Influence of thermal annealing on the microstructure evolution, fracture and fatigue behavior of nanocrystalline Cu films

In this study, the effect of thermal annealing on the microstructures, fatigue and fracture behavior of nanocrystalline Cu films on compliant substrates was investigated. Microstructure examinations revealed the void elimination, grain growth and twin formation after thermal annealing. Monotonic tensile results showed that the crack onset strain was dramatically improved from 3% to 13% after thermal annealing, while the yield strength was not reduced significantly, which were rationalized by the elimination of nanovoids and the formation of twins. In contrast, cyclic tests revealed that the fatigue tolerance was notably deteriorated by thermal annealing, which can be explained in terms of coherent twin boundaries and nanovoids. The coherent twin boundaries tend to trigger the fatigue cracks due to the elastic modulus mismatch between parent and twin as well as the difference in the Schmid factors, while the nanovoids facilitate the dynamic grain growth and delays the formation of fatigue cracks.