Freezing of Ni-Al bimetallic nanoclusters in computer simulations

Bimetallic particles have two features not observed in one-component systems: surface segregation and compositional ordering. Molecular dynamics simulations have been performed to study the freezing of Ni-Al nanoclusters at different compositions. It was found that for Al-rich and equiatomic compositions the surface segregation of Al atoms inhibits nucleation, causing supercooled clusters to solidify to an amorphous aggregate. In the case of Ni3Al particles, where there are not enough Al atoms to complete the surface layer, clusters froze to multiply twinned structures with well-defined local translational order but, owing to the fast kinetics of solidification, with compositional disorder.