Formation of slip bands in poly-crystalline nano-copper under high-cycle fatigue of fully-reversed loading

The purpose of this study is to investigate the effects of a nano-scale stress-field on fatigue damage in a nano-copper component under fully-reversed and high-cycle loading. A resonant fatigue experiment is carried out for a cantilever micro-specimen that has a polycrystalline nano-Cu sandwiched by Si, Ti and SiN. Crystallographic slip bands associated with extrusion/intrusion of about 30 nm width, which is much finer than that in the bulk copper (≈1 μm), are formed on the Cu surface owing to the high-cycle fatigue loading. The new finding, the ultra-fine extrusion/intrusion, suggests the existence of different fatigue damage mechanisms in the nano-Cu from that in a bulk counterpart. The slip bands appear only in a particular grain though some others possess slip systems with higher Schmid factor. Detailed stress analysis, taking into account the Cu grains and surrounding dissimilar materials (Si, Ti, SiN), indicates that they are formed at a slip system with the highest resolved shear stress, which is in nano-scale. The formation stress is much higher than that in a bulk.