Intermetallic Formation Reactions Research Articles

Comparing the ignition and combustion characteristics of ball-milled Al-based composites with Ti, Zr, and Mg additives

ABSTRACT The morphology/microstructure and ignition/combustion properties of ball-milled Al composite powders containing Ti, Zr, and Mg were studied and compared. The particles contained 50 at% Al or Al-8Mg alloy and 50 at% Ti or Zr, yielding four different chemistries: Al:Zr, Al-8Mg:Zr, Al:Ti, and Al-8Mg:Ti. Ignition temperatures were measured by resistively heating the powders on a nichrome wire in air. All composite particles exhibit ignition temperatures (~625-725 K) that are significantly lower than those for pure Al of similar size (~2100 K). The Ti-based composites have slightly lower ignition temperatures than the Zr-based powders. The difference is attributed to the Ti-based powders having more energetic intermetallic formation reactions as measured by differential thermal analysis (DTA). The microstructures of the as-milled composites were analyzed via powder x-ray diffraction and scanning electron microscopy and helped to explain the measured heats and ignition temperatures. The addition of Mg did not significantly affect the ignition temperatures, but it did alter the number of peaks, the peak positions and overall heat release in the DTA scans compared to the binary samples. Mg also improved refinement of both internal microstructures and the size of composite particles during milling. Spectroscopy and high-speed videography of the combusting powders revealed vapor-based emission species such as Al and AlO for the binary samples and Mg and MgO for the ternary composites. Higher average maximum combustion temperatures were measured for the Ti-based powders (3470 ± 450 K and 3300 ± 880 K) than the Zr-based powders (3340 ± 350 K and 3100 ± 400 K) with the Mg containing powders being hotter for Ti and Zr chemistries. The Ti-based composites displayed a dual-phase combustion mechanism and micro-explosions, similar to those seen for Al:Zr composites in previous studies.

Combustion in Ni–Al system with Cu additive (powder or rod) Experiment and mathematical model

Experimental studies were carried out with theoretical calculations of wave synthesis in the Ni–Al–Cu system were performed using the mathematical model developed. Approximate analytical formulas were obtained for synthesis performance evaluation. The inverse problem method was used to get kinetic constants that determine process dynamics based on the experimental data and analytical relationships. It is shown that the combustion front propagation velocity increases monotonically with an increase in the reaction sample relative density in the range of relative density values of 0.4 to 0.6. The depth of copper melt penetration from the center of the sample into the nickel-aluminum matrix depends on the relative density of the sample and copper wire diameter: higher densities and larger diameters lead to an increase in the liquid-phase impregnation area. The rate of nickel and aluminum powder frame wetting with copper melt is limited by the synthesis wave speed. Based on the experimental data and analytical ratios, we estimated the effective kinetic constants describing the high-temperature synthesis of the Ni + Al reaction mixture in the presence of copper additives. The thermal effect of the NiAl intermetallic formation reaction and the preexponential factor in the chemical transformation equation are calculated, the exponent value in the ratio for the mixture thermal conductivity is established; a constant determining the process of nickel-aluminum matrix impregnation with copper melt is found. The macroscopic approach used to analyze the NiAl intermetallic synthesis makes it possible to determine all the desired physicochemical characteristics and model parameters. The mathematical model is suitable for predictive estimates and experimental data analysis in the macroscopic approximation. Approximate analytical formulas are obtained for calculating the NiAl intermetallic synthesis characteristics. They allow for calculating the through channel characteristics and can be used in the design of NiAl products.

Evaluating the effect of microstructural refinement on the ignition of Al-Mg-Zr ball-milled powders

Next-generation composite Al-Mg-Zr fuels for bio/chem-agent defeat applications rely on intermetallic formation reactions to reduce ignition temperatures below melting point of Al. Scaling-up the ball-milling synthesis of these powders will require a detailed understanding of how milling parameters and powder composition impact the evolution of powder microstructure. In the present work, we explore the effect of milling duration on the size, microstructure, and properties of (Al-8Mg):3Zr, (Al-8Mg):Zr, 3(Al-8Mg):Zr and 4(Al-8Mg):Zr composite powders. We report compositionally dependent refinement of Zr inclusions in the Al-8Mg matrix that impacts ignition temperatures (Tig). For (Al-8Mg):3Zr and (Al-8Mg):Zr composite powders, Tig initially drops as milling time increases to 60 min and then either plateaus or rises at longer milling times. In contrast, Tig generally decreases over the full 90 min milling duration for the two Al-8Mg-rich compositions. Reductions in heterogeneity and average Zr inclusion size dominate the ignition behavior and lead to steady decreases in Tig.

