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

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 0.1 mu s) and very reactive core/shell nanostructures enables ignition at unprecedentedly low temperatures for molecular dynamics simulations. Samples ignited at low temperatures (900 K 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. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.