Shock Waves in Polycrystalline Iron

The propagation of shock waves through polycrystalline iron is explored by large-scale atomistic simulations. For large enough shock strengths the passage of the wave causes the body-centered-cubic phase to transform into a close-packed phase with most structure being isotropic hexagonal-close-packed (hcp) and, depending on shock strength and grain orientation, some fraction of face-centered-cubic (fcc) structure. The simulated shock Hugoniot is compared to experiments. By calculating the extended x-ray absorption fine structure (EXAFS) directly from the atomic configurations, a comparison to experimental EXAFS measurements of nanosecond-laser shocks shows that the experimental data is consistent with such a phase transformation. However, the atomistically simulated EXAFS spectra also show that an experimental distinction between the hcp or fcc phase is not possible based on the spectra alone.

Figure 1

Samples shocked with $u_p=0.906\mathrm{km}/\mathrm{s}$, 14.6 ps after the impact. Top (bottom) sample has an initial temperature of 50 K (300 K) and reaches a temperature behind the shock front of 296 K (622 K), pressures of about 39 GPa, and volume compressions of 19%. The samples consist of 32 grains and about 30 M atoms confined in a $57.4\mathrm{nm}\times 57.4\mathrm{nm}\times 109.9\mathrm{nm}$ box. Color coding denotes the local neighborhood of each atom: gray: bcc, blue: uniaxially compressed bcc, yellow: grain-boundary, red: hcp, green: fcc (see 5 for details). Fluctuations in the bcc structure at 300 K (bottom) wash out the color scheme analysis, and therefore make it hard to see the grain-boundary structure between the bcc grains. The hcp/fcc ratio behind the shock front is on average 1.5 for this shock strength and increases with decreasing shock strength. The initial average grain-size—as defined by the average caliper diameter—is 32.7 nm.

Figure 2

Experimental and simulated Hugoniots for polycrystalline iron. The full line is a linear fit to the experimental data [19, 20], the dashed lines are fits to the three different parts of the simulated Hugoniots: elastic precursor, transformation wave, and the overdriven part. As a reference, the $[001{]}_{\mathrm{bcc}}$ single crystalline shock data are included [5].

Figure 3

Radial distribution functions and EXAFS signal of several different cutouts (each containing about 5000 atoms) from the sample shown in Fig. 1 (bottom): purely bcc, region R1 (hcp/fcc ratio 3.2) and R2 (hcp/fcc ratio 1.03). The radial distribution and the EXAFS signals of the regions R1 and R2 are compared to ideal hcp and fcc structures simulated under the same pressure and temperature conditions. A comparison to experimental in situ EXAFS data [6] is shown in the middle panel.