Effect of Symmetric Tilt and Twist Grain Boundaries on the Void Nucleation, Growth and Spall in polycrystalline Al : Multiscale modelling

Abstract

Multiscale modelling using molecular dynamics (MD) and hydrodynamics (HD) in sequence is carried out to obtain the dynamic spall strength of aluminum (Al). Specifically, we simulate the effect of symmetric tilt and twist grain boundaries (GB) on the void nucleation and growth in Al using MD and transfer the information to HD simulations of plate impact. MD is used to create bi-crystals mimicking 11 tilt and 12 twist grain boundaries (GB) along <100>, with orientation angles spanning 0° to 90° using an NPT ensemble. The grain boundary energy (GBE) is validated with published results. MD simulations of the tri-axial tensile deformation of the Al bi-crystals show that the voids nucleate at the grain boundaries and then grow and coalesce. The time history of the bulk pressure and void volume fraction are recorded for calculating the material specific parameters of a void nucleation and growth (NAG) fracture model. 1-D Hydrodynamic flyer plate impact simulations are performed using these NAG parameters to obtain the free surface velocity vs time curve. The spall strength and spall thickness of Al, estimated from the free surface velocity profile, show a good match with published experimental values of the dynamic spall strength (relative error of 2.2%–6%) and spall thickness (relative error of 2.4%–4%) for three impact velocities viz. 518, 1588, and 2275 m/s respectively.