High-temperature oxidation behavior of NiTiNb shape memory alloys

Abstract

The NiTi shape memory alloys are smart materials of great scientific and technological importance, due to their excellent mechanical properties, high shape recovery capacity and biocompatibility. Such unique properties have made this class of materials find potential applications in several areas, such as biomedical, construction, aeronautical, automotive and naval industries. However, obtaining these alloys with low production cost is still a major challenge for the industry. The main contributing factor to this is the oxidation phenomenon. The manufacture of NiTi alloys goes through several steps involving high temperatures, and the transformations and variation of chemical composition caused by oxidation affect the shape memory effect. In addition, the formation of Ni2Ti4Ox brittle oxides may even cause ingot fracture. Therefore, processes at high temperatures must be conducted in an inert atmosphere, which makes the process more costly. Thus, ways to increase the oxidation resistance of NiTi alloys should be developed. Although still little explored, the addition of Nb, up to 7%, has already proved to be a positive factor, reducing oxidation rates. However, there are no studies verifying the influence of Nb contents higher than 7%. Based on this question, the present research proposal aims to evaluate the influence of Nb addition in 9%, 10.5% and 12% contents on the high temperature oxidation behavior of NiTi shape memory alloys. The study will be based on thermogravimetric techniques, X-ray diffraction (XRD), optical microscopy (MO) and scanning electron microscopy with dispersive energy X-ray spectroscopy (SEM/EDS). The phase changes and high-temperature oxidation mechanism of the NiTiNb alloys will be addressed based on the thermodynamic stability of the phases predicted by Thermo-Calc software using the TCHEA3 database. (AU)