Description
Ultrafine 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 films (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 films. The textured films show very little change in yield stress as well as the magnitude of early Bauschinger effect with increasing strain rate. In contrast, the nontextured films 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 films, on the other hand, significantly 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 films. In situ TEM observations show that the textured films 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 films show little or no strain rate dependence between 10–5 and 10–2 s–1. On the other hand, significant grain rotation is observed in nontextured Al films 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 films show a pronounced strain rate effect.
Recommended Citation
Izadi, E., & Rajagopalan, J. (2014). Strain rate dependence of yield stress and early Bauschinger effect in nanoscale aluminum films with different textures. In A. Bajaj, P. Zavattieri, M. Koslowski, & T. Siegmund (Eds.). Proceedings of the Society of Engineering Science 51st Annual Technical Meeting, October 1-3, 2014 , West Lafayette: Purdue University Libraries Scholarly Publishing Services, 2014. https://docs.lib.purdue.edu/ses2014/mss/mtfmm/32
Strain rate dependence of yield stress and early Bauschinger effect in nanoscale aluminum films with different textures
Ultrafine 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 films (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 films. The textured films show very little change in yield stress as well as the magnitude of early Bauschinger effect with increasing strain rate. In contrast, the nontextured films 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 films, on the other hand, significantly 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 films. In situ TEM observations show that the textured films 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 films show little or no strain rate dependence between 10–5 and 10–2 s–1. On the other hand, significant grain rotation is observed in nontextured Al films 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 films show a pronounced strain rate effect.