Abstract
The morphology and kinetics of discontinuous precipitation (DP) and discontinuous coarsening (DC) in solution treated and isothermally aged Al-Zn alloys containing 39.3 and 59.3 at.% Zn have been investigated at temperatures ranging from 323 to 523 K by light microscopy, electron microscopy and X-ray diffraction. At all aging temperatures the supersaturated alpha solid solution was observed to decompose rapidly by DP into a lamellar mixture of solute depleted alpha phase and beta phase precipitate. DP occurred so rapidly in the 59.5 at.% Zn alloy that the heat of transformation raised the temperature of the alloy significantly. With further aging a slower DC reaction transformed the lamellar DP into a coarser lamellar structure of the same two phases; however, the composition of the alpha phase of the DC was closer to the equilibrium solvus composition than that of the DP.