Effects Of The Aging-Induced Fine Tuning Of Transformation Temperatures On Thermo-Mechanical Properties and Fatigue Behavior Of Superelastic Nitinol

In the last years, a growing interest in the effects of aging on Ni-rich NiTi SMAs led researchers to focus the attention on precipitation of secondary phases and their effects on transformation temperatures (TTs), mechanical behavior and microstructural evolution. As well known, these properties vary with the alloy composition, the annealing conditions and the amount of cold-work as well as with many other parameters.

Several works studied the precipitation of the age-induced Ti₃Ni₄ phase, whose effect is mainly to generate a multi-stage transformation and thus a complex shift of the TTs. When the aging intensity is not sufficiently high to induce precipitation, other phenomena may take place inside the material. The microstructural mechanisms that a low temperature aging can activate are still not clarified, but its effects are well recognized in a slight and clear TTs tuning.

The present work systematically analyzes the effects of a secondary annealing on the properties of Ti-50.9Ni at.% straight annealed 0.3 mm wires. More than thirty different thermal treatments have been investigated and it has been possible to fine tune the transformation temperatures by independently shifting M↔R, R↔A, the gaps between them and those between the direct and the reverse transformations. After a full characterization of the wires, fatigue life of those samples that showed a fully-superelastic behavior (according to the standard requirements) have also been tested by rotary bending and compared with standard material. Results of this experimental campaign as well as still open issues will be presented and discussed.