Showing papers by "Marc A. Meyers published in 2001"

Shear localization in dynamic deformation of materials: microstructural evolution and self-organization

Abstract: The plastic deformation of crystalline and non-crystalline solids incorporates microscopically localized deformation modes that can be precursors to shear localization. Shear localization has been found to be an important and sometimes dominant deformation and fracture mode in metals, fractured and granular ceramics, polymers, and metallic glasses at high strains and strain rates. Experiments involving the collapse of a thick walled cylinder enable controlled and reproducible application of plastic deformation at very high strain rates to specimens. These experiments were supplemented by hat-shaped specimens tested in a compression Hopkinson bar. The initiation and propagation of shear bands has been studied in metals (Ti, Ta, Ti–6Al–4V, and stainless steel), granular and prefractured ceramics (Al2O3 and SiC), a polymer (teflon) and a metallic glass (Co58Ni10Fe5Si11B16). The first aspect that was investigated is the microstructural evolution inside the shear bands. A fine recrystallized structure is observed in Ti, Cu, Al–Li, and Ta, and it is becoming clear that a recrystallization mechanism is operating. The fast deformation and short cooling times inhibit grain-boundary migration; it is shown, for the first time, that a rotational mechanism, presented in terms of dislocation energetics and grain-boundary reorientation, can operate within the time of the deformation process. In pre-fractured and granular ceramics, a process of comminution takes place when the particles are greater than a critical size ac. When they are smaller than ac, particle deformation takes place. For the granular SiC, a novel mechanism of shear-induced bonding was experimentally identified inside the shear bands. For all materials, shear bands exhibit a clear self-organization, with a characteristic spacing that is a function of a number of parameters. This self-organization is analyzed in terms of fundamental material parameters in the frame of Grady–Kipp (momentum diffusion), Wright–Ockendon, and Molinari (perturbation) models. © 2001 Elsevier Science B.V. All rights reserved.

Shear localization and recrystallization in dynamic deformation of 8090 Al-Li alloy

Abstract: The microstructural evolution in localized shear deformation was investigated in an 8090 Al-Li alloy by split Hopkinson pressure bar (strain rate of approximately 10(3) s(-1)) at ambient temperature and 77 K. The alloy was tested in the peak-, over-, under-, and natural-aged conditions, that provide a wide range of microstructural parameters and mechanical properties. Two types of localized shear bands were distinguished by optical microscopy: the deformed shear band and the white-etching shear band. They form at different stages of deformation during localization. There are critical strains for the occurrence of deformed and white-etching localized shear deformation, at the imposed strain rate. Observations by transmission electron microscopy reveal that the white-etching bands contain fine equiaxed grains; it is proposed that they are the result of recrystallization occurring during localization. The deformed-type bands are observed after testing at 77 K in all heat treatment conditions, but they are not as well defined as those developed at ambient temperature. Cracking often occurs along the localized shear at ambient temperature. The decrement in temperature is favorable for the nucleation, growth and coalescence of the microcracks along the shear bands, inducing fracture. (C) 2001 Elsevier Science B.V. All rights reserved.

Anomalous elastic response of silicon to uniaxial shock compression on nanosecond time scales.

Abstract: We have used x-ray diffraction with subnanosecond temporal resolution to measure the lattice parameters of orthogonal planes in shock compressed single crystals of silicon (Si) and copper (Cu). Despite uniaxial compression along the (400) direction of Si reducing the lattice spacing by nearly 11%, no observable changes occur in planes with normals orthogonal to the shock propagation direction. In contrast, shocked Cu shows prompt hydrostaticlike compression. These results are consistent with simple estimates of plastic strain rates based on dislocation velocity data. Although the response of materials to uniaxial shock compression has been a field of study for more than a century, our understanding at the lattice level of the response of crystals to rapid loading is still far from complete. While constitutive models are useful, a full description of phenomena such as shock-induced elastic-plastic flow and polymorphic phase transitions requires a knowledge of the atomic positions and the history of their rearrangement during the passage of the shock wave. In principle, one of the most direct methods of obtaining such information is the technique of in situ time-resolved x-ray diffraction (TXRD). Indeed, the TXRD experiments of Johnson and co-workers over three decades ago gave the first direct evidence of the retention of crystallinity under shock compression [1,2]. TXRD yields information about the interatomic spacings within the crystal. The change in Bragg angle due to the shock-induced alteration of the lattice parameter for monochromatic radiation is given, for small compressions, by simple differentiation of Bragg’s law: D2dhkl2dhkl 2 cotubDu. TXRD also provides information about the degree of plastic flow within the crystal: in the limit of purely hydrostatic response, an initially cubic lattice remains cubic under shock compression (at least in the hard sphere approximation). Thus diffraction from planes with reciprocal lattice vectors orthogonal to the shock propagation direction also exhibits angular shifts: a feature that has been confirmed by TXRD for certain crystals such as LiF and KCl under sufficiently intense loading [3‐5]. We provide here similar evidence for shocked single-crystal copper, which responds approximately hydrostatically to shocks of order 180 kbar on nanosecond time scales. However, plastic flow relies on dislocation generation and transport, a process that takes a characteristic time

