Simulation of high strain rate contact of single crystal Al spheres

[Display omitted] •MD simulation of Al single crystal sphere in contact with a moving rigid plate.•Plate velocity from 10 to 1000 m/s; strain rates 8 x 108 s−1 to 8 x 1011 s−1.•Information obtained from force vs. displacement curve load drops.•High strain rate effects: increased atomic disorder at higher strain rates.•Crystal orientation effects: initial dislocation motion determines the outcome. Nanoindentation is used to probe mechanical properties of materials on a small size scale, and it is normally conducted under quasi-static conditions or at relatively low strain rates that are governed by experimental limitations. Using molecular dynamics (MD) simulations, we can now study similar contact events at strain rates up to ∼ 1 × 1012 s−1. At these high strain rates, additional deformation mechanisms, such as atomic amorphization and viscous flow, are anticipated. Here, we report MD simulations of 6 nm diameter, single-crystal aluminum (Al) spheres contacted by a rigid plate moving at velocities from 10 to 1000 m/s. Both global variables (force versus displacement) and atomic variables (dislocation activity, amorphization, and potential energy per atom) are measured as the sphere experiences plastic deformation. These measurements allow global and atomistic behavior to be correlated in a well-controlled simulation. At lower contact velocities, we observe that plastic deformation is dominated by dislocation activity accompanied by load drops. At higher contact velocities, the load drops disappear, and stiffening is observed. This behavior is accompanied by decreasing dislocation lengths and increasing numbers of atoms that are in the amorphous state and that deform via viscous flow. Dislocation-dominated deformation that occurs during the initial stages of deformation even at high contact velocities is found to play an important role in determining the final state of the sphere even when the original dislocations and previously disordered atoms have annealed.