Controllable fatigue cracking mechanisms of copper bicrystals with a coherent twin boundary.

High-angle grain boundaries are always the preferential fatigue cracking sites, while the intrinsic fatigue cracking mechanism of coherent twin boundary remains elusive. Here we systematically investigate the fatigue cracking behaviours of copper bicrystals with a coherent twin boundary as their sole internal boundary. It is found with direct experimental evidence for the first time that, unlike the random grain boundaries, the cracking behaviour of the twin boundary strongly depends on its orientation with respect to the loading direction. When the twin boundary is parallel or perpendicular to the loading direction, the fatigue cracks nucleate along the slip bands preferentially; when it is inclined at an angle to the loading direction, the fatigue crack is especially apt to nucleate along the twin boundary first. The controllable fatigue cracking mechanisms of the twin boundary may provide new and important implications for the optimized interfacial design of the high-performance materials.