Electric resistivity evolution in NiTi alloys under thermomechanical loading: phase proportioning, elasticity and plasticity effects

The well-known martensitic transformation is the main feature for almost all shape memory alloys (SMAs) usage. Meanwhile, the practical implementation of SMA in devices is not straightforward due to the evolution of their functional properties in operation. This evolution is mainly due to the different interactions between the martensite transformation (MT) or detwinning and mechanisms such as plasticity. Although these mechanisms are extensively studied by fine and precise techniques (e.g. high energy x-ray diffraction and transmission electron microscopy), their impact on a macroscopic level (usage scale) are not fully clarified. In this work, the effects of some of the most influential mechanisms in a NiTi alloy are investigated by using electric resistivity measurements at macroscopic scale. Distinct phase proportioning approaches are employed to analyze the martensitic transformation kinetic. It is found that, unlike elastic strains, plastic strains are a key influential factor on resistivity variations in SMAs. It is also shown that the use of an assumption of linearity between fraction of stress-induced martensite and strain transformation can lead to unrealistic interpretations of transformation mechanisms in NiTi wires.