Improvement of SMA-NiTi Sintering by Thermally Treated of Titanium Powder under Hydrogen Atmosphere

Powder metallurgy (PM) methods have been used for production of near net shape parts of NiTi shape memory alloys (NiTi SMAs) during last two decades. The main drawback of the methods is formation of undesirable components and intermetallics such as NiTi2, Ni3Ti and oxide phases. These compounds affect on shape memory, superelasticity and mechanical properties of NiTi parts. In this work, it is attempted to improve the production of NiTi parts by thermally treated of titanium powder under hydrogen atmosphere. X-ray diffraction (XRD) patterns have shown that the annealing temperatures of titanium powder less than 400°C reduces the oxide phase amount specially Ni3Ti3O after sintering of titanium and nickel powders. The best result was obtained for titanium powder that heat treated in 300°C for 5h. In this condition, the peaks related to NiTi are appeared as the main peaks in the XRD pattern. Micrographs of the specimens by scanning electron microscope (SEM) confirm the XRD results. Differential scanning calorimetry (DSC) curves of the specimen show distinct peaks during heating and cooling cycles.

Keywords:

Nitinol , Oxide phase , Heat treatment , Hydrogen atmosphere