Rate-limited plastic deformation in nanocrystalline Ni

Numerical simulations at nanometer scales have identified several mechanisms of plastic deformation. However, high strain rate regimes are required to resolve nanometer length scales. Extrapolating these numerical predictions at high strain rates to experimental conditions remains an unresolved challenge. Phase-field dislocation dynamics (PFDD) simulations are conducted to study the strain rate sensitivity of plastic deformation in nanocrystalline metals. The PFDD simulations involve the collective behavior of partial and extended full dislocations at strain rates ranging from 1 x 10(6) s(-1) to 1 x 10(9) s(-1) in Ni samples with an average grain size of 15 nm. Significant differences are found in the activation and glide of dislocations over this range of strain rates. At high strain rates, there are a large number of partial dislocations that begin to glide at grain boundaries across the entire sample. On the other hand, glide events are limited to a few grain boundaries at lower strain rates. Even though, the number of events is larger at high strain rates, mainly leading partial dislocations are active, and therefore, the amount of plastic deformation is smaller. This leads to an effective delay in plastic strain at high strain rates that explain the stress upturn observed at high strain rates when plastic deformation is carried out by partial dislocations. When extended full dislocations are present at lower strain rates, the yield stress is reduced by around 40%. (C) 2015 AIP Publishing LLC.