Morphology and reactivity of composite Ni·Al materials prepared by emulsion-assisted milling

Composites of nickel and aluminum have been studied extensively as model reactive materials that use intermetallic reactions. However, the correlations between the powder morphology and its reactivity are not well understood. Here, composite powders with equal mole fractions of nickel and aluminum with spherical and irregular particle shapes were prepared by milling aluminum and nickel, respectively, with and without an emulsion serving as a process control agent. These powders served as precursors for reactive structural parts consolidated by swaging. The crystallite sizes and strains identified in the milled materials from X-ray diffraction were close to one another for different powders and did not correlate with the differences in the material reactivities. Conversely, the morphological descriptors of the prepared materials obtained from analyzing their cross-sections imaged using scanning electron microscopy with backscattered electrons were found to be useful in predicting the material reactivity. When the irregularly shaped powders were heated, Al-rich intermetallic phases formed initially, consistent with previous reports. For spherical powders, Ni-rich phases formed first, suggesting a different reaction mechanism. Spherical powders were more porous and had better flowability; they formed more brittle and more porous consolidated parts. In the consolidated materials, reaction sequences observed during heating were similar to those in the initial powders; however, the reactions occurred to a greater extent at lower temperatures. The exothermic reactions observed for different powders in differential scanning calorimetry were tentatively correlated with the reactions causing ignition of the same materials exposed to heating rates exceeding 1000 K/s.