Anisotropic shock response of columnar nanocrystalline Cu

Abstract

We perform molecular dynamics simulations to investigate the shock response of idealized hexagonal columnar nanocrystalline Cu, including plasticity, local shear, and spall damage during dynamic compression, release, and tension. Shock loading ͑one-dimensional strain͒ is applied along three principal directions of the columnar Cu sample, one longitudinal ͑along the column axis͒ and two transverse directions, exhibiting a strong anisotropy in the response to shock loading and release. Grain boundaries ͑GBs͒ serve as the nucleation sites for crystal plasticity and voids, due to the GB weakening effect as well as stress and shear concentrations. Stress gradients induce GB sliding which is pronounced for the transverse loading. The flow stress and GB sliding are the lowest but the spall strength is the highest, for longitudinal loading. For the grain size and loading conditions explored, void nucleation occurs at the peak shear deformation sites ͑GBs, and particularly triple junctions͒; spall damage is entirely intergranular for the transverse loading, while it may extend into grain interiors for the longitudinal loading. Crystal plasticity assists the void growth at the early stage but the growth is mainly achieved via GB separation at later stages for the transverse loading. Our simulations reveal such deformation mechanisms as GB sliding, stress, and shear concentration, GB-initiated crystal plasticity, and GB separation in nanocrystalline solids under shock wave loading.

Shock response of nanotwinned copper from large-scale molecular dynamics simulations

“…An impact velocity of 600 m/s was chosen here, at which the NT polycrystalline shows plasticity under shock compression due to GBs while the NT single-crystalline Cu remains elastic upon shock compression. 20 This impact velocity was also found to generate enough tensile stress to induce spall in both the NT polycrystalline and single-crystalline Cu. 22 We divided the simulation cell into fine bins only along the shock direction (the 1D binning analysis) by neglecting the heterogeneities in the transverse directions, and the average physical properties were obtained in each bin, such as particle velocity (U p ) and stress (σ ij ).…”

“…The center-of-mass velocity of a bin was removed when calculating σ ij from the decomposed virial method. 20,47 The free surface velocity (U fs ) versus time profile (t) was obtained from the particle velocity evolution at the target free surface. In order to calculate the strength behind the shock front and study the pressure evolution during the spall process, we defined the maximum shear stress 2τ max as 2τ max = σ zz − (σ xx + σ yy )/2 and the hydrostatic pressure P as P = −(σ xx + σ yy + σ zz )/3.…”

“…The nucleation of voids is mainly attributed to a mechanical separation and sliding of grains at the GBs and not by dislocation behaviors in the grain interior under tensile shock loading. 12,20,22 The continue tensile shock deformation increases the size of voids, and eventually causes the coalescence of voids and completely spallation. Previous ex situ and in situ quasistatic tensile experiments on nanocrystalline metals 48,49 also showed that fracture is an intergranular manner and is initiated by the nucleation of voids at GBs and triple junctions through unaccommodated GB sliding.…”

“…Previous MD studies on the shock response of Cu were mostly limited on either single-crystal or twin-free nanocrystalline (NC) metals. [6][7][8][9][10]12,13,[17][18][19][20]22,27 Germann et al 6,7 and Bringa et al 9 studied the shock propagation along the low index directions of [001], [110], and [111] for single-crystal Cu by using Lennard-Jones (LJ) potentials and embedded atom method (EAM) potentials, respectively. The shock-induced plasticity from their results was quite similar qualitatively.…”

Shock wave loading and spallation of copper bicrystals with asymmetric Σ3⟨110⟩ tilt grain boundaries

“…Atoms in the slip bands can be in the hcp packing during plastic deformation of Cu, thus a manifestation of crystal plasticity. 10,22 To characterize the extent of deviation from planar shock, we also calculate the displacement field D i ͑x , y , z͒, simply defined for each atom relative to its preshock position.…”