Showing papers by "Marc A. Meyers published in 2002"

Constitutive description of dynamic deformation: physically-based mechanisms

Abstract: The response of metals to high-strain-rate deformation is successfully described by physically-based mechanisms which incorporate dislocation dynamics, twinning, displacive (martensitic) phase transformations, grain-size, stacking fault, and solution hardening effects. Several constitutive equations for slip have emerged, the most notable being the Zerilli–Armstrong and MTS. They are based on Becker’s and Seeger’s concepts of dislocations overcoming obstacles through thermal activation. This approach is illustrated for tantalum and it is shown that this highly ductile metal can exhibit shear localization under low temperature and high-strain-rate deformation, as predicted from the Zerilli–Armstrong equation. A constitutive equation is also developed for deformation twinning. The temperature and strain-rate sensitivity for twinning are lower than for slip; on the other hand, its Hall–Petch slope is higher. Thus, the strain rate affects the dominating deformation mechanisms in a significant manner, which can be quantitatively described. Through this constitutive equation it is possible to define a twinning domain in the Weertman– Ashby plot; this is illustrated for titanium. A constitutive description developed earlier and incorporating the grain-size dependence of yield stress is summarized and its extension to the nanocrystalline range is implemented. Computational simulations enable the prediction of work hardening as a function of grain size; the response of polycrystals is successfully modeled for the 50 nm–100 m range. The results of shock compression experiments at pulse durations of 3–10 ns (this is two–three orders less than gas-gun experiments) are presented. They prove that the defect structure is generated at the shock front; the substructures observed are similar to the ones at much larger durations. A mechanism for dislocation generation is presented, providing a constitutive description of plastic deformation. The dislocation densities are calculated which are in agreement with observations. The threshold stress for deformation twinning in shock compression is calculated from the constitutive equations for slip, twinning, and the Swegle–Grady relationship. © 2002 Elsevier Science B.V. All rights reserved.

Multiple Film Plane Diagnostic for Shocked Lattice Measurements

Abstract: Laser-based shock experiments have been conducted in thin Si and Cu crystals at pressures above the Hugoniot Elastic Limit (HEL). 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. This data showed uniaxial compression of Si (100) along the shock direction and 3-D compression of Cu (100). In the case of the Si diffraction, there was a multiple wave structure observed, which may be due to a 1-D 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 3 film cassettes that are positioned to record x-rays diffracted from a shocked crystal anywhere within a full {pi}-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 measurement, but time-resolved data may be recorded using a short duration laser pulse to create the diffraction source x-rays. This new instrument has been fielded at the OMEGA and Janus lasers to study single crystal materials shock compressed by direct laser irradiation. In these experiments,more » a multiple wave structure was observed on many different lattice planes in Si. This data provides information on the structure under compression.« less

Microstructural evolution in adiabatic shear localization in stainless steel

Abstract: Shear bands were generated under prescribed and controlled conditions in an AISI 304L stainless steel (Fe-18%Cr-8%Ni). Hat-shaped specimens were deformed in a Hopkinson bar at strain rates of ca 10(4) s(-1) and shear strains that could be varied between I and 100. Microstructural characterization was performed by electron backscattered diffraction (EBSD) with orientation imaging microscopy (OIM), and transmission electron microscopy (TEM). The shear-band thickness was ca 1-8 mum. This alloy with low-stacking fault energy deforms, at the imposed strain rates (outside of the shear band), by planar dislocations and stacking fault packets, twinning, and occasional martensitic phase transformations at twin-band intersections and regions of high plastic deformation. EBSD reveals gradual lattice rotations of the grains approaching the core of the band. A [110] fiber texture (with the [110] direction perpendicular to both shear direction and shear plane normal) develops both within the shear band and in the adjacent grains. The formation of this texture, under an imposed global simple shear, suggests that rotations take place concurrently with the shearing deformation. This can be explained by compatibility requirements between neighboring deforming regions. EBSD could not reveal the deformation features at large strains because their scale was below the resolution of this technique. TEM reveals a number of features that are interpreted in terms of the mechanisms of deformation and recovery/recrystallization postulated,. They include the observation of grains with sizes in the nanocrystalline domain. The microstructural changes are described by an evolutionary model, leading from the initial grain size of 15 mum to the final submicronic (sub) grain size. Calculations are performed on the rotations of grain boundaries by grain-boundary diffusion, which is three orders of magnitude higher than bulk diffusion at the deformation temperatures. They indicate that the microstructural reorganization can take place within the deformation times of a few milliseconds. There is evidence that the unique microstructure is formed by rotational dynamic recrystallization. An amorphous region within the shear band is also observed and it is proposed that it is formed by a solid-state amorphization process; both the heating and cooling times within the band are extremely low and propitiate the retention of non-equilibrium structures. Published by Elsevier Science Ltd on behalf of Acta Materialia Inc.

Computational Modeling of the Shock Compression of Powders

Abstract: The modeling of both inert and reactive materials at the meso‐scale with an Eulerian finite element program is discussed Issues that have an effect on the calculated response, including mixture theory and interface tracking, are briefly presented with recent calculations modeling shock initiated chemical reactions (SICR)

Plastic Deformation in Laser‐Induced Shock Compression of Monocrystalline Copper

Abstract: 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.

Evolution in the Patterning of Adiabatic Shear Bands

Abstract: The evolution of multiple adiabatic shear bands was investigated in stainless steel (different grain sizes: 30 and 140 μm), titanium, and Ti‐6Al‐4V alloy through the radial collapse of a thick‐walled cylinder under high‐strain‐rate deformation (∼104 s−1) and different global strains(up to 0.9). Ti and Ti‐6Al‐4V displayed drastically different patterns of shear bands. The shear‐band spacing is compared with one‐dimensional theoretical predictions based on perturbation (Ockendon‐Wright and Molinari) and momentum diffusion (Grady‐Kipp) concepts. The experimentally observed spacing reveals a two‐dimensional character of self‐organization, not incorporated into the existing theories. A novel analytical description is proposed, in which embryos(potential initiation sites) are activated as a function of strain (greater than a threshold) according to a Weibull‐type distribution. The model incorporates embryo disactivation by stress shielding as well as selective growth of shear bands. The imposed strain rate, emb...

Picosecond X-ray diffraction studies of shocked crystals

Abstract: Summary from only given. We present experimental time-resolved X-ray diffraction data that provide firm evidence that the response of single crystal silicon to nanosecond timescale uniaxial shock compression along the (400) axis is anomalous in that it is purely elastic.