Tension–compression asymmetry and size effects in nanocrystalline Ni nanowires

Abstract

We investigate size effects in nanocrystalline nickel nanowires using molecular dynamics and an EAM potential. Both compressive and tensile deformation tests were performed for nanowires with radii ranging from 5 to 18 nm and a grain size of 10 nm. The wires contained up to four million atoms and were tested using a strain rate of 3.33 × 10⁸ s⁻¹. The results are compared with similar tests for a periodic system, which models a bulk macroscopic sample size of the same nanocrystalline material. The importance of dislocation-mediated plasticity decreases as the wire diameter is decreased and is more relevant under compression than under tension. A significant tension–compression asymmetry was observed, which is strongly dependent on the wire size. For the bulk nanocrystalline samples and larger wire radii, the flow stresses are higher under compression than under tension. This effect decreases as the wire radius decreases and is reversed for the smallest wires tested. Our results can be explained by the interplay of nano-scale effects in the grain sizes and in the wire radii.

Acknowledgements

This work was supported by NSF, Materials Theory program. The simulations were performed using System X, Virginia Tech's Supercomputer. We are grateful to Steve Plimpton for providing and maintaining the LAMMPS simulation code.