Short-term High-energy Ball Milling Research Articles

Refractory TaTiNb, TaTiNbZr, and TaTiNbZrX (X = Mo, W) high entropy alloys by combined use of high energy ball milling and spark plasma sintering: Structural characterization, mechanical properties, electrical resistivity, and thermal conductivity

Refractory TaTiNb, TaTiNbZr, and TaTiNbZrX (X = Mo, W) high entropy alloys were synthesized by combined use of high energy ball milling (HEBM) and spark plasma sintering (SPS). Powders of predominantly bcc TaTiNbZrX (X = Mo, W) refractory high entropy alloys (RHEAs) were successfully prepared by short-term HEBM (60 min) and then SPS-consolidated at 1373 K for 10 min. TEM analysis of the TaTiNbZrW HEA powder obtained after 60 min of HEBM revealed its nanocrystalline structure with an average grain size of up to 50 nm and predominantly uniform distribution of the elements on an atomic scale. The SPS consolidation at 1300⁰C led to an increase in grain sizes up to 100–300 nm. Thus prepared bulk RHEA materials showed ultra-high Vickers hardness of 8.5 GPa and 13 GPa for TaTiNbZrMo and TaTiNbZrW alloys, respectively. The room-temperature compressive strength of TaTiNbZrW RHEA alloy sintered from HEBM powders attained a value of 2665 МPа, which is 30% higher than that for the same alloy produced from non-milled powders. Bulk samples of synthesized RHEA show a higher electrical resistivity (r) compared to the samples prepared from non-milled powder blends. Within the temperature range 298–573 K, the maximum r value for TaTiNbZrW RHEA alloy was found to vary between 132 and 143.6 Ω cm. A decrease in thermal conductivity was observed: (a) upon introduction of Zr, Mo, and W atoms to TaTiNb-based alloys due to additional phonon scattering on lattice distortions caused by different radius and mass of Zr, Mo, and W atoms and (b) for the HEBM-prepared bulk alloys because of additional phonon scattering on the surface of mechanocomposites.

Combustion synthesis: mechanically induced nanostructured materials

Combustion synthesis (CS) is a specific approach for fabrication of a variety of advanced materials through use of self-sustaining chemical reactions. Controlling over the microstructure of the material is a key factor in defining the maturity of a technology. In this work, we demonstrate that under specific conditions, morphology and microstructure of the initial reaction media do not change during the CS process. Thus, one may control the microstructure of CS materials by preparing the desired structure of the initial reaction media. Specifically, using examples from several systems, which include intermetallics (NiAl), ceramics (SiC) and refractory carbides (TiC), we demonstrate that combination of short-term high-energy ball milling and CS allows precise control over the morphology and phase composition of product powders.

Reactive Ni/Al Nanocomposites: Structural Characteristics and Activation Energy.

Stochastically structured Ni/Al reactive nanocomposites (RNCs) were prepared using short-term high-energy ball milling. Several milling times were utilized to prepare RNCs with differing internal nanostructures. These internal structures were quantitatively and statistically analyzed by use of serial focused ion beam sectioning coupled with 3D reconstruction techniques. The reaction kinetics were analyzed using the electrothermal explosion technique for each milling condition. It is shown that the effective activation energy (Eef) ranges from 79 to 137 kJ/mol and is directly related to the surface area contact between the reactants. Essentially, the reaction kinetics can be accurately controlled through mechanical processing techniques. Finally, the nature of the reaction is considered; the mechanistic effect of the reactive and three diffusive activation energies on the effective activation energy is examined.

Ni/Al Energetic Nanocomposites and the Solid Flame Phenomenon

Reactive nanocomposites (RNCs), which are comprised of stochastically layered metals, were fabricated using short-term high-energy ball milling of nickel and aluminum powders. By varying the milling conditions, the internal nanostructure of the RNCs can be controlled. Utilizing the slice and view methodology by use of a dual beam scanning electron/ion microscope, 3D reconstruction of the RNC particles was accomplished and their nanostructures were quantitatively and statistically analyzed. The reactivity, including ignition and combustion parameters, as well as microstructure of the combustion wave, for different RNCs was analyzed using high-speed infrared imaging and high-speed micro video recording. The direct relationships between the 3D structural characteristics and reactivity parameters have been determined. A comparison with existing theoretical models allows us to conclude that, for specially designed RNCs, the reaction can be initiated and self-propagates solely due to solid-state mechanisms, i.e...

