How the evolution of the dynamic elastic modulus during isothermal tensile tests can bring new information on mechanisms deformation of a NiTi superelastic wire

Superelastic behaviour of NiTi shape memory alloys thin wires are characterised mechanically using the method of Dynamic Mechanical Analysis (DMA). The method uses a non-conventional device which allows to determine storage modulus at high level of stress. Dynamic elastic modulus is measured during isothermal tensile tests at three testing temperatures above Af (the reverse martensitic transformation finish temperature). These results are analysed and modelled to highlight deformation mechanisms involved (elasticity, transformation, localization, plasticity). The method brings new information and clarifies the ambiguities we may have by analysing only stress as function of strain. It is possible to identify clearly deformation mechanisms.

The DMA analysis shows that it is unrealistic to expect having a good estimation of the macroscopic elastic moduli of Austenite (A), R phase, and Martensite (M). Lastly, dynamic elastic modulus evolution has been modelled according to the mechanism involved (phase transformation and localization). Model allows to determine the value of Clausius-Clapeyron coefficient for forward (A→M) and reverse (M→A) transformations and avoids using of stress plateau where the deformation is localised.