Reconstruction of Heat Sources Induced in Superelastically Loaded Ni-Ti Wire By Localized Deformation Processes

Abstract

Background: Shape memory alloys (SMAs) are phase transforming materials featuring strong thermomechanical couplings. Infrared thermography and heat source reconstruction (HSR) enable to track the calorific signature of deformation processes. Objective: The objective was to characterize the transformation processes in a superelastic nickel-titanium SMA wire subjected to a force-controlled superelastic tensile cycle. Methods: In-situ recorded thermographs were converted into spatiotemporal maps of heat sources using an in-house developed post-processing method based on the heat diffusion equation resolved numerically for unknown heat sources. Results: Sequentially appearing patterns of localized transformation events of four types were identified and associated with martensite bands nucleations and their subsequent merging upon tensile loading. Analogically, the events associated with austenite bands nucleations and their subsequent merging were identified upon unloading. In addition, weak heat sources observed before and after the localized transformation events were associated with the homogeneous martensitic transformation. Conclusions: The intrinsic dissipation heat associated with the nucleation and merging events is estimated to be ~ 25% of the released/absorbed latent heat.