Stability of ultrafine-grained structure of copper under fatigue loading

Abstract

Stability of microstructure of ultrafine-grained materials under cyclic loading is a crucial condition for their good fatigue performance. Changes of microstructure due to fatigue in bulk, localization of cyclic plasticity into cyclic slip bands and related development of microstructure were experimentally studied on ultrafine-grained copper prepared by equal channel angular pressing. Different reaction of ultrafine-grained structure to plastic strain-controlled and load-controlled tests was found. The different susceptibility to dynamic grain coarsening under load and plastic strain-controlled tests reported in literature cannot be explained by differences in purity or details of equal channel angular pressing. The localization of the cyclic plasticity and the development of cyclic slip bands resulting in fatigue crack initiation take place in material volumes which can be characterized as "near by oriented" regions. They correspond to the shear bands, which are characteristic for ultrafine-grained structure after equal channel angular pressing.