Atomic mechanisms of precipitate plate growth

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Abstract
In this investigation, the techniques of high-resolution transmission electron microscopy, image simulation, energy-dispersive X-ray spectroscopy and convergent-beam electron diffraction are used to characterize the atomic structure, chemistry and growth mechanisms of γ' plate-shaped precipitates in an Al-4·2 at.% Ag alloy aged for 30min at 350°C. The results of these studies show that γ' precipitates have the composition Ag2 Al and are ordered during the early stages of growth, where the A planes in the h.c.p. precipitates contain nearly pure Ag and the B planes have the composition Al2 Ag. In addition, the precipitate-matrix interphase boundary is largely coherent and faceted along low-energy precipitatematrix directions and both the lengthening and the thickening of γ' plates occur by a terrace-ledge-kink mechanism, where the limiting step in the growth process appears to be the substitutional diffusion of Ag atoms across kinks in the Shockley partial dislocation ledges, which terminate in the Ag-rich A planes. The complementary information obtained from each of these techniques allows the mechanisms of precipitate plate growth to be explained at the atomic level.

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