Naresh Thadhani | Georgia Institute of Technology (original) (raw)

Papers by Naresh Thadhani

SHOCK COMPRESSION OF CONDENSED MATTER - 2019: Proceedings of the Conference of the American Physical Society Topical Group on Shock Compression of Condensed Matter, 2020

The effect of bulk initial density (ρ 00) on the dynamic densification behavior of a brittle gran... more The effect of bulk initial density (ρ 00) on the dynamic densification behavior of a brittle granular system, cerium dioxide (CeO 2), is investigated. Specifically, the consolidation behavior of pressed powder compacts at three initial bulk densities, 33, 55, and 63% of theoretical maximum density (TMD), is examined at shock compressed densities within the compaction range via gas gun driven plate-on-plate impact experiments. The shock Hugoniot data collected from these experiments are presented and used to calibrate P-α compaction models. The dependency of these P-α model parameters on initial density are presented and discussed, particularly with their relevance to being used in a predictive manner.

Shock-induced reaction-behavior of TiSi and Ti-El powder inkx' tures T.E; Royal," N. N. Thadhani:... more Shock-induced reaction-behavior of TiSi and Ti-El powder inkx' tures T.E; Royal," N. N. Thadhani: and RA. &&amb 'School o f Materials Science and Engineering, Georgia Tech, Atlanta, GA 30332-0245

Polymers and polymer-based particulate composites are becoming increasingly used in aerospace str... more Polymers and polymer-based particulate composites are becoming increasingly used in aerospace structural applications, where they experience complex, non-static loads. Correspondingly, the high strain rate mechanical properties are of increasing importance. This paper investigates the properties of epoxy-bisphenol-A/diethanolamine epoxy (Epon 826/DEA)-and epoxy-based particulate composites across strain rates from 10-3 to 105 /s. The samples were tested using Instron, traditional split Hopkinson pressure bars (SHPBs) and a miniaturized SHPB for ultra-high strain rates. Additionally, the epoxy samples are tested with dynamic mechanical analysis to look at the effects of time-temperature superposition on the strain rate effects in the samples. The results of the testing are compared to the Hasan-Boyce model for polymers, which has shown good agreement with other epoxy studies, to develop constitutive equations for these materials.

Public reporting burden for this collection of information is estimated to average 1 hour per res... more Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.

Acta Materialia, 2011

Ball-milling Al-metal powders can result in self-sustaining high-temperature synthesis in interme... more Ball-milling Al-metal powders can result in self-sustaining high-temperature synthesis in intermetallic-forming systems. Here, Al and Ni powders with similar composition are used to investigate how microstructural differences affect the measured time to reaction between powders of different sizes processed under milling conditions specified by statistically designed experiments. Linear statistical models predicting the time to reaction (TTR) and the change in temperature (ΔT) are built from these experimental results. The time required to observe a selfsustained high-temperature synthesis of NiAl with different combinations of the powders and ball-milling conditions vary by almost an order of magnitude. Comparisons of powders milled to times corresponding to percentages of their averaged time to reaction show similar reaction initiation temperatures despite the difference in total milling time. Several distinct arrested reactions within the powder grains exhibit rapid solidification or incomplete diffusion of Ni into Al forming porous Ni-rich layered structures. The partially reacted grains suggest that the composite laminate particles are not forming intermetallic on the grain scale, but on the localized scale between layers.

The primary goals of our research were to obtain a fundamental understanding of the mechanisms an... more The primary goals of our research were to obtain a fundamental understanding of the mechanisms and kinetics of shock initiated intermetallic reactions. Several issues were addressed in this study, including investigation of (i) rate-controlling parameters of shock-initiated chemical reactions, (ii) mechanisms accounting for the extraordinarily rapid kinetics associated with shockinduced reactions, and (iii) determining how the reaction mechanisms and kinetics can be controlled by altering peak shock pressure and pulse duration, powder particle morphology, volumetric distribution or reactants, and starting porosity, in systems of highly exothermic powder mixtures. The overall approach involve:d investigation of shock initiated intermetallic reactions in powder mixtures of Ti-Si (titanium/silicon) and Al-hematite systems. Ti-Si is a typical intermetallic forming system having a large negative heat of reaction (MIR=-138 kcal/mol) with Si exhibiting a characteristic decrease in melting temperature with increasing pressure. The aluminum-hematite mixture is a typical metal-oxide thermite system (MIR =-193 kcal/mol)~ but can also be modified by reducing the oxide such that it contains various levels of oxygen until the composition approaches that of an Al-Fe intermetallic (HR=-38 kcal/ mol). The main effort in this program was focused on co.upling the shock-compressed reactant powder configuration just prior to reaction initiation with the propensity for H~action, via rnicrostructural characterization (using optical metallography and x-ray diffraction analysis) of samples obtained from recovery experiments. Realtime pressure measurements using PVDF gages were also performed. The experimental results demonstrate the influence of the shock pulse duration and initial packing density in Ti-Si powders, and the role of volumetric distribution of reactants on the~ crush-up to solid density (and therefore the configuration of reactants prior to reaction initiation), on the propensity of reaction initiation in Al + F e 2 0 3 thermite mixtures. The overall effort was coordinated with concunrent investigations at Washington State University (for controlled recovery experiments) and North Carolina State University (for numerical simulations). In addition, shock recovery and time-resolv 1 ed measurements were also performed in collaboration with and using the gas gun facilities at Sandia National Laboratories.

