computing times ($3\cdot10^4-~5\cdot10^4$ MCS), these exponents depend slightly on temperature. One possible source of these differences between simulations and the experimental data ($\nu \simeq1/2$) seems to be the existence of the low–energy type–I walls. Specifically, let us recall that our model contains only nearest neighbor interactions, so that type–I walls have zero energy and thus are extremely stable. It has been suggested previously that such a situation may lead to $\nu=1/4$.[@Castan; @Deymier] Moreover, the present model does imply a coupling of the non–conserved order parameters to the conserved density $\psi_0$. This coupling becomes active within type–II walls. The relaxation of a modified composition within type–II walls is slowed down further because type–II walls are interconnected via type–I walls which have fixed $\psi_0$ and do not allow any exchange of composition. The slight upward bending of the low–temperature data in Fig. 5 at the longest times, indicating an even slower growth, might be interpreted in this way, although this point needs to be clarified in further studies.

Summary and conclusions \[SUMMARY\]
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Implementing the vacancy mechanism in a model for the atomic dynamics in $A_3B$–type fcc–alloys, we investigated the growth of ordered domains after a temperature quench below the transition temperature 