The shape memory effects and superelastic behavior of Ni-rich NiTi alloys (> 50.6 at% Ni) are significantly dependent on microstructural changes during heat treatment. Furthermore, it is also generally known that thermal treatment of Ni-rich NiTi alloys induces diffusion processes that lead to precipitation. Depending on temperature and holding time, several types of precipitates can be formed in microstructure of the alloy. Assumed transition path is: metastable Ni4Ti3 - metastable Ni3Ti2 - stable Ni3Ti. Thus, these and other microstructural changes can have a major impact on the martensitic transformation in NiTi alloys and, essentially, on the transformation temperatures. In addition, the precipitation of secondary phases can also contribute to a substantial increase in the strength of NiTi alloys. Another critical factor affecting the properties of these materials is the composition. The experimental samples of NiTi alloy with 50.7 at% Ni were subjected to heat treatment. The thermal regimes consisted of aging treatment at 300, 350 and 400 °C for 0.5 h followed by air cooling. The microstructures before and after the heat treatment were observed by optical microscopy. The study was completed by microhardness measurement and X-ray analysis. The obtained results show that the aging temperatures led to microstructural and microhardness changes for the investigated alloy.Keywords: NiTi alloy, heat treatment, aging, microstructure
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