Molecular dynamics simulation of surface segregation in a (110) B2-NiAl thin film.

Surface segregation in (110) B2-NiAl film approximately 3 nm thick is investigated by using molecular dynamics simulation with a reliable embedded-atom potential. The simulation is performed for the stoichiometric composition at a temperature of 1500 K, just below the melting temperature of the film model. It is found that the (110) surface is structurally stable but develops adatoms, vacancies and antisites. The coverage of an adatom layer is estimated to be ∼0.07 ML (monatomic layers) and it contains on average ∼95% of Al atoms. The top (surface) and second (subsurface) layers of the (110) surface is the most enriched in Ni relative to the bulk composition. These layers contain on average ∼51% of Ni atoms. The Ni fraction in the third and forth layers of the film is estimated as ∼50.5%. The deeper layers have essentially the bulk composition. Vacancies in the film model are found only on the Ni sublattice. The vacancy concentration on the Ni sublattice in the top layer is ∼7.5%. The second layer almost does not contain vacancies. The next layers have essentially the constant bulk vacancy composition which can be estimated as ∼1.3-1.4%.