Effect of Hydrogen Absorption on Mechanical Properties of TiNi Shape Memory Alloy Thin Wire

Abstract

TiNi shape memory alloys (SMAs) have received much attention owing to their unique shape memory properties. In this study, the size effects of hydrogen absorption on the shape memory behavior and strength of TiNi alloy were investigated using thin wires with diameters of 1.0mm and 0.1mm. A brittle layer was formed on the surface of the wires owing to hydrogen absorption. The thickness of the brittle layer increased with increasing hydrogen charging time and was found to be independent of wire diameter. The shape recovery rate and tensile strength after the hydrogen charging decreased with increasing hydrogen charging time, and the rate of decrease was larger for a wire with smaller diameter. In the tensile test, a periodic array of circumferential cracks was observed in the brittle surface layer before the tensile fracture. Thus, the fracture condition of thin hydrogen-degraded wires of TiNi alloy could be successfully predicted by combining the fracture mechanics and net stress criteria.