DISCONTINUOUS COARSENING OF LAMELLAR CELLULAR PRECIPITATE IN A MONOTECTOID ZINC-40.5 AT. PCT. ALUMINUM ALLOY (GROWTH RATE, INTERLAMELLAR, LATTICE PARAMETER, SPACING, GRAIN BOUNDARY DIFFUSIVITY)

The morphology and growth kinetics of monotectoid decomposition as well as discontinuous coarsening of the lamellar product of the monotectoid reaction in a Zn-40.5 at. pct. Al alloy have been investigated by light and electron microscopy and x-ray lattice parameter measurements. On aging between 157(DEGREES) and 254(DEGREES)C the alloy was observed to decompose completely by monotectoid decomposition producing a lamellar mixture of aluminum rich and zinc rich solid solutions. With further aging the lamellar structure of the first reaction was found to decompose into a coarser lamellar structure by a discontinuous coarsening reaction. This reaction was much slower than the first reaction and the aluminum and zinc rich solid solution lamellae were closer to equilibrium than those formed by the first reaction. Several models for cellular precipitation and the Petermann and Hornbogen model for discontinuous coarsening were used to evaluate the growth kinetics of the reactions. The analysis suggests that both the first and second reactions are controlled by grain boundary diffusion in this advancing cell interface