Electrical Pulse Shape Setting of Very Thin Ni49Ti51 Wires for Shape Memory Actuators

Shape memory alloy (SMA) thin wires are employed as actuators in small devices for consumer electronics, valves and automotive applications. In order to optimize the shape memory effect, SMA wires have to undergo a shape setting heat treatment aimed to impart a straight shape to the product and to reorganize the microstructure altered by drawing process. This thermal treatment is generally carried out by wire strand annealing or by holding the sample into an oven for several minutes. In the current study a non-conventional heating method, proposed for superelastic wires [1], is investigated on Ti rich NiTi shape memory material. Low dimension Ni₄₉Ti₅₁ wires, 25 μm in diameter, were thermally treated by different high power electrical pulses of short duration under constant strain. In particular, an electrical pulse of 230 mA for 10 ms revealed to be suitable to impart good shape memory properties to the wire. An as-drawn specimen was conventionally treated at 400°C for 20 min and used as reference sample. The functional properties were characterized by thermo-mechanical cycling under constant load and correlated to shape setting parameters. Microstructural analysis was also carried out and reported.

 [1] Malard B., Pilch J., Sittner P., Gartnerova V., Delville R., Schryvers D., Curfs C. “Microstructure and functional property changes in thin NiTi wires heat treated by electric current - High energy X-ray and tem investigations” Functional Materials Letters Volume 2, Issue 2, June 2009, Pages 45-54