Showing papers by "Marc A. Meyers published in 1995"

The effect of grain size on the high-strain, high-strain-rate behavior of copper

Abstract: Copper with four widely differing grain sizes was subjected to high-strain-rate plastic deformation in a special experimental arrangement in which high shear strains of approximately 2 to 7 were generated. The adiabatic plastic deformation produced temperature rises in excess of 300 K, creating conditions favorable for dynamic recrystallization, with an attendant change in the mechanical response. Preshocking of the specimens to an amplitude of 50 GPa generated a high dislocation density; twinning was highly dependent on grain size, being profuse for the 117- and 315-μm grain-size specimens and virtually absent for the 9.5-μm grain-size specimens. This has a profound effect on the subsequent mechanical response of the specimens, with the smaller grain-size material undergoing considerably more hardening than the larger grain-size material. A rationale is proposed which leads to a prediction of the shock threshold stress for twinning as a function of grain size. The strain required for localization of plastic deformation was dependent on the combined grain size/shockinduced microstructure, with the large grain-size specimens localizing more readily. The experimental results obtained are rationalized in terms of dynamic recrystallization, and a constitutive equation is applied to the experimental results; it correctly predicts the earlier onset of localization for the large grain-size specimens. It is suggested that the grain-size dependence of shock response can significantly affect the performance of shaped charges.

High-strain, high-strain-rate behavior of tantalum

Abstract: Tantalum plate produced by a forging-rolling sequence was subjected to high plastic shear strains(γ = 1 → 5.5) at high strain rates (∼4 × 104 s-1) in two experimental configurations: (a) a special hat-shaped geometry and (b) thin disks deformed in a split Hopkinson bar. In parallel experiments, the constitutive behavior of the same material was established through quasi-static and dynamic compression tests at ambient and elevated temperatures. The microstructure generated at high strain rates and retained by rapid cooling from a narrow (200-μm) deformation band progresses from dislocated, to elongated cells, to banded structures, and finally, to subgrains as the shear strain increases from 0 to 5.5. The temperature rise predictions from the constitutive description of the material indicate that the temperature reaches values of 800 K, and it is proposed that thermal energy is sufficient to produce a significant reorganization of the deformation substructure, leading to a recovered structure.

Combustion synthesis in the Ti-C-Ni-Mo system: Part I. Micromechanisms

Abstract: Combustion-wave arresting experiments were conducted on Ti-C-Ni and Ti-C-Ni-Mo powder mixtures. The reactant powder mixtures were placed within a conical hole machined in a Cu block. The reaction was initiated at the base of the cone and proceeded down the cone axis, toward the apex, until the heat loss to the Cu block was sufficient to arrest the reaction. This enabled the postreaction characterization of the three distinct regions of the combustion wave: unreacted, partially reacted, and fully reacted. The unreacted region is characterized by removal of a surface scale on the Ti particles and Ti α→ β solid-state phase transformation. The partially reacted region is characterized by a number of physical processes and a distinct interface with the unreacted region. These processes include the formation of Ti-Ni phases, Ti-Ni melt, TiC, layer on the C particles, and TiCx spherules. The TiCx layer is composed of coarsening TiCx precipitates which are ejected into the progressively Ni-rich Ti-Ni melt. These TiCx spherules vary in size with apparent diameters of approximately 0.2 to 1 μm. No distinct interface exists between the partially and fully reacted regions. Final consumption of C is followed by TiCx spherule growth by combined Ostwald ripening and grain coalescence mechanisms resulting in an apparent diameter of 2.5 μm. The addition of Mo does not significantly affect the processes occurring within the partially reacted region. It is apparent that Mo enters into solution with the Ti-Ni melt at a rate much slower than that characteristic of the other processes(i.e., Ti-Ni melt mixing or Ti-C reaction).

Consolidation of Combustion‐Synthesized Titanium Diboride‐Based Materials

Abstract: The quasi-static consolidation in uniaxial compression of combustion-synthesized TiB{sub 2}-based materials was investigated. Consolidation was carried out in insulated containers upon completion of the combustion reaction, while the porous reaction products were ductile. Since the consolidation is not an isothermal process, the temperature change during consolidation was monitored and recorded. The effect of the addition of metallic elements to the elemental powders was established, and it was found that nickel and chromium provide the best compact integrity. The partial densification is sufficient to show significant differences between the effects of metallic additives. A phenomenological (not based on the micromechanisms of densification) constitutive model was applied to the hot and porous reaction products incorporating the temperature dependence of flow stress. The activation energy for the temperature dependence of the flow stress is established and indicates that, in addition to diffusion-induced plastic deformation, other processes occur, such as fracturing of ligaments.

The structure of controlled shear bands in dynamically deformed reactive mixtures

Abstract: The structure of controlled high-strain-rate shear bands generated in heterogeneous reactive porous materials (Nb + Si, Mo + Si + MoSi2) has been investigated using axially symmetric experimental configurations in which the source of energy is the detonation of low velocity explosives. The deformation was highly localized, with profuse formation of shear bands, which have thicknesses of 5 to 20 μm. The experimental method generated overall strains up to 100 and strain rates\(\dot \gamma \) of approximately 107 s-1. Changes in particle morphology, melting, and regions of partial reaction on three different length scales were observed. The shear band thickness is smaller than the initial characteristic particle size of the porous mixture (≤44 μm), ensuring a cooling time of the deformed material on the same order of magnitude as the deformation time (10-5 s). In the shear localization regions, two characteristic phenomena were observed: (a) a shear fracture subdividing the Nb particles into thin parallel layers and (b) the formation of vortices. A mechanism for the reaction inside the shear bands is proposed, and an expression for the largest size of chemical products as a function of shear deformation is obtained.

