FE-SEM in situ observation of damage evolution in tension-compression fatigue of micro-sized single-crystal copper

While understructures formed by fatigue strain cycling, such as veins, ladders and cell structures, bring about extrusions/intrusions on the surface of bulk metal, micro-sized metal does not have space to form an understructure, implying that another fatigue mechanism must apply in micro-sized metal. To date, information obtained on this mechanism has been very limited because of experimental difficulties. In this project, we develop an experimental method of tension–compression high-cycle loading for a micro-sized metal specimen that allows in situ observations of the fatigue mechanism by a field emission scanning electron microscope (FE-SEM). A micro-sized single-crystal copper specimen with a single slip orientation is subjected to fully reversed cyclic loading, and the damage process is successfully observed in detail. The fatigue failure is caused by extrusions/intrusions formed along the primary slip system in a slip band consisting of numerous plates with thicknesses of 15–20 nm. Detailed observations of the evolution process suggests that they are formed by the multiplication and glide of dislocations without a conventional understructure, in contrast to the case for the bulk counterpart. The fatigue process can be classified into the following three stages: (A) Evolution stage of fatigue damage, (B) steady stage and (C) cracking stage. Surprisingly, the extrusions/intrusions initiate at the reverse loading of the first cycle and continuously grow in proportion to the number of strain cycles even for less than 1% of the overall fatigue damage process.