Experimental observations on superelastic NiTi-Shape Memory Alloys and suggestions for modelling

Shape memory alloys like Nickel-Titanium (NiTi) exhibit exceptional mechanical properties due to their ability to undergo diffusionless solid-solid phase transformations. Distinguishing features are the shape memory effect and superelasticity, also referred to as pseudoelasticity. NiTi in particular additionally exhibits high strength and high transformation stresses around room temperature. These properties already led to a lot of new applications using NiTi. The talk gives an overview about different kinds of experimental observations with specimens made from NiTi: First the macroscopic behaviour of NiTi-wires in tension tests under different loading conditions is shown. Then transformation kinetics and thermo-mechanical coupling are examined using optical methods and thermo­graphy. Whereas copper-based alloys transform homogeneously, NiTi shows a distinct generation of transformation bands, which has to be taken into consideration during modelling. A simple one-dimensional model has been developed to show exemplarily methods accounting for these transformation bands. The model is based on plasticity but accounts for the thermo-mechanical coupling.