M. S. Schneider

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

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 × 10 7 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.  2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Material dynamics under extreme conditions of pressure and strain rate

Abstract Solid state experiments at extreme pressures (10–100 GPa) and strain rates (106–108s−1) are being developed on high energy laser facilities, and offer the possibility for exploring new regimes of materials science. These extreme solid state conditions can be accessed with either shock loading or with a quasi-isentropic ramped pressure drive. Velocity interferometer measurements establish the high pressure conditions. Constitutive models for solid state strength under these conditions are tested by comparing 2D continuum simulations with experiments measuring perturbation growth from the Rayleigh–Taylor instability in solid state samples. Lattice compression, phase and temperature are deduced from extended X-ray absorption fine structure (EXAFS) measurements, from which the shock induced α–ω phase transition in Ti and the α–ϵ phase transition in Fe, are inferred to occur on subnanosec time scales. Time resolved lattice response and phase can also be measured with dynamic X-ray diffraction measurements, where the elastic–plastic (1D–3D) lattice relaxation in shocked Cu is shown to occur promptly (<1 ns). Subsequent large scale molecular dynamics (MD) simulations elucidate the microscopic dislocation dynamics that underlies this 1D–3D lattice relaxation. Deformation mechanisms are identified by examining the residual microstructure in recovered samples. The slip-twinning threshold in single crystal Cu shocked along the [001] direction is shown to occur at shock strengths of ∼20 GPa, whereas the corresponding transition for Cu shocked along the [134] direction occurs at higher shock strengths. This slip twinning threshold also depends on the stacking fault energy (SFE), being lower for low SFE materials. Designs have been developed for achieving much higher pressures, P>1000 GPa, in the solid state on the National Ignition Facility (NIF) laser.

Atomistic modeling of shock-induced void collapse in copper

Nonequilibrium molecular-dynamics (MD) simulations show that shock-induced void collapse in copper occurs by emission of shear loops. These loops carry away the vacancies which comprise the void. The growth of the loops continues even after they collide and form sessile junctions, creating a hardened region around the collapsing void. The scenario seen in our simulations differs from current models that assume that prismatic loop emission is responsible for void collapse. We propose a dislocation-based model that gives excellent agreement with the stress threshold found in the MD simulations for void collapse as a function of void radius.

High-pressure, high-strain-rate lattice response of shocked materials

Laser-based shock experiments have been conducted in thin Si and Cu crystals at pressures above the published Hugoniot Elastic Limit (HEL) for these materials. In situ x-ray diffraction has been used to directly measure the response of the shocked lattice during shock loading. Static film and x-ray streak cameras recorded x rays diffracted from lattice planes both parallel and perpendicular to the shock direction. In addition, experiments were conducted using a wide-angle detector to record x rays diffracted from multiple lattice planes simultaneously. These data showed uniaxial compression of Si (100) along the shock direction and three-dimensional compression of Cu (100). In the case of the Si diffraction, there was a multiple wave structure observed. This is evaluated to determine whether there is a phase transition occurring on the time scale of the experiments, or the HEL is much higher than previously reported. Results of the measurements are presented.

Multiple film plane diagnostic for shocked lattice measurements (invited)

Laser-based shock experiments have been conducted in thin Si and Cu crystals at pressures above the Hugoniot elastic limit. In these experiments, static film and x-ray streak cameras recorded x rays diffracted from lattice planes both parallel and perpendicular to the shock direction. These data showed uniaxial compression of Si(100) along the shock direction and three-dimensional compression of Cu(100). In the case of the Si diffraction, there was a multiple wave structure observed, which may be due to a one-dimensional phase transition or a time variation in the shock pressure. A new film-based detector has been developed for these in situ dynamic diffraction experiments. This large-angle detector consists of three film cassettes that are positioned to record x rays diffracted from a shocked crystal anywhere within a full π steradian. It records x rays that are diffracted from multiple lattice planes both parallel and at oblique angles with respect to the shock direction. It is a time-integrating measur...

Plastic Deformation in Laser‐Induced Shock Compression of Monocrystalline Copper

Copper monocrystals were subjected to shock compression at pressures of 10–60 GPa by a short (3 ns initial) duration laser pulse. Transmission electron microscopy revealed features consistent with previous observations of shock‐compressed copper, albeit at pulse durations in the μs regime. The results suggest that the defect structure is generated at the shock front. A mechanism for dislocation generation is presented, providing a realistic prediction of dislocation density as a function of pressure. The threshold stress for deformation twinning in shock compression is calculated from the constitutive equations for slip, twinning, and the Swegle‐Grady relationship.

Laser‐Induced Shock Compression of Copper and Copper Aluminum Alloys

Single crystal copper and copper 2‐wt% aluminum alloy with [134] and [001] orientations are compressed by means of a high energy short pulse laser. Pressures ranging from 20 GPa to 60 GPa are achieved. The shocked samples are recovered and the residual defect substructure is analyzed by transmission electron microscopy. Results show systematic differences depending on orientation and stacking fault energy. Samples with orientations [001] are symmetrical with simultaneous activation of eight slip systems. This leads to a higher work hardening rate. The [134] orientation is asymmetrical with one dominating slip system, and thus a reduced work hardening rate due to a prolonged easy glide region for dislocations. These differences in work hardening response affect the stresses required to achieve the twinning threshold pressure. The effects of stacking fault energy on the defect substructure and threshold twinning are also characterized. Experimental results are rationalized in terms of a constitutive descr...

Direct Observation of the α‐ε Transition in Shocked Single Crystal Iron

In‐situ x‐ray diffraction was used to study the response of single crystal iron under shock conditions. Measurements of the response of [001] iron showed a uniaxial compression of the initially bcc lattice along the shock direction by up to 6% at 13 GPa. Above this pressure, the lattice responded with a further collapse of the lattice by 15–18% and a transformation to the hcp structure. The in‐situ measurements are discussed and results summarized.

Picosecond x-ray diffraction studies of shocked single crystals

The past few years have seen a rapid growth in the development and exploitation of X-ray diffraction on ultrafast time-scales. One area of physics which has benefited particularly from these advances is the the field of shock-waves. Whilst it has been known for many years that crystalline matter, subjected to uniaxial shock compression, can undergo plastic deformation and, for certain materials, polymorphic phase transformations, it has hitherto not been possible to observe the rearrangement of the atoms on the pertinent timescales. We have used laser-plasma generated X-rays to study how single crystals of metals (copper and iron) react to uniaxial shock compression, and observed rapid plastic flow (in the case of copper), and directly observed the famous alpha-epsilon transition in Iron. These studies have been complemented by large-scale multi-million atom molecular dynamics simulations, yielding significant information on the underlying physics.

Dynamic response of single crystalline copper subjected to quasi-isentropic laser and gas-gun driven loading

Single crystalline copper was subjected to quasi-isentropic compression via gas-gun and laser loading at pressures between 18GPa and 59GPa. The deformation substructure was analyzed via transmission electron microscopy (TEM). Twins and laths were evident at the highest pressures, and stacking faults and dislocation cells in the intermediate and lowest pressures, respectively. The Preston-Tonks-Wallace (PTW) constitutive description was used to model the slip-twinning process in both cases.