Acoustic Emission During Austenite → ε Martensitic Phase Transformation in TWIP/TRIP Steels

Acoustic emission in two different TWIP/TRIP steels during thermally induced γ – ε martensitic phase transformation was investigated. Large asymmetry was observed: the noise activity was considerably larger for heating than cooling. This was explained by the plastic deformation and strain-induced martensite nucleation (which usually provides much lower acoustic emission signals) in the austenite during cooling. The amplitude and energy probability distribution functions for heating followed the power-law behavior, and the critical exponents were counted as well. The amplitude and energy exponents ( α and ε , respectively) for the two different samples were the same within the errors ( α = 2.4 ± 0.2, ε = 1.7 ± 0.1), indicating the universal character. The acoustic activity vs martensite volume faction showed a maximum at around 60 pct for heating, which is most probably related to the coming apart of the elastic fields of the martensite variants.