High-resolution and energy-filtered TEM imaging of Ag segregation to planar precipitate-matrix interfaces in an Al-Cu-Mg-Ag alloy

Abstract

This paper examines the parameters that yield optimal compositional contrast in both high-resolution transmission electron microscopy (HRTEM) and energy-filtered transmission electron microscopy (EFTEM) images of planar interfaces, which contain two atomie layers of segregated Ag. It is demonstrated that the compositional contrast due to Ag segregation at the interface is highest for on-axis HRTEM images when the sample is less than 5nm, and that above this thickness the compositional contrast becomes difficult to interpret directly. It is then demonstrated that for samples with a wide range of thicknesses an off-axis orientation can be used that produces HRTEM images with more directly interpretable compositional contrast than on-axis images. Both the on-axis HRTEM images of samples thinner than 5 nm thickness and the off-axis HRTEM images show no contrast reversal at the interface containing the segregated Ag, within reasonable defocuses. EFTEM images of planar interfaces acauired in an off-axis orientation possess superior compositional contrast compared with on-axis EFTEM images. Using the Ag M_4,5 edge at 367 eV energy loss and an off-axis orientation, the two atomie layers of Ag segregated to the precipitate-matrix interface were clearly resolved, producing a detection capability of approximately 0.5 nm. During these investigations it was observed that the ω phase can have multiple orientations within one plate, while the plate as a whole retains its usual {111}_α orientation. Using EFTEM imaging, it was found that the amount of Ag segregated to the α-ω interface decreased when the plate did not have its usual orientation relationship with the matrix.