Ultrahigh Strength in Nanocrystalline Materials Under Shock Loading

Abstract

Molecular dynamics simulations of nanocrystalline copper under shock loading show an unexpected ultrahigh strength behind the shock front, with values up to twice those at low pressure. Partial and perfect dislocations, twinning, and debris from dislocation interactions are found behind the shock front. Results are interpreted in terms of the pressure dependence of both deformation mechanisms active at these grain sizes, namely dislocation-based plasticity and grain boundary sliding. These simulations, together with new shock experiments on nanocrystalline nickel, raise the possibility of achieving ultrahard materials during and after shock loading.

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

“…The major difference between shock compression and homogeneous compression is that the strain and stress in the shock front are inhomogeneous and the strain rate in the shock front is very high. 10 However, the total volumetric strain behind the shock front, ε, is constant and determined by ε = U p /U s (dε/dt = 0). The stress along the shock direction behind the shock front, σ zz , is also constant and is given approximately by σ zz = ρ 0 U p U s , where ρ 0 is the density of the preshocked material.…”

“…It was found that higher impact velocities resulted in a higher strain rate, higher values of spall strengths, and a larger number of voids in smaller times. Bringa et al 10 have studied the pressure effect on the shock compression of NC Cu. An ultrahigh strength behind the shock front was observed due to the high pressure and the suppression of GBs sliding under shock loading.…”

“…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.…”

Deformation Substructures and Their Transitions in Laser Shock–Compressed Copper-Aluminum Alloys

“…Figure 4(c) shows similar loops (marked by arrows) that formed in a Cu-2%Al alloy subjected to a higher (~35 GPa) pressure. Thus, the loop generation and expansion mechanism is being supported by a considerable amount of molecular dynamics computations ( Lomdahl and Holian [21]; Bringa [22]). …”

A viscoplastic micromechanical model for the yield strength of nanocrystalline materials

“…In a recent contribution, Bringa et al [24] interpreted the stress profiles obtained by MD simulations of shocks in nanocrystalline Cu in terms of different shock pressure dependences of slip and GB accommodation. These authors assumed a linear dependence with shock pressure of the flow stresses behind the shock front associated with both dislocation slip (i.e.…”