Creep-induced nonlinear acoustic characterizations and microstructural change in an austenitic stainless steel

We investigated the evolutions of two nonlinear acoustic characterizations; the resonant frequency shift and the mixed frequency response, with electromagnetic acoustic resonance (EMAR) throughout the creep life in austenitic stainless steel, JIS-SUS 304, and clarified the relationship between their evolutions and the microstructural changes. EMAR was a combination of the resonant acoustic technique with a non-contact electromagnetic acoustic transducer (EMAT). We used bulk-shear-wave EMAT, which transmits and receives shear-wave propagating in the thickness direction of a plate specimen. Two measured nonlinear acoustic characterizations showed the peaks at 40% and the minimum at 60% of the creep life, respectively. We interpreted these phenomena in terms of dislocation recovery, recrystallization, re-structuring, and the initiation and growth of creep void, with support from the SEM and TEM observations. This noncontact resonance-EMAT measurement can monitor the evolution of the bulk-shear-wave nonlinearity throughout the creep life and has the potential to assess the damage evolution and predict the creep life of metals.