Evaluation of generation conditions for phase transformations of Ti-Ni alloy with AE method (Comparison with results of structural analysis by MD method)

Abstract

SMA (Shape Memory Alloy) is utilized in various fields. Especially, Ti-Ni alloy has excellent features than other SMA. And, the alloy has two kinds of the phase transformations of rhombohedral (R) and martensitic (M) phase. Though R-phase transformation is not suitable for large deformation, it has been reported that its function for the cyclic deformation is steady. However, the detailed regulation for evaluating the function deterioration and life of the transformations has not been determined, because each factor affects the complexity to each other. Therefore, the establishment of the technique which evaluates the deterioration by the cyclic deformation is desired. In this study, the features of the AE (Acoustic Emission) signals detected in tensile and cyclic deformation test and the structure analysis results of the simulation with MD (Molecular Dynamics) method were compared. As the results, the following things became clear. The frequency components of the signals detected by R- and M-phase were about 750 and 350 kHz respectively. The properties of the signals in each phase showed the chaoticity, and they changed with the increase in the stress. The occurrence behaviors and features of the signals changed with the transitions of the hysteresis loop of σ-ε diagram by cyclic deformation. The generation condition of the R-phase seldom changed in the deformation until rearrangement of R-phase. On the other hand, the conditions on R- and M-phase decreased in the cyclic deformation until M transformation, because the residual phases disturbed the occurrence of M-phase. By observing the simulation results, it was possible to show the existence of the phase transformations of "Austenite (A) phase→R-phase→M-phase" and "A-phase→M-phase". And the correlations between those generation behaviors and the characteristics changes of the signals were recognized.

Keywords

• titanium-nickel alloy
• stress-induction
• rhombohedral phase transformation
• martensitic transformation
• tensile test
• cyclic deformation
• acoustic emission
• molecular dynamics
• structural analysis