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

Acoustic emission in two different TWIP/TRIP steels during thermally induced γ–e 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 e, respectively) for the two different samples were the same within the errors (α = 2.4 ± 0.2, e = 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.