Development of fatigue damage in ultrafine-grained copper Lucie Navrátilová 1a , Ludvík Kunz 2b 1 Institute of Material Science and Engineering, Faculty of Mechanical Engineering, Brno University of Technology, Technická 2896/2, 616 69 Brno, Czech Republic 2 Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Žižkova 22, 616 62 Brno, Czech Republic a navratilova@ipm.cz, b kunz@ipm.cz Keywords: fatigue, crack initiation, ECAP, ultrafine-grained copper, FIB Abstract. Equal channel angular pressing is one of severe plastic deformation methods often used for preparation of ultrafine-grained (UFG) materials in bulk. Mechanical and, particularly, fatigue properties of UFG structures has been a subject of recent intensive research. The fatigue lifetime of UFG materials is closely related to the fatigue crack initiation. For conventionally grained (CG) materials the crack initiation mechanism is well described and understood. However, this is not the case of UFG structures. Furthermore, the knowledge obtained on CG materials cannot be straightforwardly transferred to UFG structures just from the reason of the grain size, which is smaller than the characteristic dimension of dislocation structures, which develops due to fatigue in CG materials. Copper is one of the most thoroughly studied model materials as regards the investigation of fatigue crack initiation mechanisms. The cyclic slip bands, which develop on the surface of cycled Cu and which are sites of crack initiation exhibit very similar features in CG and UFG material. However, the mechanism of the cyclic slip localization known from the CG Cu cannot work in UFG structure. The characteristic dislocation structures formed by fatigue and known from CG Cu cannot simply develop in UFG material unless the grain coarsening takes place. This contribution brings results of an experimental investigation of fatigue crack initiation in UFG copper. Development of fatigue slip bands was studied by means of focused ion beam (FIB) technique in scanning electron microscope. The observation of cyclic slip bands and material microstructure just beneath them shows that the grain coarsening is not a necessary prerequisite for crack initiation and development of surface relief. Observation of dislocation structure on thin foils prepared by FIB reveals well-developed cell structure below slip bands. The technique of ion- channeling contrast indicates that slip bands develop in regions of grains, which exhibit only very small mutual disorientation (low angle boundary regions). Based on these observations the mechanism of fatigue crack initiation in UFG Cu is discussed and compared with that known from CG copper. Introduction Increasing market demands for so-called “advanced materials” having special properties and acceptable cost is a significant feature, which strongly manifests itself during the last years. There is a tendency to substitute “older” materials in constructions and in manufacture of advanced components with the aim to increase their performance and to keep economic costs. UFG materials prepared by severe plastic deformation (SPD) methods, including equal channel angular pressing (ECAP), represent a promising group of such advanced materials. They have got the grain size in the range of 100 nm to 1 m and represent a transition between CG materials and nano-materials. In