Loss in coherency and coarsening behavior of Al3Sc precipitates

The coarsening behavior of the Al3Sc particles in Al-0.2wt%Sc alloy at 673-763 K is studied on the basis of TEM observations with the numerical model. Emphasis is on the effects of coherent/semi-coherent transition of the particles. The radius for coherent/semi-coherent transition of the Al3Sc particles is determined from TEM micrographs as 15-40 nm. The average particle radius, rave, of the Al3Sc particles obeys the rave3 growth low both in the coherent stage (rave < 15 nm) and semi-coherent stage (rave > 40 nm). However, in the intermediate stage, where coherent and semi-coherent particles coexist (15 < rave < 40 nm), coarsening is delayed and particle size distribution is broadened in the experiment and also in the calculation. These results are qualitatively understood in consideration of the different growth rates of individual particles in the intermediate stage.