Tips and tricks for characterizing shape memory alloy wire: Part 1 - Differential scanning calorimetry and basic phenomena

Our intent in this series of articles is to provide recommendations for characterizing Nitinol SMA wire. We wish to forewarn experimentalists, who are relatively inexperienced with SMAs, of some pitfalls regarding experimental technique and interpretation of the data. This article provided DSC thermograms of the two Nitinol wire alloys, one that is austenite (stress free) above RT (shape memory wire) and one that is austenite at RT (superelastic wire), as a first step to characterize the materials. These were used to measure the transformation temperatures in the material, including start and finish temperatures for three martensitic transformations: A←R (Rs and Rf on cooling), R→M (Ms and Mf on cooling), and M!A (As and Af on heating). Furthermore, we showed how to extract specific latent heats of transformation and the specific heat of the material, providing recommendations for obtaining accurate data. The measured specific heat for both alloys was approximately the same, near 0.45 J/(gK), yet the latent heats of transformation were quantitatively different, 19.7 versus 15 J/g (M→R→A) for the shape memory wire and superelastic wire, respectively. The implications of this difference will be explored further in later articles in this series. In both alloys, the A↔M hysteresis was approximately 75-80°, and this is an important parameter to measure for each alloy used to quantify the inherent hysteresis in the material. We then described the phenomena of shape memory and superelasticity and the underlying microstructural mechanisms of the martensitic transformations responsible for both. Thermomechanical data were presented on the two Nitinol wires. Both exhibited shape memory and superelasticity but in different temperature regimes. The next article in the series (part 2) will address isothermal mechanical experiments over a wide range of temperatures for the same two Nitinol wire alloys and will map the various material phases in stress-temperature space. Experimental techniques will be discussed in order to achieve a consistent, high-quality data set of the fundamental temperature sensitivities involved.