| Recent advances in both electron microscopy instrumentation and technique now make it possible to study the shape-evolution of precipitate particles on an atomic level. In this investigation, the techniques of high-resolution electron microscopy, image simulation, energy-dispersive x-ray spectroscopy, electron diffraction and convergent-beam electron diffraction are used to characterize the atomic structure, chemistry and growth mechanisms of (gamma)' precipitate plates in an Al-Ag alloy. The (gamma)' precipitates were formed by aging a solid-solution Al-15 w/o Ag alloy for times of 10 to 120 min. at 350(DEGREES)C.; Energy-dispersive x-ray spectroscopy results reveal that the precipitates contain about 66 a/o Ag for the range of aging times examined and therefore, have the composition Ag(,2)Al. In addition, high-resolution electron microscopy of (gamma)' precipitates in both // and // orientations shows that all interfaces of the precipitate are largely coherent with the matrix and are faceted along low-energy 111 and 110 matrix planes, due to the influence of surface and elastic strain energies on the transformation. Further comparison between experimental and calculated high-resolution images of the precipitate/matrix interface and of Shockley partial dislocation ledges on the precipitate faces demonstrates that both thickening and lengthening of (gamma)' precipitate plates occurs by the passage of the Shockley partial dislocations along alternate 111 matrix planes by a terrace-ledge-kink mechanism. These images and electron diffraction information also indicate that the (gamma)' precipitates are ordered, where the A-planes in the precipitate contain nearly pure Ag and the B-planes have the composition Al(,2)Ag, and that the limiting reaction in the growth process is the substitutional diffusion of Ag across kinks in the Shockley partial dislocations, which termi- nate in the Ag-rich A-planes. In addition, the terraces between ledges are atomically flat and ledges are uniformly stepped-down from the centers to the edges of isolated precipitates, leading to the overall shape predicted by the general theory of precipitate morphology.; Lastly, CBED analysis of (gamma)' precipitates indicate that they have the space group P6(,3)/mmc, even though these results disagree with the ordered arrangement of atoms suggested by both the conventional diffraction patterns and high-resolution images. This difference may be due to the limited thickness of the extracted precipitates, or to the fact that the precipitates are disordering at the aging times and temperature examined. (Abstract shortened with permission of author.)... |
