The reactive interdiffusion of Co/Al bilayers was studied by atom probe tomography. For that, metallic thin films were deposited on W 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 was 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 was determined to be 0.3/nm. The Al9Co2 phase was 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 10nm before a dense layer has developed. The observed growth was understood by fast transport along incoherent interphase boundaries. For this condition, a kinetic model was discussed that describes the growth of the particles. The thickness at impingement was controlled by the ratio of the transport coefficients along and across the boundary. Choosing realistic values for the latter parameters, non-integer growth exponents ranging from 2 to 3 were 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 was observed as the second stage of reaction.

Nucleation of Product Phase in Reactive Diffusion of Al/Co. V.Vovk, G.Schmitz, R.Kirchheim: Physical Review B, 2004, 69[10], 104102 (9pp)