A study of the dislocation instability and reverse martensitic transformation in FE-NI system

Since the martensitic reaction is a strain transformation, the transformation of the lattice structure can be induced by an external stresses. This also means that the macroscopic total elastic constant should decrease as the temperature is lowered to Ms on cooling. For the reverse ii transformation an anomalous decrease of the elastic constant is to be expected as the temperature increases to As on heating. In this experiment measurements of the velocity change of an ultrasonic wave in Fe-Ni alloys were carried out for the reverse martensitic transformation. The deviation from the linear relationship of the velocity change as a function of temperature is identified to be due to dislocation motion. An analysis of the data in terms of a dislocation string model applied to a stacking fault suggests that this deviation reflects the onset of the dislocation instability.