Shock response near the elastic to plastic transition in single crystal and porous silicon

We use molecular dynamics simulation methods to study the onset of the plastic wave transition in single crystal silicon, and characterize the altered response due to various degrees of porosity from 5 to 50 percent. Non-elastic response near onset of plasticity follows a mechanism similar to one shown previously in germanium, in which a propagating densification transition is driven by the release of shear stress in the material. This transition mechanism can be characterized as a partial transition from the ambient diamond structure to a distorted body center tetragonal ($\beta$-tin) structure. We show that this onset region is strongly influenced by porosity and large scale defects. Sandia National Laboratories is a multi program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.