Enhancing NiTi Shape-Memory Effect Properties Via Warm Working Process

The NiTi Shape Memory Alloys are employed in a variety of applications (such as small-scale actuators) which require mechanical properties sufficient to provide high actuation stress and low residual strain.  In addition, implementing SMA in military products requires A_f temperature high above 70^oC (typical ambient temperature in military specifications). Yet, NiTi alloys heat treated to obtain high A_f, usually suffer from decreased mechanical properties due to dislocation annihilation during partial annealing [1].

An innovative Warm-Working process was suggested for coping with challenge of having high A_f, high yield strength and low permanent set altogether [1]. Such process is expected to create internal sub-grain structure through polygonization, contrary to partial annealing taking place during hot-working.

In the current work, two lots of NiTi alloy rod (processed from the same ingot) were tested.  The lots differ by the area-reduction methods: traditional hot-working and the suggested Warm-Working method.  The results show superiority of the warm-worked SMA in terms of yield strength and permanent set.  Additional properties, such as transformation temperatures without loading (Bend-Free Recovery tests) or under stress (Constant Load-Deflection tests), and actuation stress applied by the pre-strained material during heating, will be presented.  Suitable post-working heat treatment leads to optimization of the desired properties.  The microstructure of both materials is characterized by TEM.

References:
[1]  R. Padan , Y. Khoptiar, I. Gutman, J. Flomenblit and D. Gorni, " Effect of Pre-Stressing on Transformation Temperature (Af) of Shape Memory NiTi Alloys", IMEC 15 proceedings (2012).