Characterizing Transformation Phenomena and Elastic Moduli of Austenite and Oriented Martensite of Superelastic Thin NiTi Wire through Isothermal Dynamic Mechanical Analysis

In this paper, superelastic behavior of nickel-titanium thin wires is characterized using the method of dynamic mechanical analysis. Nominal dynamic storage modulus \( E^{{\prime }} \) is measured as function of nominal strain and stress during isothermal superelastic tensile tests at three testing temperatures above the reverse martensitic transformation finish temperature. The method brings new information on deformation mechanisms compared to the consideration of only tensile stress–strain curves. It is shown that determination of the elastic moduli E, especially at high strain, requires to calculate the true storage modulus \( E_{\text{t}}^{{\prime }} \). Using \( E_{\text{t}}^{{\prime }} \) and not \( E^{{\prime }} \), elastic modulus of oriented martensite \( E_{\text{M}} \) is determined equal to 73 GPa of the same order than the elastic modulus of austenite \( E_{\text{A}} \) equal to 70 GPa. Two models are then proposed to simulate experimental storage moduli evolution during the tests. A first model explains the \( E^{{\prime }} \) evolution during stress plateau by the localization phenomenon; it leads to express \( E^{{\prime }} \) as function of the nominal strain. A second model describes the evolution of \( E_{\text{t}}^{\prime } \) after the stress plateau as function of true stress and test temperature. This model permits to determine the Clausius–Clapeyron coefficient of the forward transformation.