Enhancement of energy release performance of Al–Ni composites by adding CuO

Abstract Al–Ni composites, which have excellent energy densities and mechanical properties, are promising novel energetic materials for improving the damaging effects of ammunition. To enhance their energy release performance under impact loading, CuO was introduced to form the thermite. The results of differential scanning calorimetry (DSC), heat treatment, and ballistic tests indicated that Al and Ni were more prone to donate their free electrons after the addition of CuO. A decrease in the number of free electrons as well as the weak metal-oxygen bonds at the Al/Ni interfaces caused by the thermal decomposition of CuO, impeded the activity of the Al–Ni intermetallic formation reaction, increasing its onset temperature from 766 K to 820 K. The reduced difference in the onset temperatures for the intermetallic formation and thermite reactions caused the thermite reaction between Al and CuO to be ignited by the Al–Ni intermetallic formation reaction. The overlap of these two exothermic reactions greatly improved the reaction intensity, and then accelerated the oxidation of Al and Ni, finally enhancing the energy release performance during high-speed impact. This work demonstrated that the energy release performance of Al–Ni composites can be effectively modulated by introducing a metal oxide. Moreover, the effect of a metal oxide on the energy release behavior of Al–Ni composites accounted for the interaction of aluminum and nickel with the added copper oxide.

Composite Al∙Ti powders prepared by high-energy milling with different process controls agents

Aluminum-rich Al-Ti composite powders with controllable particle size distributions were prepared by staged mechanical milling using polar and nonpolar organic process control agents. Staged milling allowed the separation of formation of composites in the first stage from control of particle size and ignition behavior in the second stage. Using the polar solvent acetonitrile in the second stage resulted in more finely dispersed particles with lower onset temperatures for intermetallic formation reactions, and lower ignition temperatures, compared with using the non-polar hexane. All composites showed lower ignition temperatures than reference aluminum. Combustion rates for all composites were greater than for reference aluminum, while net energy release remained comparable. The results suggest that staged milling with process control agents of different polarity is a viable method to control particle size distributions and ignition behavior in mechanically milled, metal-based energetic composite powders while maintaining the energy content. Particle size control is critical for adopting such materials in practical systems.

Thermodynamic study on the formation of order-disorder structure of the NiAl intermetallic compound by semi-empirical model

In this study, the mixture of nickel and aluminum powders has been mechanically milled for different times in a planetary mill and its thermodynamics were investigated. Phase and structural changes of the mixture powders have been investigated by XRD and SEM analysis. The prediction of a stable phase of the milling process was carried out using the Miedema's semi-empirical method. The results showed that Ni55Al45 intermetallic was more stable than a solid solution and amorphous phase after the mechanical milling process. In order to determine the mechanism of forming an intermetallic mixture of milled and unmilled powders, they have been subjected to DTA thermal analysis and were compared the experimental and the theoretical results. The results of the thermal analysis have been showed that the intermetallic formation reaction was completed in four steps, with the formation of the middle phases of Ni3Al and Al2Ni3. The grain size and lattice strain changes have been calculated using the Williamson-Hall equation during mechanical alloying.

Design and functionality of a high-sensitivity bomb calorimeter specialized for reactive metallic foils

We have designed and built a unique non-adiabatic bomb calorimeter to measure heat generated from intermetallic formation reactions and the subsequent partial combustion of nanocomposite metallic foils. Reactions are initiated with a low-energy electrical spark and can be performed in 1 atm of air, oxygen, nitrogen, or argon, in order to investigate reaction and burning characteristics in various environments. The bomb was designed to hold the foil samples with minimal thermal contact in order to mitigate heat losses and maximize surface area available for oxidation and nitridation. Samples are limited to the milligram range, and are much less energetic than an equivalent mass of organic material such as benzoic acid. Therefore, to maximize instrument sensitivity, heat capacity was minimized by designing the bomb to be as small as possible and by using low-viscosity silicone oil for the bath instead of water. The calorimeter has an energy equivalent of e (calor) = 279 ± 6 J K−1, allowing us to measure heat generation on the order of tens of Joules. Calibrations were performed in argon using Al:Ni nanocomposite foils with well-known heats of reaction, which were measured using differential scanning calorimetry.

