Naresh N. Thadhani

Grain structure effects on the lattice thermal conductivity of Ti-based half-Heusler alloys

Half-Heusler alloys with the general formula TiNiSn1−xSbx are currently being investigated for their potential as thermoelectric (TE) materials. A systematic investigation of the effect of Sb doping on the Sn site and Zr doping on the Ti site on the electrical and thermal transport of the TiNiSn system has been performed. Unexpectedly, lattice thermal conductivity κL appears to increase somewhat randomly with small amounts (x<5%) of Sb doping. Subsequently, an investigation of grain structure in these Sb-doped materials has been found to correlate with the anomalous behavior of κL. Furthermore, effects of submicron grain sizes on κL in ball milled and shock compressed samples are also presented.

Shock-induced chemical reactions in titanium-silicon powder mixtures of different morphologies: Time-resolved pressure measurements and materials analysis

The response of porous titanium (Ti) and silicon (Si) powder mixtures with small, medium, and coarse particle morphologies is studied under high-pressure shock loading, employing postshock materials analysis as well as nanosecond, time-resolved pressure measurements. The objective of the work was to provide an experimental basis for development of models describing shock-induced solid-state chemistry. The time-resolved measurements of stress pulses obtained with piezoelectric polymer (poly-vinyl-di-flouride) pressure gauges provided extraordinary sensitivity for determination of rate-dependent shock processes. Both techniques showed clear evidence for shock-induced chemical reactions in medium-morphology powders, while fine and coarse powders showed no evidence for reaction. It was observed that the medium-morphology mixtures experience simultaneous plastic deformation of both Ti and Si particles. Fine morphology powders show particle agglomeration, while coarse Si powders undergo extensive fracture and entrapment within the plastically deformed Ti; such processes decrease the propensity for initiation of shock-induced reactions. The change of deformation mode between fracture and plastic deformation in Si powders of different morphologies is a particularly critical observation. Such a behavior reveals the overriding influence of the shock-induced, viscoplastic deformation and fracture response, which controls the mechanochemical nature of shock-induced solid-state chemistry. The present work in conjunction with our prior studies, demonstrates that the initiation of chemical reactions in shock compression of powders is controlled by solid-state mechanochemical processes, and cannot be qualitatively or quantitatively described by thermochemical models.

Shock-Induced Chemical Reactions and Synthesis of Nickel Aluminides

Chemical reactions in Ni and Al powder mixtures, initiated by the passage of shock waves, are used for the synthesis of nickel aluminides. Mechanistic investigations reveal that the extent of these shock-induced chemical reactions and the type (stoichiometry) of shock-synthesized compound formed depend on shock-loading conditions and the initial powder particle morphology. More intense shock conditions and irregular powder morphology assist in attaining an intimately (mechanically) mixed and activated closed-packed mass, thereby favoring bulk chemical reactions and resulting in the synthesis of compounds. While the Ni3Al compound is the preferred reaction product at lower shock conditions, more intense shock conditions favor the formation of the equiatomic B2-phase NiAl compound (having highest melting temperature and highest heat of reaction in contrast to other nickel aluminides), in spite of the starting powders mixed in a volumetric ratio corresponding to the Ni3Al compound.

Shock compression of reactive powder mixtures

Abstract The shock compression of reactive powder mixtures can yield varied chemical behaviour with occurrence of mechanochemical reactions in the timescale of the high pressure state, or thermochemical reactions in the timescale of temperature equilibration, or simply the creation of dense packed highly reactive state of material. The principal challenge has been to understand the processes that distinguish between mechanochemical (shock induced) and thermochemical (shock assisted) reactions, which has broad implications for the synthesis of novel metastable or non-equilibrium materials, or the design of highly configurable next generation energetic materials. In this paper, the process of shock compression in reactive powder mixtures and the associated role of various intrinsic and extrinsic characteristics of reactants in the triggering of ultrafast shock induced chemical reactions are discussed. Experimental techniques employing time resolved diagnostics and results which identify the occurrence of shock induced reactions are reviewed. Conceptual and numerical models used to describe the heterogeneous nature of such reactions through mesoscopic details of shock compression are presented. Finally, a discussion of the application of recent results for the design of reactive material systems with controlled reaction initiation and energy release characteristics is provided.

Reaction synthesis of high-temperature silicides

An attempt has been made to understand the reaction mechanisms involved in the synthesis of high-temperature silicides by combustion synthesis of MoSi2, reactive sintering and simultaneous hot pressing of MoSi2, shock-assisted reaction synthesis of NiSi and Ni2Si, and shock-induced reaction synthesis of Ti5Si3. The experimental results on combustion synthesis, and on the reactive sintering and simultaneous hot pressing of Mo + 2Si, indicate a heterogeneous reaction between liquid Si and Mo powder. In contrast to the above techniques, Mo5Si3 is observed as a secondary phase along with MoSi2 when the mixture is heated at low heating rates. Densities of reactive-sintered and simultaneously hot-pressed MoSi2 are close to 85% of MoSi2 density. The results indicate that densification and synthesis can effectively be combined, eliminating the need to hot-press a pre-alloyed powder. Shock-processed mixtures of Ni + Si and 2Ni + Si result in single-phase NiSi and Ni2Si, respectively, in the solid state upon subsequent reaction sintering. Mixtures of 5Ti + 3Si also result in single-phase ti5Si3 compound by solid-state shock-induced reaction mechanism.

