Cyclic deformation induced strengthening and unusual rate sensitivity in Cu/Ru nanolayered films

In this work, we have systematically investigated the effect of cyclic deformation on the strength and rate sensitivity of Cu/Ru multilayers with different individual layer thickness (h) by nanoindentation tests. It was found that cyclic deformation remarkably enhances the hardness of Cu/Ru multilayers comparing with the specimens by monotonic loading. The rate sensitivity (m) of multilayer exhibits an anomalous size dependence after nanoscale cyclic deformation. When h > 10 nm, the m linearly increases with increasing cycle number of loading-unloading (s). However, the m sharply decreases with increasing s when h < 10 nm, presenting an inverse cyclic deformation effect on m. An obvious Bauschinger effect is observed during cyclic loading, where the evolution of effective stress is consistent with the m. Cyclic deformation induced dislocation accumulation and arrays at the heterogeneous interface are the intrinsic plastic mechanism for the enhanced rate sensitivity. The formation of amorphous layers at the critical h is mainly responsible for the inverse size m.