Self-sustaining oxidation initiated by rapid formation reactions in multilayer foils

Here we report that a self-sustaining oxidation of a multilayer foil can be ignited by an intermetallic formation reaction, releasing ∼4× the energy of the formation reaction. We examine foils with overall chemistries of 3Al:2Ni, 3Al:Zr, and Al:Zr and find that only the latter experiences significant oxidation. The Al:Zr samples initially react to form intermetallics and reach ∼1500 K in <10 ms. The samples then oxidize in air, absorbing ∼30 at. % O and remaining at ∼1400 K for >2.0 s. The phases within the Al:Zr foils are characterized and temperature-time profiles are examined to predict the heat generated by the oxidation reaction.

Characterization of self-propagating formation reactions in Ni/Zr multilayered foils using reaction heats, velocities, and temperature-time profiles

We report on intermetallic formation reactions in vapor-deposited multilayered foils of Ni/Zr with 70 nm bilayers and overall atomic ratios of Ni:Zr, 2 Ni:Zr, and 7 Ni:2 Zr. The sequence of alloy phase formation and the stored energy is evaluated at slow heating rates (∼1 K/s) using differential scanning calorimetry traces to 725 °C. All three chemistries initially form a Ni–Zr amorphous phase which crystallizes first to the intermetallic NiZr. The heat of reaction to the final phase is 34–36 kJ/mol atom for all chemistries. Intermetallic formation reactions are also studied at rapid heating rates (greater than 105 K/s) in high temperature, self-propagating reactions which can be ignited in these foils by an electric spark. We find that reaction velocities and maximum reaction temperatures (Tmax) are largely independent of foil chemistry at 0.6±0.1 m/s and 1220±50 K, respectively, and that the measured Tmax is more than 200 K lower than predicted adiabatic temperatures (Tad). The difference between Tmax and Tad is explained by the prediction that transformation to the final intermetallic phases occurs after Tmax and results in the release of 20%–30% of the total heat of reaction and a delay in rapid cooling.

Time-resolved x-ray microdiffraction studies of phase transformations during rapidly propagating reactions in Al/Ni and Zr/Ni multilayer foils

We showed how intermetallic formation reactions can be studied under rapid heating (106–107 K s−1) using x-ray microdiffraction with temporal resolution on microsecond time scales. Rapid heating was achieved by initiating an exothermic reaction in multilayer foils comprising alternating nanoscale layers of elemental metals. The reaction occurred in a front ∼100 μm wide which propagated across the foil at ∼1–10 m s−1. By using synchrotron x-rays focused to a small spot (60 μm diameter) and a fast pixel-array detector, we were able to track the evolution of phases in the reaction front during the initial heating transient, which occurred in approximately 1 ms, through cooling over a period of hundreds of milliseconds. In Al/Ni multilayer foils, the first phases to form were an Al-rich liquid and the cubic intermetallic AlNi (which likely formed by nucleation from the liquid). In foils of overall composition AlNi, this is the stable intermetallic and the only phase to form. In foils of composition Al3Ni2, during cooling we observed a peritectic reaction between AlNi and the remaining liquid to form Al3Ni2, which is the stable phase at room temperature and the final product of the reaction. This is in contrast to the sequence of phases under slow heating, where we observed formation of nonequilibrium Al9N2 first and do not observe formation of a liquid phase or the AlNi intermetallic. We also observed formation of an amorphous phase (along with crystalline ZrNi) during rapid heating of Zr/Ni multilayers, but in this system the temperature of the reaction front never reached the lowest liquidus temperature on the Zr–Ni phase diagram. This implies that the amorphous phase we observed was not a liquid arising from melting of a crystalline phase. We suggest instead that a Zr-rich amorphous solid formed due to solid-state interdiffusion, which then transformed to a supercooled liquid when the temperature exceeded the glass transition temperature. Formation of the supercooled liquid presumably facilitated continued rapid intermixing, which may be necessary to sustain a self-propagating reaction front in this system.

Reactive processing of nickel-aluminide intermetallic compounds

NiAl have been fabricated by reactive sintering compacts of ball-milled powder mixtures containing Ni and Al. The reaction mechanism, as well as phase and microstructural development, were investigated by analyzing compacts quenched from different temperatures during reactive hot compaction. It was found that the reaction process was strongly affected by pressure, heating rates, heat loss from the sample to the environment. The application of 50 MPa prior to the reaction resulted in the intermetallic-formation reaction initiating at a temperature (480°C) much lower than that (∼550°C) when no pressure was applied. At high heating rate (50°C/min), when the heat loss is small, the formation of NiAl occurs rapidly via combustion reaction. On the other hand, if the heat loss is significant as in slow heating rate (10°C/min), the reaction process is controlled by solid-state diffusion. The phase formation sequence for the slow solid-state reaction was determined to be: NiAl3→ Ni2Al3→ NiAl → NiAl (Al-rich) + Ni3Al → NiAl.