Bulletin of the American Physical Society
2005 14th APS Topical Conference on Shock Compression of Condensed Matter
Sunday–Friday, July 31–August 5 2005; Baltimore, MD
Session H3: Materials Science III: Fundamentals |
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Chair: George Gray III, Los Alamos National Laboratory Room: Hyatt Regency Constellation D |
Tuesday, August 2, 2005 9:00AM - 9:15AM |
H3.00001: The Bond Modulus and the Stability of Solids John Gilman The chemical stabilities of molecules are determined by their LUMO-HOMO energy gaps. For solids the of these are their energy band gaps. However, solids are poly-molecules (i.e., polymers). But, the stabilization energy of a monomer cannot be used to describe the stability of a polymer. An intensive para-is needed. Such a parameter is the gap energy per molecular volume. The author has coined the name ``bond modulus'' for this parameter because it tends to be proportional to elastic moduli and it has the same dimensions. It applies primarily to covalent solids with localized bonding (i.e., Group IV elements, III-V, and II-VI compounds. A related parameter is electronegativity difference density. It correlatesmobilities, indentation hardnesses, and critical compressions for structure transformations. It is proportional to chemical hardnesses, and bulk moduli, as well as octahedral shear moduli, and inverse polarizabilities. [Preview Abstract] |
Tuesday, August 2, 2005 9:15AM - 9:30AM |
H3.00002: Hot Spots from Dislocation Pile-up Avalanches Ronald Armstrong, William Grise The model of hot spots developed at dislocation pile-up avalanches has been employed to explain both: greater drop- weight heights being required to initiate chemical decomposition of smaller crystals [1]; and, the susceptibility to shear banding of energetic and reference inert materials, for example, adiabatic shear banding in steel [2]. The evidence for RDX (cyclotrimethylenetrinitramine) is that few dislocations are needed in the pile-ups thus providing justification for assessing dynamic pile-up release on a numerical basis for few dislocation numbers [3]. For release from a viscous obstacle, previous and new computations lead to a local temperature plateau occurring at the origin of pile-up release [4], in line with the physical concept of a hot spot. [1] R.W. Armstrong, C.S. Coffey, V.F. DeVost and W.L. Elban, J. Appl. Phys. 68 (1990) 979. [2] R.W. Armstrong and F.J. Zerilli, Mech. Mater. 17 (1994) 319. [3] R.W. Armstrong, Proc. Eighth Intern. Seminar: New Trends in Research of Energetic Materials, April 19- 21, 2005, Pardubice, CZ. [4] W.R. Grise, NRC/AFOSR Summer Faculty Fellowship Program, AFRL/MNME, Eglin Air Force Base, FL, 2003. [Preview Abstract] |
Tuesday, August 2, 2005 9:30AM - 9:45AM |
H3.00003: X-ray Diffraction Measurements in Silicon Shocked Along [100] and [111] Orientations Below the Elastic Limit Y.M. Gupta, Stefan J. Turneaure Plate impact experiments were performed to obtain uniaxial strain loading along [100] and [111] in Si compressed below the Hugoniot elastic limit (HEL). X-ray diffraction measurements were obtained to determine the interplanar lattice spacing along the wave propagation direction in the shocked state. Additionally, the continuum response of the Si crystals was obtained for these orientations by monitoring the rear surface velocity history using laser interferometry. Both sets of measurements are in good agreement with the Si response, calculated using second and third order elastic constants. Our results will be compared with earlier measurements [1]. Work supported by DOE. [1] A. Loveridge-Smith et al., Phys. Rev. Lett. \textbf{86}, 2349 (2001). [Preview Abstract] |
Tuesday, August 2, 2005 9:45AM - 10:00AM |
H3.00004: Influence of Grain Boundary Crystallography on the Nucleation Characteristics of of Dynamic Failure Mukul Kumar, Roger Minich The scaling of mechanical properties with a microstructural length scale such as grain size is well known. However, the role of grain boundary crystallography is only recently starting to emerge. For instance, it has been shown recently that the Hall-Petch scaling of yield stress with grain size needs to be reformulated to take into account a parameter called grain boundary character distribution, which is related to the frequency of crystallographically ``special'' boundaries in the microstructure. Less well developed is an understanding of the role of microstructures in the process of void nucleation and growth leading to failure during shock loading of materials. In this paper, we shall report on the scaling recently observed in the case of dynamic failure or spall under shock deformation conditions for different microstructures in high purity copper. The spall strength is observed to increase as the length scales coarsen, which is counter to the Hall-Petch relationship, eventually leveling off for single crystals. The role of nucleation site density and grain boundary character distribution in understanding this behavior as a function of impact pressure will be explored in the context of the scaling laws that emerge from this data. This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. [Preview Abstract] |
Tuesday, August 2, 2005 10:00AM - 10:15AM |
H3.00005: Effects of Explosive Shock Prestraining on the Microstructural Evolution and Shear Localization of 304 and 316L Stainless Steels Qing Xue, Ellen Cerreta, George Gray III Initiation and development of adiabatic shear bands in explosively pre-shocked 304 and 316L stainless steels was investigated to characterize the influence of shock prestraining on the onset of shear bands. Forced shear tests on hat shaped specimens were conducted using a compressive split-Hopkinson pressure bar. The mechanical responses and the shear localized behaviors under the force shear condition in these preshocked materials were examined and compared. Shear band initiation was found to be very sensitive to the preshocked microstructures, especially to the strong interactions among defects such as deformation twin networks. The evolution of microstructure before and during the formation of shear bands has been characterized using transmission electron microscopy(TEM). Dynamic and quasistatic recovery is verified to be a dominant mechanism in the formation of ultra fine substructures within the shear bands generated in these preshocked steels. [Preview Abstract] |
Tuesday, August 2, 2005 10:15AM - 10:30AM |
H3.00006: Metallization of selenium under shock compression Sergey Gilev Time-resolved electrical conductivity measurements on solid and powder selenium are performed under shock compression followed by release. Measurement technique takes account of electromagnetic skin effect in conductive matter that allows one (unlike known schemes) to study dielectric-metal transition in single shock wave. Pressure dependence of the conductivity $\sigma (P)$ is obtained for amorphous selenium up to $30\,GPa$. With increasing the pressure the conductivity of selenium increases monotonically by 12 orders of magnitude. For $P<20 \, GPa$ the conductivity is of semiconductor nature. The conductivity data combined with temperature calculations give energy gap of compressed semiconductor selenium about 1.8 eV. For $P>20 \, GPa$ the conductivity shows pronounced saturation at $\sim 10^4 \ Ohm^{-1}cm^{-1}$. Such large conductivity testifies to metallic state of selenium. Experiments with solid and powder specimens bring to light temperature effect on state of selenium. Metallization mechanism is polymorphic transition or melting (it depends on specimen density). The metallization transition goes without noticeable time delay. At the same time releasing pressure causes temporary conservation of the metallic phase that points to its metastable nature. Comparison of conductivity data obtained for two elemental semiconductors (selenium, silicon) reveals fundamental features of shock metallization and its principal distinctions from static metallization. [Preview Abstract] |
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