  NiTi-based high-temperature shape memory alloys (HTSMAs) such as NiTiHf have
been utilized in a broad range of applications due to their high strength and
work output, as well as, their ability to increase the transformation
temperatures (TTs). Recently, additive manufacturing techniques (AM) have been
widely used to fabricate complex shape memory alloy components without any
major modifications or tooling and has paved the way to tailor the
manufacturing and fabrications of microstructure and critical properties of
their final parts. NiTi alloys properties such as transformation temperatures
can be significantly altered due to oxidation, which can occur during the
manufacturing process or post-processing. In this work, the oxidation behavior
of Ni-rich NiTi20Hf shape memory alloys, which was fabricated by the selective
laser melting (SLM) method, is evaluated. Thermogravimetric analysis (TGA) is
used to assess the kinetic behavior of the oxidation at different temperature
ranges of 500, 700, and 900 C for 20 hours in the air. After oxidation, to
evaluate the microstructure and chemical composition X-ray diffraction (XRD),
scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy
(EDS) was conducted. The isothermal oxidation kinetics of conventional NiTi20Hf
alloys were studied, and the results were compared to AM samples. Results show
a two-stage oxidation rate at which oxidation increased with the high rate at
the initial stage. As the oxidation time increased, the oxidation rate
gradually decreased. The oxidation behavior of NiTiHf alloys initially obeyed
logarithmic rate law and then followed by parabolic rate law. SEM results
showed the formation of a multi-layered oxide scale, including TiO2, NiTiO3,
and Hf oxide.
