DISCONTINUOUS COARSENING OF LAMELLAR DISCONTINUOUS PRECIPITATE IN AN ALUMINUM-39.3 AT. % ZINC ALLOY
The phase transformations of discontinuous precipitation and discontinuous coarsening in an Al - 39.3 at .% Zn alloy have been investigated morphologically and kinetically by light microscopy, electron microscopy, and X-ray diffraction. At aging temperatures ranging from 50 to 250(DEGREES)C the alloy was observed to decompose completely by a discontinuous precipitation reaction into fine alternate lamellae of Al-rich (alpha) and Zn-rich (beta) phases. With subsequent aging the fine lamellar product of discontinuous precipitation was further decomposed by a much slower discontinuous coarsening reaction which resulted in a coarser lamellar product. Both discontinuous reactions are characterized by the observation of a moving boundary which separates the initial state from the steady-state growth reaction product and invariably serves itself as a diffusional path. The growth rates, lamellar spacings, and phase compositions for both discontinuous reactions were determined and evaluated with the aid of several boundary diffusion-control theories. Comparisons of data for discontinuous reactions in Al-Zn alloys containing 29.0, 39.3, and 59.5 at .% Zn showed that the reaction growth rate increases and the lamellar spacing decreases with an increase of zinc content and that the grain boundary diffusivities calculated for the discontinuous precipitation and discontinuous coarsening reactions in these alloys are of the same order of magnitude as those for grain boundary diffusion in stationary boundaries.
"DISCONTINUOUS COARSENING OF LAMELLAR DISCONTINUOUS PRECIPITATE IN AN ALUMINUM-39.3 AT. % ZINC ALLOY"
(January 1, 1986).
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