Atomistic mechanism of the formation of a nanometer-sized amorphous metal by Kirkendall effect

Molecular dynamics methods have been employed to study the structural and chemical stability of Ni–Zr core–shell particles in the size range between 3 and 6 nm. It is shown that a gradual temperature rise determines the intermixing of Ni and Zr atoms as a consequence of vacancy-mediated displacements mostly involving Ni atoms. In the initial stages, vacancies form preferentially at the Ni–Zr interface, which stores excess free volume due to its incoherent character. The displacement of Ni atoms exhibits a square-root dependence on time and promotes the formation of an amorphous layer. Starting from the interface region, this progressively extends to the free surface. On the inner side, the formation of voids and the possible observation of a residual Ni layer depending on the particle composition suggest for diffusion a Kirkendall mechanism. The results obtained point out that Kirkendall diffusion could be exploited on the nanometer scale to produce hollow shells with an amorphous structures.