Molecular dynamics study of reaction pathways in an Al-coated Ni nanoparticle

Using molecular dynamics simulation in combination with an embedded atom method potential we analyze the alloying reaction in an Al-coated Ni nanoparticle with equi-atomic fractions and a diameter of about 9.5 nm. The first stage of the alloying reaction is controlled by interdiffusion between the f.c.c. Al-shell and f.c.c. Ni-core. Then, the large driving force for further alloying of Ni and Al promotes solid state amorphization of a supersaturated metastable f.c.c. Ni–Al solid solution in the vicinity of the interface region and, eventually, the whole shell of the nanoparticle. It is shown that there are at least two further possible pathways of the reaction. The first pathway occurs through crystallization of the amorphous shell into an Al-rich B2-NiAl phase before the complete dissolution of the f.c.c. Ni-core followed by interdiffusion between the Al-rich B2-NiAl shell and the f.c.c. Ni-core. A prediction is made that under certain conditions, interdiffusion between an Al-rich B2-NiAl shell and an f.c.c. Ni-core in such a nanoparticle may result in the formation of a hollow B2-NiAl nanoparticle. The second pathway leads to the complete dissolution of the f.c.c. Ni core in the growing amorphous Ni–Al shell. In order for this scenario to be realised the rate of temperature increase at the stage of dissolution of the f.c.c. Ni core in the growing amorphous Ni–Al shell should be higher than in the previous case. We also demonstrate some possibilities to control the self-heating rate at the stage of dissolution of the f.c.c. Ni core in the growing amorphous Ni–Al shell, which is important for a switching of the reaction pathways in a desired direction