Effect of electrical heating conditions on functional fatigue of thin NiTi wire for shape memory actuators

Abstract

Shape memory alloys (SMAs) are used in several technological applications thanks to their unique properties: superelasticity and shape memory effect. In recent decades, many efforts have been made to improve performance of SMA wires in order to utilize them as thermal actuators for hundreds thousands cycles. Near-equiatomic Nickel-Titanium alloys are the most employed materials for this purpose because of their cycling stability and their large recoverable strain values. Thermo-mechanical cycling were carried out to study the change of functional properties of the material in using condition (shape memory fatigue or functional fatigue). In particular, thin Ni49Ti51 wires (76μm) for actuators were heated by Joule effect with different current impulses (step and ramp) under constant stress (190 MPa) and then cooled by natural convection air cooling. During these test, SMAs accumulate microstructural defects and nano-scale precipitates which induce significant modifications in functional properties. DSC scans were carried out before and after 3x105 cycles in order to show changes in transformation temperatures caused by different current impulse thermo-mechanical cycling. Important alterations in terms of thermal hysteresis and deformation recovering were also noticeable by constant load test executed by DMA. Finally fracture surfaces were analysed by scanning electron microscope.