Combustion synthesis in the Ti-C-Ni-Mo system: Part II. Analysis

Abstract: Combustion-wave arrest experiments provide the means of greater understanding of the physical phenomena which occur during the propagation of a combustion wave within a Ti-C-Ni-Mo powder mixture. The apparent activation energy for the process (E ≈ 120 ± 40 kJ/mol) and the observations reported in the companion article indicate that the rate-limiting step in the reaction between Ti and C is the dissolution of C into the Ti-Ni-C melt. Temperature profile analysis indicates that the 2μm C flakes are completely consumed within approximately 0.2 seconds. The formation of TiC x spherules and their subsequent detachment is explained in terms of compressive stresses established in the growing TiC x layer on the C particle. The compressive stresses are estimated to exceed 1 GPa, and an energy balance analysis predicts the formation of spherules for layer thicknesses on the order of 1μm, consistent with the experimental results.

Effect of shock pressure and plastic strain on chemical reactions in NbSi and MoSi systems

Abstract: NbSi and MoSi elemental powder mixtures contained within cylindrical capsules were subjected to co-axial shock-wave loading at varying pressures (2.8–70 GPa). Shock-induced or shock-assisted chemical reactions were observed in these powder mixtures along the capsule axis. Three concentric regions with the capsules were observed: (1) fully reacted (Mach stem region); (2) partially reacted; and (3) unreacted. These results confirm the Krueger-Vreeland concept of threshold energy for shock-induced chemical reactions. Analysis of partially reacted regions enabled the identification of the reaction micromechanisms in accordance with the model proposed by Meyers, Yu and Vecchio (Acta Metall. Mater., 42 (1994) 715). Asymmetric shock-wave loading experiments on the above powder mixtures were also conducted. Significant macroscopic plastic deformation (i.e. ϵ ≌ 0.2–0.5 ) along with consolidation were achieved by modifying the explosive loading configuration. Because of the asymmetric loading, regions of shear localization were produced. These regions were also characterized by the onset of the chemical reaction resulting from the local thermal excursion due to both the frictional dissipation of kinetic energy and plastic deformation. The results obtained in this investigation confirm the earlier hypothesis that the shock energy dissipated by plastic deformation does play an important role in the initiation of the chemical reaction. It is proposed that the Krueger-Vreeland threshold energy be modified to take into account the plastic deformation energy.

Synthesis of nanocrystalline titanium carbide by spark erosion

Abstract: Interest in nanocrystaJline ceramic powders is due to their attractive processing and mechanical properties. The disadvantages of conventional ceramics, such as large internal flaws, negligible ductility, hightemperature processing, can possibly be eliminated or minimized by starting with finegrained powders. A previous study on the sinterability of nanocrystalline ceramic powders showed that high densities can be achieved at lower temperatures, both with and without the’ application of pressure [ 11. In addition, it is remarkable that superplastic behavior has been reported in nanocrystalline ceramics at room temperature [2]. One of the novel methods for nrenaring nanocrvstalline ceramic powders is reactive spark erosion [3-51. An important feature of this method is the-capability to synthesize new materials by reacting the eroded particles with the dielectric liguid. This technique utilizes two metal or conducting electrodes sparked in a suitable dielectric liquid that reacts with the pa&les to produce the desired ceramic powders. The high temperature provided by the spark results in vaporization of the electrodes and the surrounding dielectric liquid; the reaction products formed between the various vapor species are quenched in the liquid and produce spherical particles. The aim of the present note is to report on the titanium carbide particle formation in a carbonic dielectric liquid (pentane) with titanium electrodes and charges. The nanocrystalline titanium carbide was produced in a wide range of particle size distribution and in the form of single phase spheres.

Metallurgical and materials applications of shock-wave and high-strain-rate phenomena : proceedings of the 1995 International Conference on Metallurgical and Materials Applications of Shock-Wave and High-Strain-Rate Phenomena (EXPLOMET '95)

Abstract: Dynamic consolidation of materials dynamic fracture phenomena materials aspects of ballistic and hypervelocity impact and penetration extreme plastic deformation - adiabatic shear bands, dynamic recrystallization, shaped charges and explosively formed penetrators modelling and simulation of shock-wave and high-strain-rate phenomena in materials shock-induced reactions and synthesis shock-wave modification of materials structure and properties explosive welding, high-strain-rate deformation and material working.

The one-step synthesis of dense titanium-carbide tiles

Abstract: Self-propagating, high-temperature synthesis (SHS), which is an attractive process for forming alloys, cermets, ceramics, and composites, has been combined with a rapid quasi-isostatic consolidation technology called the CeraconTM process. This one-step synthesis and densification route has been applied to the rapid fabrication of large 15 cm × 15 cm × 2.5 cm titanium carbide tiles. A cost analysis of this process based on prototype quantities shows that the cost of the process is 30–50% of that of current manufacturing processes.