TEM studies of transformation interfaces and substructures in some copper-based shape-memory alloys

The characterization of interfaces and substructures resulting from solid-to-solid phase transformations is reviewed in the context of three examples in cooper-based shape-memory alloys. The first of these concerns the interfacial structure of martensite intervariant plate boundaries in a Cu−14.6Zn−16.1Al (at%) alloy. The atomistic structure of such interfaces, which is important to their mobility and to shape memory behavior, is deduced from the examination of one-dimensional lattice images together with selected-area diffraction data. The second example considers the mechanism of bianitic transformation in a Cu−25.1Zn−9Al (at%) alloy. In this case, the relative degree of inheritance of parent phase order by the bainite transformation product, which is relevant to the distinction between displacive and reconstructive mechanisms of formation of the bainite, is revealed by the use of dark field imaging techniques using two different types of superlattice reflections. The third case discusses dynamic strain contrast effects and in-situ cooling effects observed in “tweed” microstructures in an aged Cu−47.7Mn−3.5Al (at%) high damping alloy. The tweed contrast consists of irregular lines of strain contrast lying approximately parallel to traces of {110} planes of the parent FCC phase, and arises in this case due to a tetragonal distortion which occurs, upon cooling, in a dispersion of Mn-enriched regions which have formed upon aging. In addition, a unique “flickering” contrast effect is observed within the tweed contrast. This consists of repetitive movements of short (about 10 nm) lines of contrast, and apparently corresponds to changes in the orientation of the FCC-to-FCC distortion due to the excitation of the incident electron beam. Individual sites of this activity, which are believed to be associated with the mechanism of high damping in these aged alloys, are isolated in weak-beam images. In-situ cooling and video-taping experiments are also described.