Structural transformations in Fe-Ni-alloy nanoclusters: Results of molecular-dynamic-simulation

Size effect in Fe-Ni-alloy nanoclusters has been studied by the molecular-dynamics (MD) method using multiparticle interatomic interaction potentials. It is shown that the alpha-gamma transformation in nanoparticles with sizes d > 3.5 nm proceeds by the mechanism of nucleation at grain boundaries and propagation of fcc-phase plates. As a result of the transformation, a twinned lamellar domain structure is formed. In particles with sizes 3.0 < d < 3.5 nm, the alpha-gamma transformation is accompanied by radial-symmetry atomic movements that are close to those characteristic of the Bain scheme. This results in the formation of a single-domain fcc phase. In nanoparticles with sizes 1.5 < d < 3.0 nm, the alpha-gamma transformation proceeds via an intermediate state that is retained within a temperature range of a few hundreds of kelvins and is characterized by an incomplete phase transformation. It has been found that in Fe-Ni clusters with sizes <= 1.5 nm, the alpha-gamma transformation does not occur. During heating, the initial bcc configuration turns into an icosahedral one through polytetrahedral or amorphous-like configuration.