AN IN-SITU HIGH-RESOLUTION TRANSMISSION ELECTRON-MICROSCOPY STUDY OF THE GROWTH AND DISSOLUTION OF THETA((111)) PRECIPITATE PLATES IN AN AL-CU-MG-AG ALLOY

In-situ hot-stage high-resolution transmission electron microscopy inv estigations were performed to determine the atomic mechanisms and grow th kinetics of theta-Al2Cu precipitate plates with a {111} habit plane in an Al-3.9 wt% Cu-0.5 wt % Mg-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 d irection, along [001](theta)//[1 1(2) over bar1$](alpha), the theta({1 11}) plate faces were parallel to the direction of observation. This a llowed the motion of ledges moving across the faces and at the edges o f the precipitates to be recorded. In the second direction, along [(1) over bar 10](theta)//[111](alpha), the theta({111}) plate faces were perpendicular to the direction of observation and data on the nucleati on and propagation of kinks on the growth ledges were obtained. The re sults from these studies allowed the behaviour of single-plane and mul tiple-plane ledges, as well as multiple-plane ledge interactions, to b e described in terms of the nucleation and propagation behaviour of ki nks on the ledges. A nucleation rate and a free energy for kink nuclea tion were determined by analysing the velocities of ledges and kinks. The lengthening rate of plates contained within the foil was consisten t with diffusion control limited by the nucleation of kinks at the pla te edge although kinetic analyses of ledge motion indicate that surfac e diffusion may dominate the growth kinetics of theta({111}) plates wh ich intersect the transmission electron microscope foil surface in thi n foils. These studies also show that ledge motion and interface migra tion involve the cooperative motion of many atoms and that these local processes at the interface can occur at rates up to six orders of mag nitude faster than those predicted by long-range volume diffusion cont rol.