The growth Kinetics of discontinuous precipitation in three Al-Zn alloys containing 18.7, 22.8, and 26.8 at % Zn were investigated at temperatures ranging from 323K to 523K by measuring cell growth rate, interlamellar spacing, and degree of segregation using light microscopy, electron microscopy, and X-ray diffraction.
At all aging temperatures the alloys were observed to decompose completely by a discontinuous precipitation reaction, which results in a fine lamellar structure of aluminum rich and zinc rich solid solutions.
The results of X-ray study showed that the composition of the depleted α phase was far from the equilibrium composition except at temperatures, 473K and above.
Analysis of cell growth kinetics showed that solute transport occurred by boundary diffusion in the advancing cell interface and that the diffusivity was of the same order of magnitude as that of grain boundary diffusion in stationary boundaries.
The first fine lamellar structure was further developed into coarser lamellar structure at all aging temperatures by a second cellular or discontinuous coarsening reaction.
The fraction of total free energy change released by discontinuous and interlamellar spacing are increased with aging temperature and are decreased with initial zinc concentration.
It appears that the cell growth rate is independent of the amount of available free energy change. However the cell growth rate increases with the increase in the amount of total free energy change.
