Nanotwin-induced strengthening in silicon: A molecular dynamics study

H Nobarani and N Zhang and MA Zaeem, INTERNATIONAL JOURNAL OF MECHANICAL SCIENCES, 189, 105990 (2021).

DOI: 10.1016/j.ijmecsci.2020.105990

Mechanical performance of silicon nanopillars with homogeneous and gradient nanotwinned structures are investigated through a series of molecular dynamics simulations. The most observed Sigma 3 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 Sigma 3 TBs when compared to the single crystalline nanopillar without any TBs. The increase in strength of Sigma 3 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-Perch strengthening behavior is observed as the twin thickness decreases to a critical value in silicon nanopillars with homogenous Sigma 3 111 or Sigma 3 001 TBs. Below the critical twin thickness, an inverse Hall-Perch 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.

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