Molecular dynamics simulation of the alloying reaction in Al-coated Ni nanoparticle

Using molecular dynamics simulation in combination with the embedded atom method we analyze the alloying reaction in Al-coated Ni nanoparticle with equi-atomic fractions and diameter of ∼4.5 nm. The alloying reaction in the nanoparticle is accompanied by solid state amorphization of the Al-shell and Ni-core in the vicinity of the interface region. The large driving force for alloying of Ni and Al promotes the solid state amorphization of the nanoparticle because it makes intermixing of the components much easier compared with the crystalline state. Though, a fraction of Al atoms is retained to be segregated to the surface of the nanoparticle because Al has a lower surface energy than Ni. Then, the crystallization of the Ni–Al amorphous alloy into the B2-NiAl ordered crystal structure is observed. The energy release from the transformation of the initial Al-coated Ni nanoparticle into the B2-NiAl ordered nanoparticle can be estimated as ∼0.46 eV/at. The B2-NiAl ordered nanoparticle melts at a temperature of ∼1500 K. The adiabatic temperature for the alloying reaction in the initial Al-coated Ni nanoparticle can be estimated to be below the melting temperature of the B2-NiAl ordered nanoparticle. It is shown that very rapid intermixing and Ni–Al amorphous alloy formation with the reaction self-heating rate ∼1 K/ps occurs when the reaction is ignited before the formation of thin Ni–Al layer will take place at the interface. In this case, the ignition temperature can be as low as ∼100 K. The alloying reaction will occur much more slowly if thermal explosion is not ignited at the stage of Ni–Al layer formation at the interface, and the reaction time will be at least one-two orders of magnitude greater. The formation of the thin Ni–Al layer at the interface results in the appearance of a stronger interfacial diffusion barrier. The barrier slows down the alloying reaction in the nanoparticle.