Nucleation of product phase in reactive diffusion of Al/Co

The reactive interdiffusion of Co/Al bilayers is studied by atom probe tomography. For that, metallic thin films were deposited on tungsten substrates preshaped by field evaporation. Owing to the outstanding resolution of the method and its real three-dimensional analysis, the nucleation of the first product could be characterized in detail. It is clearly seen that interdiffusion of the initial Al and Co layers precedes the formation of the first nuclei. A critical gradient for the onset of nucleation is determined to be 0.3 nm(-1). The Al9Co2 phase is identified as the first reaction product by direct chemical analysis. Instead of a layer, this phase forms as globular nuclei, which quickly grow to a thickness of about 10 nm before a dense layer has developed. The observed growth is understood by fast transport along incoherent interphase boundaries. For this condition, a kinetic model is discussed that describes the growth of the particles. The thickness at impingement is controlled by the ratio of the transport coefficients along and across the boundary. Choosing realistic values for the latter parameters, noninteger growth exponents ranging from 2 to 3 are predicted. Thus, the reaction proceeds much faster than in the case of ordinary parabolic or linear growth. After consumption of pure Al, the formation of B2-ordered AlCo is observed as the second stage of reaction.