Conjoint Influence of Thermal and Stress Cycling on Functional Fatigue Behavior of the NiTiZr Shape Memory Alloys

Abstract

Shape memory alloys (SMAs) are excellent candidates for selection and use in a spectrum of engineering applications, spanning actuators, hydraulic couplings, morphed aircraft wings, to name a few, and medical applications, spanning the domain of endovascular stents and orthodontic arch-wires. This is primarily because they exhibit the two distinct characteristics of shape memory effect (SME) and super-elastic effect (SE). The shape memory alloys (SMAs) are often required to function in a cyclic manner between their transition temperatures when chosen for use in a variety of applications. The cycling that they often go through can be broken down into the three distinct categories, namely: (i) thermal, (ii) mechanical (stress), and (iii) thermomechanical. These categories are essentially determined by the following: (i) functional properties of the material (SME or SE), (ii) temperature, and (iii) loading conditions. The phenomenon, known as functional fatigue, often occurs when shape memory alloy (SMA) actuators are subjected to repeated use due to thermal cycling or thermomechanical cycling. This causes the two functional properties, namely: (i) transformation temperatures and (ii) recovery strain, to be affected, which in turn causes an observable degradation in the shape memory characteristics of the chosen alloy. A noticeable fluctuation in the functional qualities is often dependent on the type of cycling that is performed. For instance, the transformation temperatures often tend to fall during thermal cycling but reveal an observable increase during thermomechanical cycling. These alterations are thought to be caused by the creation and presence of dislocations during the cycling process. A sizeable number of dislocations are often present during the early stages of the cycling process. However, as a result of dislocation–dislocation interactions that take place during sustained use of the device under cyclic conditions, the material will gradually become work-hardened. The thermal actuators often work under different upper cycle temperatures while the application is in use. This is essentially because the thermal actuators tend to gradually pick up heat from the surrounding atmosphere. Under these conditions, it is anticipated that the shape memory alloys (SMAs) will be able to function by undergoing partial transformation rather than full cycling with no detrimental influence on their overall performance. To build a SMA actuator having a combination of high performance and improved fatigue life, it is both essential and desirable to delve deeper into studying and evaluating the influence of both operational and testing parameters during thermomechanical cycling. In this paper, the results of a recent study aimed at investigating the impact of thermal cycling and stress cycling on NiTiZr high temperature shape memory alloy and its effect on functional fatigue are presented and briefly discussed.