The work performed to-date focused on investigation of dynamic pressure induced transformations i... more The work performed to-date focused on investigation of dynamic pressure induced transformations in simple binary metallic glasses including a Ni80P20 binary alloy in the form of discs cut from an electrodeposited rod made at the Los Alamos National Laboratory, and a Ce75Al25 alloy fabricated via melt spinning at the Ames Laboratory. A Tibased metallic glass composite was also investigated, in collaborative work with the Jet Propulsion Laboratory, to determine the influence of the in-situ formed crystalline phase on the dynamic compressive and tensile spall strength properties. A number of materials physics and mechanics based fundamental questions were targeted,

Le Journal de Physique Colloques, 1988

Resume-Des reactions induites par une onde de choc ont

Journal of Applied Physics, 2016

The nanometer size of CdTe quantum dots (QDs) and their unique optical properties, including size... more The nanometer size of CdTe quantum dots (QDs) and their unique optical properties, including size-tunable narrow photoluminescent emission, broad absorption, fast photoluminescence decay, and negligible light scattering, are ideal features for spectrally tagging the shock response of localized regions in highly heterogeneous materials such as particulate media. In this work, the timeresolved laser-excited photoluminescence response of QDs to shock-compression was investigated to explore their utilization as mesoscale sensors for pressure measurements and in situ diagnostics during shock loading experiments. Laser-driven shock-compression experiments with steady-state shock pressures ranging from 2.0 to 13 GPa were performed on nanocomposite films of CdTe QDs dispersed in a soft polyvinyl alcohol polymer matrix and in a hard inorganic sodium silicate glass matrix. Time-resolved photoluminescent emission spectroscopy was used to correlate photoluminescence changes with the history of shock pressure and the dynamics of the matrix material surrounding the QDs. The results revealed pressure-induced blueshifts in emitted wavelength, decreases in photoluminescent emission intensity, reductions in peak width, and matrix-dependent response times. Data obtained for these QD response characteristics serve as indicators for their use as possible time-resolved diagnostics of the dynamic shock-compression response of matrix materials in which such QDs are embedded as in situ sensors. Published by AIP Publishing.

AIP Conference Proceedings, 1994

Shock-compression recovery experiments were performed on mixtures of Ti and Si powders of fine, m... more Shock-compression recovery experiments were performed on mixtures of Ti and Si powders of fine, medium, and coarse morphology, and packed at different initial densities, using the Sandia Momma and Poppa Bear fixtures with Baratol explosive. The shock-compressed configuration revealed characteristi_ typical of either chemically reacted material with fine equiaxed grains, or unreacted material with densely packed Ti and Si particles. The unreacted configuration showed that Ti particles were extensively deformed, irrespective of powder morphology and shock conditions generated by either fixture. In contrast Si particles showed different characteristics depending on the powder morphology, packing density, and shock conditions. The microstructural characteristics of unreacted configuration of Ti and Si powder mixtures were investigated. Mechanistic processes occurring prior to the inception of shock-induced chemical reactions in this system are described.

