Thermomechanical behavior of NiTiPdPt high temperature shape memory alloy springs

Abstract

Transformation strains in high temperature shape memory alloys (HTSMAs) are generally smaller than for conventional NiTi alloys and can be purposefully limited in cases where stability and repeatability at elevated temperatures are desired. Yet such alloys can still be used in actuator applications that require large strokes when used in the form of springs. Thus there is a need to understand the thermomechanical behavior of shape memory alloy spring actuators, particularly those consisting of alternative alloys. In this work, a modular test setup was assembled with the objective of acquiring stroke, stress, temperature, and moment data in real time during joule heating and forced convective cooling of Ni19.5Ti50.5Pd25Pt5 HTSMA springs. The spring actuators were subjected to both monotonic axial loading and thermomechanical cycling. The role of rotational constraints (i.e., by restricting rotation or allowing for free rotation at the ends of the springs) on stroke performance was also assessed. Finally, recognizing that evolution in the material microstructure can result in changes in HTSMA spring geometry, the effect of material microstructural evolution on spring performance was examined. This was done by taking into consideration the changes in geometry that occurred during thermomechanical cycling. This work thus provides insight into designing with HTSMA springs and predicting their thermomechanical performance.

Thermomechanical behavior and microstructural evolution of a Ni(Pd)-rich Ni24.3Ti49.7Pd26 high temperature shape memory alloy

“…Even though a wide range of NiTiPd alloys have been developed and studied over the years, they have not been fully exploited in engineering applications, but they have been considered for a few aerospace-related components [50][51][52][53][54]. Besides the high cost of Pd, the unsuccessful commercialization of this system is largely due to unresolved issues with dimensional/thermal stability, and lack of data concerning fatigue and durability, which are of prime importance to actuator applications at elevated temperatures (above 100 °C).…”

An Intermetallic NiTi-Based Shape Memory Coil Spring for Actuator Technologies

“…The alloys produced by adding ternary elements, such as palladium (Pd), platinum (Pt), and gold (Au), to NiTi-based SMAs can widen the transformation temperature range to 100-800 • C [85,86]. Nicholson et al [87] studied the thermomechanical behavior of NiTiPdPt HTSMA springs. The authors observed that the transformation strains are generally smaller compared to conventional NiTi alloys.…”

Engineering design tools for shape memory alloy actuators: CASMART collaborative best practices and case studies

“…2. Nicholson et al (2014) utilizes the full-form force-deflection equations to determine apparent shear modulus, G M , of the as-shape-set spring. This apparent shear modulus, G M , was then used to decouple the effects of geometry and material evolution before and after thermal cycling under constant load.…”

“…reduced, modified, and/or full form) as the spring tool. The Ni 19.5 Ti 50.5 Pd 25 Pt 5 spring provided in Table 3 was used in Stebner et al (2009) and Nicholson et al (2014). The experimental results provided here are from Nicholson et al (2014).…”

“…Both versions use the same calculations and input and output labels (Table 5). The GUI-based (Dhakal et al, 2016) Ni 19.5 Ti 50.5 Pd 25 Pt 5 (Nicholson et al, 2014;Stebner et al, 2009) Ni 47.1 Ti 49.6 Fe 3.3 (Benafan et al, 2013) version is described in this section, while additional details on the DoE version can be found in Wheeler et al (2016).…”