NiTi shape memory alloy helixes through the hydriding–dehydring method

Abstract

Shape memory alloys have been adopted in many industries, including the biomedical and electronic industries, due to their unique properties. Actuator design is one of the areas in which the thermomechanical properties of shape memory alloys have played a major role in technological advances. Helical spring shape memory alloy actuators, a common type of actuator, are generally made by torsional loading of a straight wire; however, a stress concentration can result from the loading leading to a reduction of the recovery force. In this study, a low pressure hydriding–dehydriding method for producing shape memory alloy helixes from wires is presented. The helix pattern was obtained by hydrogen charging NiTi shape memory alloy 500 µm diameter wires for different time intervals, where the wires are embrittled and go through martersite-to-austenite phase transformation resulting in a helical crack. The wire hydrogen charging is then followed by dehydriding to remove the hydrogen. The wires and resulting helixes were characterized using scanning electron microscopy, differential scanning calorimetry, and synchrotron x-ray radiation diffraction. The relationship between the dimensions of the helixes and charging time was investigated.