The influence of the continuous precipitation on the discontinuous precipitation in an Al-14.6at%Zn alloy has been studied using optical microscopy, resistivity measurement, high angle x-ray study, small angle x-ray study, and transmission electron microscopy.
Using resistometric technique, the kinetics of continuous precipitation is extracted from the total transformation kinetics. Results show that a considerable amount of decomposition (up to 8 ~ 14 vol.%) takes place in the matrix through the prior continuous precipitation before the onset of the discontinuous precipitation at the grain boundary. The amount of this prior continuous precipitation decease with the increase in the solution treatment temperature (in the range of 350℃ to 450℃). However the growth kinetics of the cellular structure due to the discontinuous precipitation was observed to be faster in the case of low solution treatment temperature, contraly to the traditional concept that the chemical free energy is the driving force for the growth of cellular structure. The reversion treatment of this prior continuous precipitates for short time at high temperature resulted in a decrease of the growth rate of cellular structure, consistantly with the above observations.
All these observations and calculations of various energies involved indicated that the growth rate of the cellular structure due to the discontinuous precipitation appears not to be directly related with the available chemical free energy, nor with the available total free energy (sum of available chemical free energy and surface energy and strain energy), but to be directly related with the strain energy associated with the prior continuous precipitation of coherent GPzones.
A Random Average' method is newly developed in this study in order to obtain the exact growth length of discontinuous precipitates. Employment of this new method suggested that the growth rate of the cellular structure at 70℃ is low during the initial aging time and that it tends to increase with the aging time. This result is also believed to be consistent with the above conclusion that the driving force for the cellular structure is the misfit strain energy associated with the prior continuous precipitation of coherent GPzones.
Diffusion equation for discontinuous precipitation has been reanalyzed by taking into account solute diffusion not only along the moving grain boundary, but also across the moving grain boundary. The result showed that the plot of Ln($GS^2$) (where G is growth rate and S is the lamellar spacing of the cellular structure) vs 1/T can deviate from a linearity at low temperatures where grain boundary diffusivity across the grain boundary can be significant as compared to the growth rate. Careful examination of various published experimental data indicated that the above theoretical consideration is indeed observable.
A model of growth kinetics of discontinuous precipitation has been developed by applying the Hillert's approach in calculating the free energies dissipated into various sinks, by assuming that the driving force is not the chemical free energy, but the misfit strain energy associated with the continuous precipitation. The prediction of the model was qualitatively in good agreement with the experimental observation that the degree of segregation increases, while the lamellar spacing decreases, with the increase in the growth rate.
