Diffusion of zinc in the lamellar eutectic,Al-CuAl2

Diffusion of Zn in the aligned lamellar Al-CuAl₂ eutectic has been studied. Couples were prepared to observe diffusion either parallel or perpendicular to eutectic interfaces at temperatures of 193, 240, and 287° and times to 1.66 × 10⁸ sec. Both optical and electron microprobe techniques were used to study the resultant microstructures. Zn is present in the α Al platelets at distances far greater than predicted by lattice diffusion. The diffusion of Zn into 0-CuAl₂ causes a rapid transformation of θ to a two phase mixture of α particles in a matrix of T-Al₃ZnCu₃. Advance of the growth front (θ → Τ + α) is parabolic with time, and the growth rate increases with increasing temperature. The transformation to Τ + α is observed to be slower in the 0 platelets of the eutectic than in bulk 0. In the eutectic, less Zn reaches the advancing transformation front because of lateral diffusion of Zn into adjacent a platelets. An experimentally determined activation energy for the combined transformation and diffusion processes suggests that the α -Τ boundaries act as high-diffusivity paths for Zn diffusion. This paper is based upon a thesis submitted by M. R. Jackson in partial fulfillment of the requirements of the degree of Doctor of Philosophy at Lehigh University.