An in-situ high-resolution transmission electron microscopy study of the growth and dissolution of θ_{111} precipitate plates in an Al-Cu-Mg-Ag alloy

Abstract

In-situ hot-stage high-resolution transmission electron microscopy investigations were performed to determine the atomic mechanisms and growth kinetics of θ-Al₂Cu precipitate plates with a {111} habit plane in an Al-3.9 wt% Cu-0.5 wt % Ag-0.5 wt % Ag alloy. In order to obtain a three-dimensional description of precipitate growth mechanisms, the studies were performed along two principal directions. In the first direction, along [001]₀\\[121]_α the θ_{111} plate faces were parallel to the direction of observation. This allowed the motion of ledges moving across the faces and at the edges of the precipitates to be recorded. In the second direction, along [110]_θ\\[111]_α the θ_{111} plate faces were perpendicular to the direction of observation and data on the nucleation and propagation of kinks on the growth ledges were obtained. The results from these studies allowed the behaviour of single-plane and multiple-plane ledges, as well as multiple-plane ledge interactions, to be described in terms of the nucleation and propagation behaviour of kinks on the ledges. A nucleation rate and a free energy for kink nucleation were determined by analysing the velocities of ledges and kinks. The lengthening rate of plates contained within the foil was consistent with diffusion control limited by the nucleation of kinks at the plate edge although kinetic analyses of ledge motion indicate that surface diffusion may dominate the growth kinetics of θ{111} plates which intersect the transmission electron microscope foil surface in thin foils. These studies also show that ledge motion and interface migration involve the cooperative motion of many atoms and that these local processes at the interface can occur at rates up to six orders of magnitude faster than those predicted by long-range volume diffusion control.