The growth kinetics of discontinuous precipitation in three Al-Zn alloys containing 18.7, 22.8 and 26.8at\% Zn were studied by means of optical microscopy, electron microscopy, X-ray diffraction, electrical resistance measurement, and small angle X-ray scattering(SAXS) method.
A significant amount of decomposition has been already occurred through the continuous precipitation before the apparent onset of the discontinuous precipitation at grain boundary. The growth rate of the celluar structure due to the discontinuous precipitation appeared to have no direct relationship with the available chemical free energy for the discontinuous precipitation. The variation of the growth rate of the cellular structure was observed to be rather proportional to that of the volume fraction of the metastable phase in the matrix produced by continuous precipitation prior to the onset of the discontinuous precipitation.
Controlled heat treatments have been designed in order to vary the amount of the prior continuous precipitation of the coherent phase before the onset of the discontinuous precipitation. The results showed that the growth rate of the cellular structure increase with the increase in the volume fraction of coherent phase. It is believed that the growth rate of cellular structure in this alloy is best explained by the variation of the misfit strain energy in the matrix associated with the continuous precipitation.
The influence of the additions of Mg and Cu on the discontinuous precipitation and continuous precipitation in Al-22.8at\% Zn alloy have been investigated by means of metallographic observation and electrical resistance measurement. Discontinuous precipitation is markedly suppressed on adding small amount of both Mg and Cu at low temperature(i.e., below $\approx 150\,^\circ\!C$). However this effect become increasingly less effective at high temperature. This temperature sensitive effect was more pronounced in the case of Cu and that the growth rate of cellular structure was comparable with that in the binary alloy. These observations was believed to indicate the importance of the segregation effect of the third elements on the grain boundary.
The growth rate of the cellular structure of the alloy contains 0.01Mg was faster at temperature below $\approx 100\,^\circ\!C$, but slower at temperature above $\approx 100\,^\circ\!C$ as compared to the alloy contains 0.1Cu. The analyses of the decomposition kinetics suggested that this trend is consistent with the comparative decomposition kinetics of prior continuous precipitation, similarly as in the case of vinary alloy.
