Transport processes and structural properties of the Cu3Au and Ni3Al(1 1 1) surface by molecular dynamics simulations

Using an effective potential model in analogy to the tight-binding scheme in the second-moment approximation, we investigated the transport processes and the structural properties of the Cu3Au and Ni3Al(1 1 1) surfaces by molecular dynamics simulations. We found that in the case of the Cu3Au(1 1 1) surface both Cu and Au adatoms are unstable and penetrating into the bulk (already at room temperature) induce local surface disorder that is propagating into the substrate. It is interesting to note that the interlayer diffusion takes place exclusively via Cu atoms and therefore, only Au adatoms stimulate local disorder through this process. Unlike, in the Ni3Al(1 1 1) surface the adatoms are stable, they diffuse via a multitude of hopping mechanisms, the Ni being more active than Al adatom. Below Ts=900 K exchange mechanisms are lacking for both adatoms, while above this temperature exchange diffusion of the Al adatom with Ni surface atoms is present inducing local disorder in the surface layer. Interestingly, no exchange events were detected for the Ni adatom. Consequently, below Ts the presence of adatoms on this surface does not affect the surface order, in agreement with the available experimental observations referring to Ni3Al. In addition, contrary to the behaviour found in the cases of the (0 0 1) and (1 1 0) surfaces, the presence of surface vacancies has no serious effect on the surface order in either of the alloys. This is essentially attributed to the structural difference of this surface, the second layer (Au or Al) atoms appearing to be an effective restraining element of any surface vacancy hopping mechanism.