Solid-flame: Experimental validation

The tantalum–carbon reactive system possesses a high energy of reaction with an adiabatic combustion temperature of 2743 K, which is significantly below the melting points of the reactants, as well as any intermediate phases and final products. It was suggested that a combustion wave could propagate in Ta+C mixtures solely owing to a solid–solid reaction. However, this combustion process has never been shown to occur without gas-assisted transport. Here, we report preparation of highly pure and pore-free Ta/C composite particles, which were used for experimental validation of the solid-flame. Preparation of these composite particles involves short term high-energy ball milling (HEBM) of tantalum and carbon powders. High-resolution microscopy coupled with three-dimensional reconstruction techniques were used to characterize the volume nanostructure of mechanically fabricated composite particles. It was quantitatively shown that the particles have nano-scale mixing of the reagents and possess high contact surface area between tantalum and carbon. Experiments revealed that the ignition temperature of as fabricated composite particles is 1243 ± 15 K and maximum combustion temperature was shown to be 2487 ± 50 K, which is well below any possible solid–liquid transitions. Utilizing results obtained with composite particles prepared under different HEBM conditions, it is shown that carbon diffusion through the tantalum grain boundaries and subsequent formation of a Ta(C) solid solution defines low temperature ignition of mechanically fabricated particles. The high surface area contact between the Ta and C nano-scale reagents allows the reaction to propagate in a self-sustained manner, owing solely to a solid-state diffusion mechanism.

Transition from Impact-induced Thermal Runaway to Prompt Mechanochemical Explosion in Nanoscaled Ni/Al Reactive Systems

The effect of microstructure on ignition sensitivity and reaction behavior is investigated for nanoscaled Ni/Al gasless reactive systems. Nanometric homogeneity of the reactive media was achieved through (a) conventional mixing of nanometric powders; (b) short-term high-energy ball milling (HEBM) of micrometer-sized powders. Sensitivity to thermal inputs is investigated by differential thermal analysis and mechanical sensitivity is studied by high-rate shear impacts. The composite Ni/Al particles prepared by HEBM were extremely thermally sensitive, with reaction ini- tiating at 2208C, compared to 5598C for nanometric powder samples and 6408C for un-milled, micrometer-sized Ni + Al powder mixture. In contrast, nanometric powder mixtures were more susceptible to ignition through me- chanical means, exhibiting a high-speed reaction mode that is not observed in HEBM samples. The high-speed mode preferentially appears in high-shear regions and is in- terpreted as a mechanically-induced thermal explosion. Its progression is tied to the passage of a stress wave in the heterogeneous media that heats and mixes the materials, rather than being propagated due to chemical energy re- lease. The microstructures unique to each material are con- sidered responsible for their individually ignition sensitivi- ties. Specifically, the finely interspersed porosity in nano- metric powder mixtures allows direct heating of the reac- tive interface between Ni and Al particles during compres- sion through pore collapse and plastic deformation, which leads to exceptionally high mechanical sensitivity. The HEBM materials have high specific reactant interface area in the bulk of each composite particle that enhances thermal sensitivity, but the relatively low specific interface area be- tween particles is unfavorable to mechanical ignition.

Dynamics of phase transformation during thermal explosion in the Al–Ni system: Influence of mechanical activation

Time-resolved X-ray diffraction (TRXRD) techniques, which allowed for in-situ investigation of the dynamics of phase transformation during rapid, gasless heterogeneous reactions, were used to study the thermal explosion (TE) process in the stoichiometric Ni–Al system. Comparison of the phase transformation sequences for initial mixtures of the precursors and those after short-term high-energy ball milling shows that while for the former case the necessary condition for TE is melting of the reactive media, in the latter case TE occurs solely through solid- state reactions. It is also concluded that the rates of solid-state reaction are significantly enhanced due to an increase of oxygen-free contact surface area and a decrease of the scale of heterogeneity in the composite particles formed during mechanical treatment.