MATERIALS TRANSACTIONS, 2005

Dynamic consolidation of powders was studied to fabricate exchange-coupled Pr 2 Fe 14 B/-Fe nanoc... more Dynamic consolidation of powders was studied to fabricate exchange-coupled Pr 2 Fe 14 B/-Fe nanocomposite bulk magnets, using explosively generated shock waves transmitted through water. The planar shock wave propagating into the powder had a peak pressure calculated to be 12 GPa. Extensive plastic deformation of the powders and solid-state interfacial bonding of the ribbon flakes was obtained during shock compaction, resulting in fabrication of bulk compacts with nearly full density. Retention of nano-scale structure, and in fact further refinement of the grain size, ensured exchange coupling between the hard and soft phases, resulting in magnetic properties better than those of resin-bonded commercially available magnets. Post-shock annealing of the compacts at 750 and 850 C resulted in deterioration of the magnetic properties due to slight grain growth and decoupling of exchange interactions between hard and soft phases. The results illustrate that dynamic shock consolidation employing explosive loading is a viable method for fabricating bulk nanocomposite magnets and the rapid thermal excursions can be controlled to minimize and in fact eliminate the detrimental effects otherwise observed during high temperature sintering and annealing of the powders.

The REU+RET site at Georgia Tech has served to provide opportunities for talented undergraduate s... more The REU+RET site at Georgia Tech has served to provide opportunities for talented undergraduate students and enthusiastic high school teachers to experience hands-on participation in research for 8-weeks during the summer. In 2008, a diverse group of 19 science, math, and engineering students were recruited from across the country, ensuring participation of women and minority groups, and those from predominantly undergraduate institutions. The site also included participation of 5 RET teachers, recruited from various high schools in the Atlanta area. The REU student and RET teacher participants were mentored by faculty from various schools in Science and Engineering at Georgia Tech.

This paper is published in the interest of the scientific and technical information exchange. Pub... more This paper is published in the interest of the scientific and technical information exchange. Publication of this paper does not constitute approval or disapproval of the ideas or findings.

This work has been submitted for publication in the interest of the scientific and technical exch... more This work has been submitted for publication in the interest of the scientific and technical exchange. Publication of this report does not constitute approval or disapproval of the ideas or findings.

Experiments using a laser-accelerated flyer impact system were conducted on various forms of nick... more Experiments using a laser-accelerated flyer impact system were conducted on various forms of nickel-aluminum over a range of velocities in attempt to characterize the processes leading to reaction initiation and product formation. The laser-accelerated flyer system consists of an Nd:YAG, 3-Joule, 1064nm seeded laser, beam shaping optics, recovery chamber, and velocity interferometry diagnostics. Using various flyer materials, impact velocities up to 1.4 km/sec have been measured. The laser-accelerated flyer system was used to conduct impact experiments on Ni-Al powder compacts, Ni-Al vapor-deposited foils, and single Ni-Al interface configurations. Subsequently, post-mortem characterization was performed to identify the processes that lead to reaction initiation and products formed in the various Ni-Al configurations.

International Journal of Modern Physics B, 2008

Gd -doped ceria nano powder(~5 nm) was shock-loaded by a plate impact experiment using a single s... more Gd -doped ceria nano powder(~5 nm) was shock-loaded by a plate impact experiment using a single stage light gas gun. A computational model was used to simulate the shock state (pressure and density changes along time) of the sample in two dimensional Eulerian code. The predicted density of compacted sample from the simulation was about 90%. To reveal the effect of shock compaction on sintering behavior, the recovered sample was heat-treated and the microstructure was compared with that of a conventionally compacted and heat-treated sample.

Metallurgical and Materials Transactions A, 1996

Shock compression was used to make dense compacts of Ti and C elemental powder mixtures for subse... more Shock compression was used to make dense compacts of Ti and C elemental powder mixtures for subsequent reaction sintering in near-net form. The reaction sintering experiments were performed in an induction-heated hot press at temperatures below the melting point of Ti, with hold times of less than a few hours. The unique combination of defect states and packing characteristics introduced during shock compression results in significant enhancement in the solid-state chemical reactivity of the powder mixtures. Consequently, the reaction behavior of the powders is altered, and the reaction mechanism is dominated by solid-state diffusion, resulting in a microstructure reminiscent of solidstate processes. Reaction-sintered TiCx compacts, with bulk density in the range of 3.9 to 4.2 g/cm 3 (80 to 85 pct TMD of TIC), were produced in near-net form. The compacts had a highly refined microstructure (<6-/xm grain size) and microhardness in the range of 1360 to 1934 KHN. In this article, reaction sintering mechanisms involving solid-state diffusion in Ti + C powder mixture compacts will be described, along with characteristics of the titanium carbide produced by the combined shock modification and reaction sintering approach.