Ni/Al reactive multilayer foils were fabricated by cold rolling method and
self-propagating reactions in these foils were investigated. A two-stage
phase formation process was observed in the ignition experiment. The first
step is the lateral growth of Al3Ni phase from isolated
nucleation sites and the following coalescence into a continuous layer; the
second step is the growth of such Al3Ni layers in the
perpendicular direction of the interface until all Al is consumed. As there
is still Ni available, Al3Ni will continuously react with Ni until no Ni is
left, and the final reaction product turns out to be ordered B2 AlNi
compound. X-ray diffraction (XRD) experiments showed that the reaction
product of the cold rolled foil was the same as the physical vapor
deposition (PVD) foil. The reaction process was studied by differential
scanning calorimetry (DSC). Three peaks can be identified from the DSC
curve. Cold rolled foils were heated to different peak temperatures obtained
from DSC curve with the same heating rate as DSC. XRD results for such foils
showed that the first two peaks were the exothermic formation of Al3Ni,
while the last one was for the formation of AlNi. The enthalpy of the
reaction for the cold rolled foil was calculated to be -57.5 KJ/mol, which
was in good agreement with the formation enthalpy of AlNi (-59 KJ/mol). The
reaction velocities of the first formation stage were measured to be 7mm/s
for cold rolled foils, which were much smaller than the reaction velocities
of PVD foils (which range between 1-30 m/s).