Abstract
The kinetics of discontinuous coarsening (DC) of a discontinuous precipitate in an Al-29.5at.%Zn alloy have been investigated over the temperature range from 323 to 523 K by optical and scanning electron microscopy and X-ray diffraction. A lamellar discontinuous precipitate having a range of lamellar spacings and phase compositions was formed by aging at temperatures ranging from 323 to 523 K. These lamellar structures were subsequently isothermally aged at other temperatures in the range from 323 to 523 K and were observed to coarsen discontinuously. The reaction front migration rates, lamellar spacings and phase compositions for the DC were determined and analyzed according to the model of Petermann and Hornbogen, and the reaction was found to be controlled by boundary diffusion in the migrating reaction front. Boundary diffusivity ( sδD b) values calculated for the DC reaction front were 2–15 times lower than those calculated for discontinuous precipitation, an observation which is consistent with the fact that the boundary diffusivity of zinc in AlZn alloys decreases with decreasing zinc content.
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