Reaction front propagation in nanocrystalline Ni/Al composites: A molecular dynamics study

This work focuses on a class of nano-laminated Ni/Al composites fabricated by a rolling-stacking procedure. This method results in nano-laminated composites with a complex structure. Their reactivity was compared with that of reactive multilayer nanofoils with constant bilayer thickness. Typical samples are composed of randomly distributed grains of Ni and Al. The self-propagating reactive front presents peculiarities that are directly related to the initial microstructure. The combustion mode is unsteady, with shifts in velocity. The temperature profile is characterized by broad combustion and post-combustion zones. The instantaneous shape of the front is tortuous, with the appearance of hot spots correlated with the underlying grain configuration. Local dynamics was studied in order to detect all the elemental processes occurring at nanoscale that trigger propagation: melting of Al, coarsening of Al grains, progressive exothermic dissolution mixing, heterogeneous nucleation, and growth of the intermetallic phase, B2-NiAl. The role of the complex initial microstructure was clearly demonstrated.