Precipitation in superelastic Ni-rich NiTi alloy affects material properties such as fatigue life, tensile strength, or thermos-mechanical response. In this work, a nonconventional method of pulse heating by the electric current was employed to recover cold-worked NiTi wires and set the initial microstructure without precipitation. Thus, the impact of recrystallization, and aging on mechanical properties can be separated. After the initial pulse heating, the NiTi wires were subjected to various aging treatments at 250 °C – 400 °C for 1 h – 50 h. The aging at an appropriate temperature allows a fluent and predictable adjustment of superelastic response with increased tensile strength. However, the aging of samples with an increased gain size (d>250 nm) yields lower improvement in tensile strength and stability of cyclic superelastic response as the larger grain size offers a lower number of nucleation sites.
Moreover, TEM analysis was performed to determine mean size and distribution of Ni-rich precipitates in the NiTi matrix. Precipitation occurs predominantly on grain boundaries during aging at 400 °C, and functional stability in cyclic loading remains insufficient. Thus, aging at low temperatures (T ~ 300 °C) was employed, which yields approx. 5nm precipitates and substantially improves the homogeneity of precipitates distribution improving the stability of superelastic response and increasing tensile strength. Low-temperature aging is beneficial, especially in microstructures with large grain sizes, as these would be otherwise severely unstable in a cyclic thermo-mechanical loading.