Unveiling the Atomistic Mechanisms of Shear-Induced LDA$\leftrightarrow$HDA Transformations and Shear Banding in Amorphous Silicon under High Pressures

Abstract

Large-scale molecular dynamics simulations of shear deformation under constant pressures of amorphous silicon, PT from low-density-amorphous (LDA) to high-density-amorphous (HDA) Si, and formation of shear bands (SBs) are performed using the state-of-the-art Gaussian Approximation Potential. The simulations reveal that LDA$\leftrightarrow$HDA shear-induced PTs occur simultaneously until reaching steady state. The developed mechanism-based analytical model well describes shear-strain-governed kinetics and steady states at all pressures, independent of shear stresses. Shear reduces the pressure for initiation and completion of LDA$\to$HDA PT by $4.36$ and $5.10$ GPa, respectively. Without PT at low pressure, shear-banding occurs, which is partially suppressed by PT at higher pressure with uniform deformation-PT at $9.8$ GPa. Despite the much larger shear and expected fraction of HDA, surprising sharp drop in the HDA atomic fraction within the SB was discovered. In bulk, Si deforms by atomic rearrangement in localized shear transformation zones with high nonaffine displacements, which trigger nucleation of HDA clusters within LDA and, concurrently, of LDA clusters within HDA, without growth and coalescence. In SB, a turbulent-like flow with swirls is revealed, which promotes reverse PT from HDA$\to$LDA more effectively. Transformation-induced plasticity in amorphous Si is revealed. The findings open up basic research into the mechanisms and kinetics of plastic strain-induced PTs in amorphous materials under high pressure, with numerous important applications.

18 pages, 10 figures