Strain rate effects on the mechanical behavior of nanocrystalline Au films

Strain rate sensitivity of nanocrystalline Au films at room temperature

“…These values are smaller than the modulus of bulk Au, reported as 78 GPa [22,23], but very consistent with modulus values reported for most tensile experiments with Au films in the literature [3,7,13,14,19,24,25]. Different authors have attributed the smaller elastic modulus to microstructure [14] and grain boundary relaxation [24], and, in the case of significantly smaller stiffness values, to film porosity [3,26]. Film porosity has been reported to be the reason for low elastic modulus, yield strength and tensile strength 3.…”

“…Earlier works by Emery and Povirk [13,14], Chasiotis et al [3] and Wang and Prorok [7] presented strain rate data for Au films that spanned strain rates between 10 À6 and 10 À3 s À1 . We extended this range to about eight decades of strain rate, between 6 Â 10 À6 and 20 s À1 , spanning time scales prone to different inelastic deformation mechanisms, such as creep and dislocation plasticity.…”

“…However, the beneficial effect of nanocrystallinity is weighed by the increased strain rate sensitivity [2][3][4][5][6][7] and increased creep rates compared to films with larger grain size. Nanocrystalline metals allow for large material volumes occupied by grain boundaries that control dislocation nucleation [4,8] and grain boundary mediated creep [9] while affecting the relative contribution of thermal and stress driven inelastic processes [10][11][12].…”

“…To date, experiments have been limited to a small range of strain rates, typically 10 À6 -10 À3 s À1 [3,7,13,14], which, although very indicative of the existence of rate sensitivity, investigated a narrow range of strain rates and did not clearly distinguish the effects of room temperature creep [15] and anelastic stress relaxation [16], which are present at slow strain rates, on the overall deformation. Steadystate creep rates for nanocrystalline face-centered cubic (fcc) metals at room temperature have been reported to be of the order of 10 À10 -10 À7 s À1 [17,18].…”

“…Therefore, unless experiments span regimes of strain rate significantly broader than 10 À6 -10 À3 s À1 , it may not be possible to separate the effects of stress induced plasticity and creep. There are additional considerations that need to be accounted for: the mechanical response of metallic films has been shown to vary with film thickness [3,7,19]. Espinosa et al [19] reported on an inverse thickness effect on the yield strength of Au and Al films.…”