Audio-frequency internal friction characteristics associated with the γ→ε martensitic transformation in Fe-Mn-Si based alloys

Audio-frequency internal friction and elastic modulus (square of frequency) of Fe-Mn-Si based shape memory alloys were measured as a function of temperature from 150 to 600 K using an electrostatic audio-frequency internal friction instrument. The internal friction characteristics of the martensitic and its reverse transformations have been investigated. The results show that the mechanisms of gamma-->epsilon martensitic and epsilon-->gamma reverse transformations may be different. The elastic modulus softening does not occur during these transformations, implying that the spontaneous nucleation by stacking faults would not need the lattice softening. The changes in modulus during the transformations are due to the fact that the modulus of the epsilon phase is higher than the gamma phase. The amount of the epsilon martensite after the gamma-->epsilon transformation decreases with the number of thermal cycling between 150 and 600 K.