Influence of dynamic densification on nanostructure formation in Ti5Si3 intermetallic alloy and its bulk properties

Abstract Dynamic densification was used to consolidate mechanically amorphized Ti–Si alloy powders, using a three-capsule, plate–impact, gas–gun loading system at velocities of 300, 500, and 700 m s −1 . The dense compacts were subsequently crystallized at annealing temperatures in the range of 800–1200°C, for time periods of 1–12 h. The compacts were observed to crystallize to a typically single-phase Ti 5 Si 3 compound and an ultra-fine grain microstructure, based on TEM and XRD analysis. The average grain size changed from ∼50 nm upon heat treatment at 800°C for 1 h, to ∼160 nm at 1200°C for 3 h, however, it remained stable (∼115–125 nm) during annealing at a constant temperature of 1000°C and increasing heat treatment time from 1 to 12 h. In-situ crystallization studies performed by heating the dynamically-densified samples in the TEM at temperatures up to 900°C, revealed that the increase in fraction of amorphous to crystalline phase was occurring by an increase in the number density of nucleating crystallites, and not via significant growth of existing crystallites since their growth is inhibited by the impingement of the crystals. Vickers microhardness measurements showed values of 1200–1400 kg mm −2 for grain size ranging from ∼60 to 160 nm. While these microhardness values are ∼80% higher than those for microcrystalline shock-densified Ti 5 Si 3 alloy, the fracture toughness values measured using the indentation method were observed to be ∼2–4 MPa√m, which is typical of that of brittle ceramics.

Investigation of shock-induced reaction behavior of as-blended and ball-milled Ni+Ti powder mixtures using time-resolved stress measurements

The shock-compression response of as-blended and ball-milled elemental Ni and Ti powder mixtures is investigated in this study. An 80-mm diameter single-stage gas-gun was employed to perform time-resolved measurements using piezoelectric stress gauges to monitor the stress wave profiles at the front (input) and back (propagated) surfaces of the ∼50% dense Ni+Ti powder mixture compacts. Shock-compression experiments performed on as-blended Ni+Ti powder mixtures in the range of 522m∕sto1046m∕s impact velocities, showed characteristics of powder densification at measured input stress of 1.12GPa, and shock-induced chemical reaction indicated by volume expansion and wave speed increase at measured input stress of 3.2GPa. Ball-milled powder mixtures also showed similar evidence of shock-induced chemical reaction at stresses exceeding 3GPa, with the degree of expansion depending on the energy release associated with the reaction in the powder mixtures ball-milled for various times. The measured high-pressure exp...

Discrete particle simulation of shock wave propagation in a binary Ni+Al powder mixture

Numerical simulations of shock wave propagation through discretely represented powder mixtures were performed to investigate the characteristics of deformation and mixing in the Ni+Al system. The initial particle arrangements and morphologies were imported from experimentally obtained micrographs of powder mixtures pressed at densities in the range of 45%–80% of the theoretical maximum density (TMD). Simulations were performed using these imported micrographs for each density compact subjected to driver velocities (Up) of 0.5, 0.75, and 1km∕s, and the resulting shock velocity (Us) was used to construct the Us-Up equation of state. The simulated equation of state for the 60% TMD mixture was validated by matching results obtained from previous gas-gun experiments. The details of shock wave propagation through the Ni+Al powder mixtures were explored on several scales. It is shown that the shock compression of mixtures of powders of dissimilar densities and strength is associated with heterogeneous deformatio...

Bulk nanocomposite magnets produced by dynamic shock compaction

Exchange-coupled R2Fe14B/α-Fe (R=Nd or Pr) nanocomposite bulk magnets with nearly full density have been successfully produced by shock compaction of melt-spun powders. X-ray diffraction and transmission electronic microscopy analyses of the shock-consolidated compacts showed no grain growth upon compaction, in fact, a decrease in the crystallite size of both the hard and soft phases was observed. As a consequence, magnetic properties were retained and even improved after compaction. Hysteresis loops of the shock-consolidated powder compacts showed a smooth single-phaselike behavior, indicating effective exchange coupling between hard and soft magnetic phases.

High-pressure shock activation and mixing of nickel-aluminium powder mixtures

The adiabatic chemical reaction behaviour of shock-compressed Ni-Al powder mixtures of varying morphology and different volumetric distributions has been investigated by microstructural and differential thermal analysis (DTA) to understand the mechanistic changes responsible for chemical reactions occurring during shock treatment. Mechanically mixed Ni-Al powders undergo exothermic chemical reactions at temperatures close to the melt-temperature of AI. In contrast, shock-treated Ni-Al powder mixtures exhibit a “pre-initiation” exothermic event, before the main exothermic reaction. Different forms (reaction start and peak temperatures) of the preinitiation exotherm are observed depending on the degree of macroscopic mixing, contact intimacy and activation, accomplished during shock compression of the powder mixtures of different morphology and volumetric distribution, all shock-treated under the same conditions. Mixtures containing equimolar volumetric distribution of powders of more irregular (flaky) morphologies undergo a significant extent of configuration change during shock-compression, resulting in the formation of an activated, intimately mixed and close-packed state. In such a state, chemical reaction is readily initiated by external thermal stimulation, such as heating during DTA. In fact, a greater degree of configuration change, activation and more intense mixing occurring during shock-compression can even lead to reaction initiation and completion in the shock duration itself.