Elsevier

Acta Materialia

Volume 51, Issue 5, 14 March 2003, Pages 1211-1228
Acta Materialia

Laser-induced shock compression of monocrystalline copper: characterization and analysis

https://doi.org/10.1016/S1359-6454(02)00420-2Get rights and content

Abstract

Controlled laser experiments were used to generate ultra-short shock pulses of approximately 5 ns duration in monocrystalline copper specimens with [001] orientation. Transmission electron microscopy revealed features consistent with previous observations of shock-compressed copper, albeit at pulse durations in the μs regime. At pressures of 12 and 20 GPa, the structure consists primarily of dislocation cells; at 40 GPa, twinning and stacking-fault bundles are the principal defect structures; and at a pressure of 55–60 GPa, the structure shows micro-twinning and the effects of thermal recovery (elongated sub-grains). The results suggest that the defect structure is generated at the shock front; the substructures observed are similar to the ones at much larger durations. The dislocation generation is discussed, providing a constitutive description of plastic deformation. It is proposed that thermally activated loop nucleation at the front is the mechanism for dislocation generation. A calculational method for dislocation densities is proposed, based on nucleation of loops at the shock front and their extension due to the residual shear stresses behind the front. Calculated dislocation densities compare favorably with experimentally observed results. It is proposed that simultaneous diffraction by Laue and Bragg of different lattice planes at the shock front can give the strain state and the associated stress level at the front. This enables the calculation of the plastic flow resistance at the imposed strain rate. An estimated strength of 435 MPa is obtained, for a strain rate of 1.3×107 s−1. The threshold stress for deformation twinning in shock compression is calculated from the constitutive equations for slip, twinning, and the Swegle–Grady relationship. The calculated threshold pressure for the [001] orientation is 16.3 GPa.

Introduction

Askaryon and Morez [1] demonstrated in 1963 that shock pulses could be generated in metals from laser-pulse induced vaporization at the surface. The use of surfaces covered by a laser-transparent overlay was introduced by Anderholm [2]; this enabled the confinement of the vapor products resulting in an increase of the peak pressure of the shock incident on the metal. Shock amplitudes as high as those generated by explosives or planar impact devices could be generated with a basic difference: the duration of the shock pulse was in the nanosecond range. Fairand et al. [3] and Clauer et al. [4] used these laser-induced shock pulses to modify the microstructure of engineering alloys, increasing their strength and fatigue resistance.

Simultaneous shock compression and X-ray diffraction experiments were introduced by Johnson et al. [5] and continued by Zaretsky et al. [6]. Wark et al. [7] used laser-generated X-rays to produce shock compression at the Nova laser facility. These shock compression experiments on silicon monocrystals were coupled with X-ray diffraction that successfully measured the compression both perpendicular (Laue) and parallel (Bragg) to the shock propagation direction. The X-rays that generated the shock pressure were created by eight laser beams focused into an internally shielded hohlraum [8], and the X-rays used for X-ray diffraction were provided by two lasers incident on a separate metal foil.

The use of simultaneous shock compression and X-ray diffraction offers a very attractive means of observing the distortions in the lattice as it is being compressed. These measurements are essential to unravel the mechanisms of plastic deformation in shock compression. A number of proposals for dislocation generation in shock compression have been advanced over the years [9], [10], [11], [12], [13], [14], but none of them have been critically tested.

This paper describes a series of experiments carried out on copper single crystals at the Omega ICF (Inertial Confinement Fusion) Facility, University of Rochester. Some of these experiments were carried out to record the simultaneous Bragg and Laue diffraction, while others were carried out to shock compress and then recover the sample for post-mortem analysis of the plastic deformation microstructure. The initial shock amplitudes varied from approximately 10 to 60 GPa and pulse durations were on the order of 5 ns, one order of magnitude lower than earlier shock laser experiments (20–100 ns) and two orders of magnitude lower than plate impact experiments (0.1–0.2 μs). Thus, these experiments explore a new regime of shock compression. In this paper, we describe laser recovery experiments and the deformation substructures generated, and compare these observations with analytical predictions.

Section snippets

Experimental techniques

For the recovery experiments, single crystals of Cu with a [100] orientation were obtained from Goodfellow in the form of disks with 2.0–3.0 mm diameter and 1 mm thickness. They were mounted into foam-filled recovery tubes shown in Fig. 1(a). Foam with a density of 50 mg/cm3 was used to decelerate the samples for recovery.

The shock amplitude at the surface of the Cu crystal can be obtained from the laser energy and the computed values (using hydrocode calculations). Results are shown in Fig. 2.

Experimental results and discussion

Copper has been the object of numerous shock recovery experiments and its response is fairly well understood. It has a stacking-fault energy of 57±8 mJ/m2. The shock-induced structure consists of dislocation cells up to a critical pressure. At higher pressures, twinning is prevalent. For single crystals, De Angelis and Cohen [15] found that the crystal twinning stress was 14 GPa, for the shock wave propagating along [100] while it was 20 GPa for [111]. This is consistent with the findings by

Analysis

The dislocations are envisioned to be generated as loops and this is supported by Fig. 13 (two-dimensional schematic representation). The edge (positive and negative) components of the loops are shown. The sheared area (within the loop) is indicated by a thinner line connecting the positive and negative edge dislocations. Whereas one of the loop legs moves towards the shock front, the other (opposite) is repelled from the front. Xu and Argon [24] calculated the activation energy for loop

Conclusions

It is demonstrated that laser-driven shock compression experiments can provide unique information on the processes of defect generation at high strain rates. The results indicate that all critical processes of defect generation operate at the shock front. The pulse duration in the current experiments was on the order of nanoseconds, two orders of magnitude lower than plate impact experiments. Nevertheless, the substructures observed by transmission electron microscopy are very similar. The

Acknowledgements

Research supported by the Department of Energy Grants DEFG0398DP00212 and DEFG0300SF2202, and by the US Army Research Office MURI Program.

References (37)

  • E. Hornbogen

    Acta Met

    (1962)
  • M.A. Meyers

    Scripta Met

    (1978)
  • J. Weertman

    Mech of Matls

    (1986)
  • R.L. Nolder et al.

    Acta Met.

    (1963)
  • R.L. Nolder et al.

    Acta Met

    (1964)
  • L.E. Murr

    Scripta Met

    (1978)
  • J.R. Rice

    J Mech Phys Solids

    (1992)
  • W. Tong et al.

    J Mech Phys Sol

    (1992)
  • P.S. Follansbee et al.

    Matls Sci and Eng

    (1991)
  • L.E. Murr et al.

    Acta Mater

    (1997)
  • M.A. Meyers et al.

    Acta Mat

    (2001)
  • G.A. Askaryon et al.

    JETP Lett

    (1963)
  • N.C. Anderholm

    Appl Phys Lett

    (1970)
  • B.P. Fairand et al.

    Appl Phys Lett

    (1974)
  • A.H. Clauer et al.
  • Q. Johnson et al.

    Phys Rev Lett.

    (1970)
  • E. Zaretsky

    J Appl Phys

    (1995)
  • J.S. Wark

    Phys Rev B

    (1989)
  • Cited by (245)

    • Role of micro-alloying element in dynamic deformation of Mg-Y alloys

      2024, International Journal of Mechanical Sciences
    • Dynamic deformation of Al under shock loading

      2022, Computational Materials Science
    View all citing articles on Scopus
    View full text