Understanding the strain rate sensitivity of nanocrystalline copper using molecular dynamics simulations

A Rida and M Micoulaut and E Rouhaud and A Makke, COMPUTATIONAL MATERIALS SCIENCE, 172, 109294 (2020).

DOI: 10.1016/j.commatsci.2019.109294

Strain rate studies of polycrystalline materials can provide information on atomic scale mechanisms in tensile strain experiments. In this work, we use molecular dynamics simulations to investigate the influence of the strain rate on the mechanical properties of nanocrystalline copper systems at a fixed average grain size (9 nm). The samples were obtained by a melting-cooling of a perfect monocrystal. A series of uniaxial tensile tests were performed at 3 orders of magnitude strain rates lower than that usually used in atomic simulations ranging from 104 to 1010 s(-1) at ambient temperature. First of all, we found that the increase of the flow stress with the strain rate is caused by the delay in the onset of dislocation propagation. Furthermore, we show that while using a <(epsilon)over dot> >= 5.10(5) s(-1) in atomic simulations the system is not able to reach the equilibrium state with the ongoing deformation over the ns timescale. Even in the elastic regime, the effective Young modulus of the material was found to depend on the strain rate above this threshold. In addition, The strain rate sensitivity and the flow stress activation volume are calculated, their values are consistent with experimental studies. Finally, the strain rate dependence of dislocations, twinning and grain boundaries processes is quantitatively discussed at the atomic level.

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