Heat treatment study and properties of quaternary SMA spring actuators

The properties of shape memory alloy (SMA) springs are influenced by many factors, including the alloy composition, the thermomechanical processing and the geometry (wire diameter, spring diameter, number of coils, pitch).

Quaternary SMAs offer a wide array of advantages over binary or ternary SMAs due to their versatile und customizable properties. By precisely adjusting the proportions of the four alloying elements the alloy for the spring can be customized in terms of mechanical properties, transformation temperatures, thermal and thermomechanical stability.

While the composition defines the base property range, especially, with the final heat treatment (shape setting) in combination with the prior deformation the microstructure can be fine tuned to fit the mechanical properties und transformation temperatures for the intended application of the SMA spring.

In this study the effect of the final heat treatment on the properties of quaternary SMA spring actuators was investigated. The results show that the properties of the actuator springs can be varied in a wide range by shifting the phase transformation temperatures and widen or narrow the transformation hysteresis. Just as with binary and ternary SMA overheating (temperature too high, time too long) leads to a coarser microstructure with low dislocation density which is known to be less resistant against functional fatigue. A higher final degree of deformation leads to higher phase transformation temperatures for the same heat treatment. At a constant degree of deformation, the phase transformation temperatures can be set in a range +/-5°C.