IN-SITU HRTEM STUDY OF OMEGA-PRECIPITATE DISSOLUTION IN AN AL-CU-MG-AG ALLOY

A JEOL 4000EX high-resolution transmission electron microscope (HRTEM) equipped with a hot-stage pole piece was used to perform in situ studies of early-stage OMEGA precipitates in an Al-3.9Cu-0.5Mg-0.5Ag alloy. Three types of precipitate behavior were observed for thirty-six OMEGA precipitates examined during a 160-degrees-C in situ study: (1) the precipitates dissolved at various linear rates, (2) the precipitate diameters remained constant throughout the experiment, and (3) sporadic growth and dissolution of the precipitates occurred. The first type of behavior appears to be due to the interaction of edge-on OMEGA precipitates with the foil surface while the second type resulted from local coarsening. Regardless of whether the OMEGA precipitates grew or dissolved in the in situ study, their thickness was one unit-cell throughout the experiment. This thickness remained until some precipitates completely disappeared in the matrix. The average size of the minimum observable fl precipitates taking into account the effects of overlapping matrix was 1.79 nm, and this may represent a size just above the critical nucleus size r*. Image simulations were conducted to understand the visibility of an OMEGA precipitate embedded in the Al matrix in HRTEM. It was found that the visibility limit is about 25% and 15% of the foil thickness in [110]alpha and [112]alpha orientations, respectively, for foil thicknesses up to 17.2 nm.