Growth and Collapse of Nanovoids in Tantalum Monocrystals Loaded at High Strain Rate

Shock-induced spall in ductile metals is known to occur by the sequence of nucleation, growth and coalescence of voids, even in high purity monocrystals. However, the atomistic mechanisms involved are still not completely understood. The growth and collapse of nanoscale voids in tantalum are investigated under different stress states and strain rates by molecular dynamics (MD) simulations. Three principal mechanisms of deformation are identified and quantitatively evaluated: shear loop emission, prismatic loop formation, and twinning. Dislocation shear loops expand as expected from a crystallographic analysis, and their extremities remain attached to the void surface in tension (if there is no dislocation reaction or cross slip), but can detach in compression and form prismatic loops due to cross slip and reactions. Prismatic loops that detach from the void are also formed by reaction of multiple shear loops sharing the same <111> slip direction during hydrostatic loading. Nanotwins form preferably upon both uniaxial and hydrostatic tensile stress. The void-size effect on plasticity is studied via MD simulations and is modeled based on the shear loop emission mechanism. The stresses required for generation of a free surface step, dislocation and bow are calculated by continuum dislocation theory. The predictions agree well with MD simulation results.