Investigation of the Precipitation Processes in NiTi Filaments

Abstract

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.