Strain rate dependence of yield stress and early Bauschinger effect in nanoscale aluminum films with different textures

Ultrafi ne grained (UFG) and nanocrystalline (NC) metals, unlike coarse-grained metals, often show inelastic strain recovery during unloading, which is referred to as early Bauschinger effect. Several experimental and simulation studies have focused on the microscopic origins of this unusual mechanical behavior. However, the strain rate dependence of early Bauschinger effect in UFG and NC metals has not been explored. In this study, the stress–strain response of textured (bicrystalline) and nontextured (fully polycrystalline) aluminum fi lms (thickness 150–400 nm, mean grain size 100–300 nm) subjected to monotonic and cyclic tensile deformation was measured over a range of strain rates (10–5–10–2 s–1). For each experiment, both the residual plastic strain and recovered strain was extracted from the stress–strain response. The results show substantial differences in the strain rate dependent response of textured and nontextured fi lms. The textured fi lms show very little change in yield stress as well as the magnitude of early Bauschinger effect with increasing strain rate. In contrast, the nontextured fi lms show a large inelastic strain recovery at low strain rates (10–5–10–4 s–1) but the recovery nearly vanishes at higher strain rates (10–2 s–1). The yield stress of the nontextured fi lms, on the other hand, signifi cantly increases with increasing strain rate. These differences in macroscopic behavior are a direct consequence of the underlying deformation mechanisms operating in the textured and nontextured fi lms. In situ TEM observations show that the textured fi lms primarily deform by dislocation slip and very little grain rotation or sliding is present. Because dislocation slip is affected only at very high strain rates (> 10 s–1) even in nanocrystalline metals, yield stress and early Bauschinger effect in textured fi lms show little or no strain rate dependence between 10–5 and 10–2 s–1. On the other hand, signifi cant grain rotation is observed in nontextured Al fi lms during in situ TEM straining. This grain rotation appears to be accommodated by slow diffusive processes, which makes it highly rate dependent. Hence, both the early Bauschinger effect and yield stress of nontextured fi lms show a pronounced strain rate effect.