Molecular dynamics simulations of the reaction mechanism in Ni/Al reactive intermetallics

Abstract

We present an atomistic level description of the reaction mechanisms in thermally ignited samples of reactive core/shell Ni/Al wires. The combination of very long simulation times (up to 0.1μs) and very reactive core/shell nanostructures enables ignition at unprecedentedly low temperatures for molecular dynamics simulations. Samples ignited at low temperatures (900K or less) follow a multi-stage reaction process involving solid-phase diffusion and the formation of the B2 NiAl phase, while samples ignited at higher temperatures, follow a direct reaction path with accelerated diffusion of Ni into molten Al, leading to a completely molten final state. Interestingly, NiAl nucleates and grows under a significant concentration gradient, which may explain the absence of other phases like AlNi3. The formation of the B2 intermetallic slows atomic diffusion and consequently the reaction rates.