Nanotwin-induced strengthening in silicon: A molecular dynamics study

Abstract

Mechanical performance of silicon nanopillars with homogeneous and gradient nanotwinned structures are investigated through a series of molecular dynamics simulations. The most observed Σ3 twin boundary (TB) with two preferable (lowest surface energy) planes of {111} and {001} are used to generate homogeneous and gradient nanotwinned structures. Simulations of compression and tension of nanotwinned pillars reveal an extra strengthening behavior due to the addition of Σ3 TBs when compared to the single crystalline nanopillar without any TBs. The increase in strength of Σ3 nanotwinned pillars is attributed to a high density of dislocations in grains caused by reducing the twin thickness or increasing the number of TBs in a homogenous nanotwinned structure. Moreover, Hall-Petch strengthening behavior is observed as the twin thickness decreases to a critical value in silicon nanopillars with homogenous Σ3 {111} or Σ3 {001} TBs. Below the critical twin thickness, an inverse Hall-Petch phenomenon is observed, which results in softening of nanotwinned pillars. The pile-up of dislocations and their interactions with TBs cause enhancement in strength, while the nucleation and movement of partial dislocations parallel to the TBs cause the softening. Furthermore, the nanotwinned gradient structures with different configurations are examined, and an optimal nanotwinned gradient structure is obtained for maximizing the strength. Our simulation results provide fundamental understanding of twinning effect on mechanical deformation of homogenous and gradient nanotwinned silicon pillars.