Alternating current potential drop studies on localization phenomena in ultra fine grained NiTi shape memory alloys

When a pseudoelastic NiTi specimen is subjected to uniaxial tension, it is often observed that the stress-induced martensitic transformation occurs in a localized manner, where macroscopic transformation bands form and propagate. Distinct transformation interfaces separate the specimen into regions of transformed martensite and austenite yet to transform. We have recently established alternating current potential drop (ACPD) measurements as a new experimental technique to characterize and track the positions of these interfaces during pseudoelastic cycling. In the present contribution, we apply this ACPD method to study localization events in ultra fine grained NiTi ribbons subjected to a wide range of thermo-mechanical load cases: (1) uniaxial tension at constant temperatures (pseudoelasticity), (2) cooling and heating through the transformation range under constant load (actuator load case), (3) combinations of mechanical and thermal loading. We monitor the ACPD signals in several zones along the gage length of our NiTi specimen, and we additionally characterize localized transformation events by thermal imaging and optical microscropy. Localization of transformation plays an important role under shape memory/actuator conditions, where stress-induced martensite is formed during cooling under a constant applied load. Moreover, changes in electrical resistance clearly highlight differences between thermally induced martensite (formed during cooling at no or very small loads), stress-indcued martensite (formed at higher loads), and reoriented martensite (formed when loads are applied at low temperatures). We discuss our results on a microstructural basis and we demonstrate that localized transformation events are a generic feature of pseudoelastic and shape memory deformation. We discuss the implications of our findings for actuator applications.