Shock deformation of face-centred-cubic metals on subnanosecond timescales

Abstract

Despite its fundamental importance for a broad range of applications, little is understood about the behaviour of metals during the initial phase of shock compression. Here, we present molecular dynamics (MD) simulations of shock-wave propagation through a metal allowing a detailed analysis of the dynamics of high strain-rate plasticity. Previous MD simulations have not seen the evolution of the strain from one- to three-dimensional compression that is observed in diffraction experiments. Our large-scale MD simulations of up to 352 million atoms resolve this important discrepancy through a detailed understanding of dislocation flow at high strain rates. The stress relaxes to an approximately hydrostatic state and the dislocation velocity drops to nearly zero. The dislocation velocity drop leads to a steady state with no further relaxation of the lattice, as revealed by simulated X-ray diffraction.

Picosecond X-Ray Diffraction from Laser-Shocked Copper and Iron

“…The first one was located at ∼ 0.18 µm and the second one at ∼ 0.4 µm along the z axis. Further details of the simulation are provided elsewhere, [7] and much was learnt about the role and interplay between ramped compression waves and dislocation sources. However, for the present discussion it suffices to say that large numbers of homogeneously-generated dislocations (> 1 − 2 × 10 13 cm −2 ) were found in the first part of the crystal before the first prismatic loop.…”

“…Above the transition pressure (image B) a two-wave structure is seen, with an initial wave corresponding to compressed bcc, and then, within a few lattice spacings, the material transforms to the hcp phase. The mechanism of the martensitic transformation corresponds to the [110] (or, degenerately, the [1][2][3][4][5][6][7][8][9][10]) plane of the original bcc lattice becoming the c-axis of the hcp phase. On these timescales there is negligible movement of the atoms in a direction normal to the shock propagation direction.…”

Discrete particle simulation of shock wave propagation in a binary Ni+Al powder mixture

“…[9][10][11] Modeling of the shock-compression of granular systems has been performed by a number of investigators. [12][13][14][15][16][17][18][19][20][21][22] However, still several questions remain unanswered. Some of these include the following.…”

Collective nature of plasticity in mediating phase transformation under shock compression

“…It is well known that if a material undergoes shock compression beyond the Hugoniot elastic limit, it exhibits rapid plastic flow which is expected to occur via the generation and propagation of defects (twin, dislocations, etc.) [1,[6][7][8][9][10], and possible phase transformations [11][12][13][14]. In the presence of large peak stresses, strain rates, and significant inelastic strain due to shock, these plastic deformation modes can differ from those observed under longer time scales or more quasistatic conditions [1,6], and may interact with the phase transformations [15,16].…”