The formation of outgrowths in CO-evaporated Ni-Al films

Abstract

Films with compositions ranging from pure nickel to nickel with approximately 55 at.% Al were prepared by co-evaporation onto substrates held at 360–790 K. Their microstructure and morphology were investigated by transmission and scanning electron microscopies. For aluminium contents of less than 30 at.% the film surface is smooth with few irregularities. In the composition range 30–45 at.% Al and for substrate temperatures between 360 and 700 K the films are covered with surface outgrowths. In the same ranges the films consist of a mixture of nickel, α'-Ni 3Al and β'-NiAl, the last material being the major phase present. A model of the formation of the outgrowths by directed atomic migration under the influence of a chemical potential is discussed.