Quasi-static and dynamic mechanical response of Strombus gigas (conch) shells

Abstract: Quasi-static and dynamic compression and three-point bending tests have been carried out on Strombus gigas (conch) shells. The mechanical response is correlated with its microstructure and damage mechanisms. The mechanical response is found to vary significantly from specimen to specimen and requires the application of Weibull statistics in order to be quantitatively evaluated. The conch exhibited orientation dependence of strength as well as significant strain-rate sensitivity; the failure strength at loading rates between 10×103 and 25×103 GPa s−1 was approximately 50% higher than the quasi-static strength. Quasi-static compressive failure occurred gradually, in a mode sometimes described as ‘graceful failure’. Crack deflection, delocalization of damage, and viscoplastic deformation of the organic layers are the most important mechanisms contributing to the unique mechanical properties of these shells.

On the effect of grain size on yield stress: extension into nanocrystalline domain

Abstract: Four principal factors contribute to grain-boundary strengthening: (a) the grain boundaries act as barriers to plastic flow; (b) the grain boundaries act as dislocation sources; (c) elastic anisotropy causes additional stresses in grain-boundary surroundings; (d) multislip is activated in the grain-boundary regions, whereas grain interiors are initially dominated by single slip, if properly oriented. As a result, the regions adjoining grain boundaries harden at a rate much higher than grain interiors. A phenomenological constitutive equation predicting the effect of grain size on the yield stress of metals is discussed and extended to the nanocrystalline regime. At large grain sizes, it has the Hall–Petch form, and in the nanocrystalline domain the slope gradually decreases until it asymptotically approaches the flow stress of the grain boundaries. The material is envisaged as a composite, comprised of the grain interior, with flow stress σfB and grain boundary work-hardened layer, with flow stress σfGB. The predictions of this model are compared with experimental measurements over the mono, micro, and nanocrystalline domains. Computational predictions are made of plastic flow as a function of grain size incorporating differences of dislocation accumulation rate in grain-boundary regions and grain interiors. The material is modeled as a monocrystalline core surrounded by a mantle (grain-boundary region) with a high work hardening rate response. This is the first computational plasticity calculation that accounts for grain size effects in a physically-based manner.

Combustion synthesis/densification of an Al2O3–TiB2 composite

Abstract: The self-propagating gasless combustion reaction 3TiO2+ 3B2O3+ 10Al 5Al2O2+ 3TiB2 was used to produce an Al2O3 –TiB2 composite, which was densified by uniaxial loading immediately following completion of reaction. The densification was enabled by the high temperatures produced by the combustion reaction ( 2000°C) which rendered the reaction product ( 70% porosity) plastic. The microstructure was characterized by columnar TiB2 grains with a diameter of 1–2 m and length of 5–10 m embedded in equiaxed A12O3 (grain size 50m); the TiB2 phase tended to agglomerate in clusters. A few of the TiB2 grains exhibited dislocations, while the A12O3 was annealed. This indicates that recovery processes took place after the plastic deformation involved in densification. Several constitutive models (corresponding both to rigid-plastic and power-law creep material behavior) were used to describe the mechanical response of the porous and ductile ceramic product and compared to the experimental results, with satisfactory agreement for power-law creep models. These constitutive models have a temperature-dependent term that incorporates the effect of specimen cooling, that occurs concurrently with densification; thus, it was possible to obtain a flow stress dependence of temperature which is in reasonable agreement with values interpolated from literature experimental results. © 2001 Elsevier Science B.V. All rights reserved.

Fundamental issues and applications of shock-wave and high strain-rate phenomena : proceedings of the 2000 International Conference on Fundamental Issues and Applications of Shock-Wave and High-Strain-Rate Phenomena (EXPLOMET '2000)

Abstract: Foreword. Preface. Materials Issues in Shock and High Strain Rates. Shock Consolidation, Reactions, and Synthesis. Material Aspects of Ballistic and Hypervelocity Impact. Modelling and Simulation. Novel Applications of Shock and High-Strain-Rate Phenomena. Contributing Author Index. Subject Index.

Self-organization of adiabatic shear bands in Ti, Ti-6Al-4V and stainless steel

Abstract: The self-organization of multiple adiabatic shear bands (SB) was investigated through the radial collapse of a thick-walled cylinder under high-strain-rate deformation (∼104 s−1). Materials with different properties, stainless steel 304L, Ti, and Ti-6Al-4V alloy, were used to examine the shear-band initiation, propagation, as well as spatial distribution. Shear-band spacing is compared with existing theories.