Investigation of the martensitic transformation and the damping behavior of a superelastic Ti-Ta-Nb alloy

In this study the α′′ stress-induced martensitic transformation and damping behaviour of the superelastic β-Ti–25Ta–25Nb alloy are investigated by tensile tests at room temperature and by dynamic mechanical analysis (DMA) in tensile mode for different applied stresses. Tensile tests show a fully non-linear elastic domain and, consequently, a specific method is proposed to determine the elastic modulus. Due to the wide range of temperature over which the martensitic transformation occurs in this class of alloys, the martensitic start temperature, Ms, is not a relevant parameter to characterize the transformation and the temperature Mmax corresponding to the temperature of maximum transformation is used. The important gap between these two temperatures explains the fully non-linear elastic behaviour of this alloy during conventional tensile tests. It is observed that two main damping sources occur in this alloy: friction at austenite/martensite interfaces during the martensitic transformation and friction at martensite/martensite interfaces at lower temperature. However, a third unexpected damping peak is also observed at high stress. Its origin is discussed with respect to the orientation of the applied stress and with regard to the most favourably oriented martensite variants determined by Schmid factor analysis.