Initiation of fatigue damage in ultrafine grained metal films

The aim of the current research work is to understand how individual local microstructure features such as grain size, grain orientation, grain boundary characteristics, as well as the global parameters of the microstructure (texture, grain size distribution, twin boundary fraction) influence the process of fatigue damage initiation in thin films. Cyclic strain with an amplitude of 1% was applied to polymer-supported ultrafine grained gold and copper films allowing for observation of numerous independent localized plasticity events within a single macrosample. Detailed electron backscatter diffraction analysis of the microstructure in the vicinity of these plasticity events was performed to gain statistical information and elucidate robust correlations. On the local scale, the damage was initiated in large grains (>1 µm) in the vicinity of coherent twin boundaries specifically oriented with respect to the loading direction. When appropriate large grains were not present in the initial microstructure, mechanically induced grain coarsening preceded fatigue damage initiation. If the initial microstructure did not contain properly oriented coherent twin boundaries, then the local grain size of dynamically coarsened grains was the sole parameter controlling plastic slip localization. On the global scale, the major parameters of the microstructure influencing the fatigue damage initiation process are the fraction of twin boundaries, texture, and the width of the grain size distribution. Based on the presented results a clear strategy for improving the fatigue life of thin films is proposed.