Isothermal coarsening of dendritic equiaxial grains in Al15.6wt.%Cu alloy

The isothermal coarsening of equiaxial dendritic grains in Al15.6wt.%Cu alloy at 826 K (5 K above the eutectic temperature) was studied. For very short times (less than 60 s), the structure ripened by the usual increase in the secondary dendrite arm spacing. The secondary arms also coalesced and liquid inclusions formed within the grains. Simultaneously there was some fragmentation, in that the average center-to-center distance λ between grains at first decreased and then, at approximately 400 s, λ increased chth time. For times greater than 900 s, λ increased very slowly, and the morphology of the grains changed from dendritic globular to globular, when the original dendritic structure was no longer discernible. Using the surface-area-to-volume ratio S_v of the solid as the characteristic metric, a kinetic analysis showed that Ostwald ripening could not explain the results. Coalescence was identified as a mechanism, but unlike previous work, a t^{- 1 3} dependence of S_v on time t was not observed.