We studied microstructure evolution in a 4Npolycrystalline copper subjected to zero-to-tension fatigue throughin situ monitoring of shear-wave attenuation and velocity usingelectromagnetic acoustic resonance (EMAR). Contactlesstransduction based on the Lorentz force mechanism is the key toestablishing a continuous monitor for the microstructural changein the bulk of metals with a high sensitivity. In a short interval,between 20 and 40 percent of the total life in the order of l0~4-l0~5cycles, attenuation experiences a large peak and ultrasonic velocityshows a depression, being independent of the cyclic stress ampli-tude. This novel phenomenon is interpreted in terms of drasticchange in dislocation mobility and rearrangement, which issupported by the replication for slip bands and TEM observationsfor dislocation structure. At this particular period, the densedislocation structure starts to transform to cells, which temporallyaccompanies long, free dislocations absorbing much ultrasonicenergy to produce the attenuation peak. The possibility ofremaining-life prediction is discussed.
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