to the alloys where strengthening can be achieved via fine oxide-dispersion, i.e., to make oxide dispersion strengthened (ODS) alloys. In this study, NiTi-20Hf (at. %) alloy powder particles mixed with Y2O3 and HfO2 particles were used to additively manufacture a dispersion strengthened NiTi-20Hf alloy composite using Laser Powder Bed Fusion (LPBF). The microstructure of the powder particles before AM, and of the composites after AM were examined for bulk defects such as porosity and cracking. Differential scanning calorimetry and thermomechanical data were collected for each of the two alloy composites (NiTiHf-Y2O3 and NiTiHf-HfO2) to examine the effects of dispersoids on transformation temperatures and strength. Detailed microstructures of the composites, mechanical data and correlation between the two will be presented.