Metastable phase transformation and deformation twinning induced hardening-stiffening mechanism in compression of silicon nanoparticles

Y Hong and N Zhang and MA Zaeem, ACTA MATERIALIA, 145, 8-18 (2018).

DOI: 10.1016/j.actamat.2017.11.034

The compressive mechanical responses of silicon nanoparticles with respect to crystallographic orientations are investigated by atomistic simulations. Superelastic and abrupt hardening-stiffening behaviors are revealed in 110-, 111- and 112-oriented nanoparticles. The obtained hardness values of these particles are in good agreement with the experimental results. In particular, 111-oriented particle is extremely hard since its hardness (similar to 33.7 GPa) is almost three times greater than that of the bulk silicon (similar to 12 GPa). To understand the underlying deformation mechanisms, metastable phase transformation is detected in these particles. Deformation twinning of the metastable phase accounts for the early hardening-stiffening behavior observed in 110-oriented particle. The twin phase then coalescences and undergoes compression to resist further deformation, and leads to the subsequent re-hardening and re stiffening events. The same metastable phase is also detected to form in 111- and 112-oriented particles. The compression of such metastable phase is responsible for their hardening-stiffening behavior. In contrast, the crystal lattice of diamond cubic silicon is merely elastically deformed when compressing along 100 direction. Throughout the simulations, no perfect tetragonal beta-tin silicon phase formed due to the deconfinement status of nanoparticle comparing to the bulk silicon. A size effect on hardness of silicon nanoparticles, i.e., "smaller is harder", is also revealed. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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