Bifurcation of the Kirkendall plane during interdiffusion in the intermetallic compound β-NiAl

An experimental verification of the phenomenological approach introduced earlier [Phys. Rev. Lett. 86 (2001) 3352] to rationalize possible bifurcation of the Kirkendall plane inside a diffusion-grown compound layer is presented. A reaction couple, in which a single-phased layer of -NiAl intermetallic is growing during interdiffusion from its adjacent phases is used as a model system. The corresponding Kirkendall velocity diagram was constructed on the basis of inter- and intrinsic diffusion data obtained with the diffusion couple technique. The agreement between the predicted and experimentally determined position of the marker planes was found to be good within the range of uncertainty of the experimental results, which demonstrates the validity of the proposed model. It is observed that the position of a stable Kirkendall plane is characterized not only by the presence of inert markers, but also by a different crystal morphology at both sides of this plane. The bifurcation of the Kirkendall plane within the product layer of -NiAl is directly related to the growth of grains of this phase at a location in between (and not at) the interfaces with starting materials.