The effect of strain rate on the inelastic properties of nanocrystalline Au films was quantified with 0.85 and 1.76 lm free-standing microscale tension specimens tested over eight decades of strain rate, between 6 106 and 20 s1. The elastic modulus was independent of the strain rate, 66 ± 4.5 GPa, but the inelastic mechanical response was clearly rate sensitive. The yield strength and the ultimate tensile strength increased with the strain rate in the ranges 575–895 MPa and 675–940 MPa, respectively, with the yield strength reaching the tensile strength at strain rates faster than 101 s1. The activation volumes for the two film thicknesses were 4.5 and 8.1 b3, at strain rates smaller than 104 s1 and 12.5 and 14.6 b3 at strain rates higher than 104 s1, while the strain rate sensitivity factor and the ultimate tensile strain increased below 104 s1. The latter trends indicated that the strain rate regime 105–104 s1 is pivotal in the mechanical response of the particular nanocrystalline Au films. The increased rate sensitivity and the reduced activation volume at slow strain rates were attributed to grain boundary processes that also led to prolonged (5–6 h) and significant primary creep with initial strain rate of the order of 107 s1.
Nanocrystalline materials; Thin films; Ductility; Microvoids; Creep tests
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