Enhancing NiTi Shape-Memory Effect Properties Via Warm Working Process

Tuesday, May 21, 2013: 14:00
Congress Hall 2 (OREA Pryamida Hotel)
Yuri Khoptiar , Rafael Advanced Defense Systems Ltd., Haifa, Israel
Joseph Flomenblit , Rafael Advanced Defense Systems Ltd., Haifa, Israel
Royi Padan , Rafael Advanced Defense Systems Ltd., Haifa, Israel
Itamar Gutman , Rafael Advanced Defense Systems Ltd., Haifa, Israel
David Gorni , Rafael Advanced Defense Systems Ltd., Haifa, Israel
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 Af temperature high above 70oC (typical ambient temperature in military specifications). Yet, NiTi alloys heat treated to obtain high Af, 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 Af, 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).