Surface Damage of Ultra-Fine Grain Copper in High-Cycle Fatigue Regime
Masahiro GOTO, Seung-Zeon HAN, Terutoshi YAKUSHIJI, Cha-Yong LIM, Norio KAWAGOISHI and Sang-Shik KIM
Abstract:In order to study the formation process of surface damage of ultrafine grain copper, rotating bending fatigue tests of smooth specimens have been carried out. After a total of 8 passes of ECA pressing with Bc route, grains with about 250-300nm diameter were formed. Specimens were fatigued at three constant stress amplitudes; a = 240, 120 and 80 Mpa (corresponding fatigue lives were Nf 2x105, 4x106 and 5x107 cycles, respectively). Significant differences in morphological feature in post-fatigued surfaces between high and low cyclic stress amplitudes were observed. To clarify the formation process of surface damage, morphological changes in surface damage caused by cyclic stresses were monitored successively by an optical microscope. Putting those OM observations and SEM observations of post-fatigued surfaces together, the formation processes of surface damage were discussed. In addition to those experiments, SEM observation of fracture surface and measurement of surface hardness during fatigue tests have been made. Key Words:Fatigue damage, ECAP, Ultra fine grained copper, Shear bands, Grain boundary sliding, Hardness