Analogies and Differences Between Tensile and Free Recovery (TFR) and The Common Techniques For Superelastic Nitinol Characterization

In 2006, M. H. Wu, M. Polinsky and N. Webb proposed a critical approach to the “big deal” about the active Af temperature and they concluded that “Materials having a similar active Af_BFR temperature can exhibit diversely different Young’s modulus, plateau stresses, UTS as well as fatigue endurance. Therefore, active Af_BFR temperature alone cannot be interpreted as the sole transformation parameter for specifying Nitinol devices”. This original (and unique) work did not have any further development.

A novel characterization methodology for the evaluation of thermo-mechanical properties of NiTi-based superelastic alloys has been proposed and developed: on Ø 0.3 mm straight annealed commercial wires, TFR is found to be a powerful, fast, easy and reliable test. The present work analyzes analogies and differences between TFR and other well-known characterization techniques such as Differential Scanning Calorimetry (DSC) and Bend and Free Recovery (BFR). As the data confirm, the transformation temperatures from TFR are found to be much more correlated with the mechanical properties (plateaus stress levels) of the material. Since the relationship between them appears very strong, it can be concluded that the method is characterized by a very high resolution. The measurement of the electrical resistance upon heating highlights another interesting aspect: The full shape recovery seems to be reached by the material before the completion of the thermal transformation. Finally, a critical analysis on the role of the transformation temperatures is provided with the aim to contribute in answering to one of the still open questions around Nitinol.