Virtual melting and cyclic transformations between amorphous Si, Si I, and Si IV in a shear band

TL;DR

This study uses molecular dynamics simulations to validate virtual melting as a deformation mechanism in silicon under shear, revealing cyclic phase transformations within shear bands that influence plasticity.

Contribution

It introduces a thermodynamic criterion for virtual melting and uncovers cyclic amorphous-crystalline transformations in shear bands, a phenomenon previously overlooked.

Findings

Virtual melting occurs immediately after shear instability.

Cyclic transformations between amorphous Si, Si I, and Si IV are observed.

Transformation-induced plasticity contributes to deformation.

Abstract

Virtual melting (VM) as alternative deformation and stress relaxation mechanisms under extreme load is directly validated by molecular dynamics (MD) simulations of the simple shear of single crystal Si I at a temperature 1,383 K below the melting temperature. The shear band consisting of liquid Si is formed immediately after the shear instability while stress drops to zero. A thermodynamic criterion for VM, which depends on the ratio of the sample to shear band widths, is derived analytically and confirmed by MD simulations. With further shear, the VM immediately transforms to a mixture of low-density amorphous a-Si, Si I, and IV, which undergo cyclic transformations a-Si to and from Si I, a-Si to Si IV, and Si I to and from Si IV with volume fraction of phases mostly between 0.2 and 0.4 and non-repeatable nanostructure evolution. Such cyclic transformations produce additional important carriers for plastic deformation through transformation strain and transformation-induced plasticity due to volume change, which may occur in shear bands in various material systems but missed in experiments and simulations.