Effect of milling temperature on structure and reactivity of Al–Ni composites

Reactive Al–Ni composites are of interest for combustion synthesis, for joining, and as components of energetic formulations. Mechanical milling is one of the most practical techniques used for their preparation. This study investigated the effect of ball milling at cryogenic temperatures on the reactivity and structure of Al–Ni composites. Stoichiometric mixtures of Al and Ni powders were milled both at room temperature and cooled by liquid nitrogen. Products were characterized using scanning electron microscopy, differential scanning calorimetry, and x-ray diffraction. Aluminum and nickel were mixed on a finer scale in cryogenically milled powders compared to those prepared at room temperature. DSC traces of the cryogenically milled powder showed three exothermic peaks, while only two clearly resolved peaks were observed for the powders milled at room temperature. A low-temperature reaction preceding the first peak was detected only for the cryomilled material. Apparent activation energies, obtained by model-free analysis, were consistent with previous work on nanofoils. Ignition temperatures of powders prepared at room temperature decreased with increasing structural refinement. This effect was even stronger for the cryogenically milled powder. Energetic characteristics of the prepared materials were correlated with their respective structural refinement. These correlations suggest that the cryomilled material has a narrower zone separating Al and Ni in which the elements are mixed together compared to the similar material prepared by room temperature milling.