Investigation of the Precipitation Processes in NiTi Filaments
Precipitation in Ni-rich NiTi alloy is an influential process affecting material properties such as fatigue life, tensile strength or thermomechanical response. In this work, a nonconventional method of pulse heating by electric current was employed to recover cold-worked NiTi filaments (50 µm in diameter) and set the initial microstructure without precipitation. The impact of recovery processes and Ni-rich precipitates on mechanical properties can be separated to some extent, owing to the ultrafast electric current annealing. Afterwards, the NiTi filaments were subjected to an aging at 350–520 °C for 2–120 min. The aging at appropriate temperature allows a fluent and predictable adjustment of superelastic response together with an increase in tensile strength of approx. 20%. Small angle neutron scattering experiments were also performed to determine the mean size of Ni-rich precipitates. Less than 10 nm precipitates are created at 350 °C and 2 h aging, which yields a stable superelastic response and higher tensile strength. On the other hand, the filaments annealed at 520 °C show significant instability during superelastic cycling, a decrease in tensile strength and a precipitate mean size that significantly increases up to 500 nm.
KeywordsNiTi alloy Pulse heating Precipitation hardening Superelasticity
We kindly acknowledge the financial support from the student grant SGS ČVUT no. SGS16/249/OHK4/3T/14 and the Czech science foundation through project no. 16-20264S.
- 4.Otsuka K, Wayman CM (1998) Shape memory materials. University Press, CambridgeGoogle Scholar
- 12.Pilch J, Heller L, Sittner P (2009) Final thermomechanical treatment of thin NiTi filaments for textile applications by electric current. Esomat 2009. EDP Sciences. https://doi.org/10.1051/esomat/20090524