Influence of heat treatments on the mechanical properties of high-quality Ni-rich NiTi produced by powder metallurgical methods

NiTi alloys produced by different powder metallurgical (PM) methods are investigated concerning transformation behavior and shape memory characteristics. Effects of purity, heat treatments and processing conditions are taken into account. It is shown that the deformability of PM-NiTi is increased after a long-term treatment at 1250 °C when the two different types of secondary phases resulting from impurities of oxygen and carbon become larger in size, and their distribution is therefore more isolated. In the case of Ni-rich NiTi material, the mechanical behavior is further influenced by the precipitation of metastable phases (Ni4Ti3) at intermediate temperatures (350 and 450 °C), which reduce the deformability, again resulting in reduced elongations at fracture. The stress level and the elongation at fracture are reduced by a residual porosity.