Effect of 160.DEG.C. Annealing on the Fatigue Damage and Small Crack Growth Behavior of Ultrafine Grained Copper

Abstract

Ultrafine grained (UFG) copper was processed by equal channel angular pressing. After the processing, some samples were annealed at 160°C (3 min). Materials with and without annealing are referred to UFG-A and UFG, respectively. Although the tensile strength of UFG-A was about 20% less than that of UFG, fatigue strength in the long-life field in excess of 107 cycles was higher in UFG-A than in UFG. The change in surface damage was monitored successively, showing the difference in formation behavior of surface damage between UFG and UFG-A. The surface hardness of both materials was severely decreased with stressing. The decreasing trend of hardness was closely related with the formation process of surface damage. The higher fatigue strength of UFG-A in the long-life field was discussed based on the morphological feature of surface damage and its formation behavior. Moreover, the growth behavior of a small crack was studied. The crack growth rate, dl/dN, of UFG was determined by a term σnal for all stress amplitudes examined (σa/σu≥0.24, σu : tensile strength). For UFG-A, however, dl/dNwas determined byΔK and σnal according to the magnitude of stress amplitudes.