Strain rate effects on the mechanical behavior of nanocrystalline Au films

The effect of fabrication, film thickness, and strain rate on the mechanical behavior of Au films with 100 nm (evaporated gold) and 200 nm (electroplated gold) average grain sizes was investigated. Uniaxial tension was imposed at 10− 3–10− 6 s− 1 strain rates on evaporated 0.5 μm and 0.65 μm thick Au specimens, and at 10− 2–10− 5 s− 1 on electroplated 2.8 μm thick Au specimens. Strain rates between 10− 3 and 10− 5 s− 1 had a marked impact on the ultimate strain of evaporated films and less significant effect on their yield and saturation stress. The ductility increased with decreasing strain rate and it varied between 2–4.5% for 500–650 nm thick films and 3.4–10.6% for 2.8 μm thick films. When compared at the same strain rate, the thick electroplated films were more ductile than the thin evaporated films, but their yield and saturation stresses were lower, possibly due to their larger grain size. Qualitatively, the stress–strain behavior was consistent at all rates except at the slowest that resulted in significantly different trends. A marked decrease of the maximum strength, effective Young's modulus, and yield strength occurred at 10− 6 s− 1 for thin, and at 10− 5 s− 1 for thick films, while for 500 nm thin films multiple stress localizations per stress–strain curve were recorded. Because of temperature, applied stress, and grain size considerations this behavior was attributed to dislocation creep taking place at a strain rate comparable to the applied strain rate.