Although metal fatigue has been intensively investigated for over 50 years, surprisingly little is known about the very early stage of fatigue damage initiation especially for ultra-fine grained (UFG) and nanocrystalline thin films. In this work we will try to find out what are the crucial parameters of local microstructure which “decide” where the fatigue damage will be initiated.
Polymer-supported thin Au and Cu films with UFG microstructures were subjected to cyclic strain with different amplitudes. In order to track the microstructural changes, cyclic tests were interrupted after given cycle numbers and the microstructure of the films was analyzed by electron backscatter diffraction (EBSD) and compared to the initial microstructure. In 250 nm thick gold films with strong (111) texture and log-normal grain size distribution the fatigue damage is initiated within large grains which have neither a low Taylor factor nor a high Schmid factor. By means of interrupted EBSD analysis it was possible to show that damage initiation in these gold films is a two-step process: first the room-temperature grain coarsening occurs and then the slip bands, extrusions and cracks are formed within the coarsened grains. In 250 nm thick copper films which have initially a bi-modal grain size distribution the initiation of fatigue damage was observed in the larger grains with preferable (100) orientation normal to the surface. By considering further examples of UFG thin films it is suggested that the local grain size plays more important role for the initiation of fatigue damage than the local grain orientation. It is important to note that in most cases the grain sizes were not constant being a dynamically changing factor which can be seen as an alternative mechanism of microplasticity. Not only initiation but also the propagation of crack/extrusion couples in polymer-supported films is often, if not always, accompanied by the local grain coarsening. Besides the grain size and grain orientation there are several indications that specifically oriented twin boundaries can play a role in fatigue damage initiation.