Acoustic emission during cyclic deformation of ultrafine-grain copper processed by severe plastic deformation

Abstract

Acoustic emission (AE) is investigated during cycling at a constant plastic strain amplitude of ultrafine-grain (UFG) copper produced by severe plastic deformation, aiming at detailed characterization of AE in the time and frequency domain and clarification of fatigue damage mechanisms. The results of AE analysis in UFG copper are compared with those obtained in conventional polycrystals and single crystals. It is shown that fatigue damage and corresponding AE in UFG copper is controlled by three main mechanisms: large-scale shear banding, mode I tensile crack opening and mode II shear crack propagation. No AE that could be associated with ordinary dislocation mechanisms typical for conventional coarse-grain metals was detected in the UFG state. A close connection between strain localization, crack nucleation and grain boundaries in UFG materials is emphasized. It is shown that annealing at a moderate temperature of UFG materials fabricated by severe plastic deformation can reduce the degree of strain localization, modify the fatigue mechanisms and improve the fatigue life significantly.