Ultrasonic attenuation peak during fatigue of polycrystalline copper

We studied microstructure evolution in a 4N polycrystalline copper subjected to zero-to-tension fatigue through in situ monitoring of sheer-wave attenuation and velocity using electromagnetic acoustic resonance (EMAR). Contactless transduction based on the Lorentz force mechanism is the key to establishing a continuous monitor for the microstructural change in the bulk of metals with a high sensitivity. In a short interval, between 20 and 40% of the total life in the order of 10(4)-10(5) cycles, attenuation experiences a large peak and ultrasonic velocity shows a depression, bring independent of the cyclic stress amplitude. This novel phenomenon is interpreted in terms of drastic change in dislocation mobility and rearrangement, which is supported by the replication for slip bands and TEM observations for dislocation structure. At this particular period, the dense dislocation structure starts to transform to cells, which temporally accompanies long, free dislocations absorbing much ultrasonic energy to produce the attenuation peak. The possibility of remaining-life prediction is discussed. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.