In this study, we investigated the localization of the macroscopic deformation during tensile test of nitinol at room temperature. The specimens were fully austenitic and therefore, the martensitic phase transition under deformation had place. It is established that the deformation phase transformation is realized by the formation and motion of fronts of localized deformation. Fronts moves at constant velocities and annihilated at the meeting. The velocities of their movement are determined by crosshead velocity and the duration of stress plateau. In this respect, the behavior of the phase-transition fronts is completely similar to the kinetics of the Lüders bands fronts observed in mild steel, but distinctive feature of this phenomenon is that the level of localization of the deformation at the phase transition front is an order of magnitude smaller than at the front of the Lüders band. In addition, the phase transition front has a more complex structure than the front of the Lüders band. These features of the kinetics and morphology of the transformation front lead to the fact that its motion takes place under hardening conditions, and inclined strain plateau is observed on the deformation curve.
Author: Gorbatenko V.
A study was made of the macro-scale plastic flow non-homogeneities, which occur in metals in the form of Luders bands or Portevin-Le Chatelier effect. The motion kinetics was investigated for the mobile fronts of Luders bands observed for the yield plateau as well as localized plasticity fronts traveling in the course of serrated plastic flow behavior (Portevin-Le Chatelier effect). It is shown that the propagation of the above two kinds of band fronts can be regarded as macro-scale auto-wave processes of switching and excitation, respectively, which frequently occur in active media of different kinds.