Interface structure of faceted lath-shaped Cr precipitates in a Cu-0.33 wt% Cr alloy

The interface structure of lath-shaped, faceted Cr-rich precipitates in a Cu-0.33 wt%Cr alloy was investigated by transmission electron microscopy using diffraction contrast and weak beam dark field imaging techniques. The lath axis was aligned precisely with the [5 (6) over bar 1](f) invariant line direction, and its cross-sectional shape was characterized by four distinct facets with different defect contents. The major facet near (335)(f) appeared defect-free. The (111)(f) and (552)(f) facets showed a single set of ledges with a step height of similar to 1.1 nm and an associated displacement vector along similar to[335](f). The curved similar to(443)(f) side facets contained an array of dislocations spaced similar to 8.1 nm apart with a Burgers vector that could reasonably be characterized as 1/3[111](f) or 1/2[110](b), i.e. as d.s.c. lattice dislocations accommodating the misfit in interplanar spacing of the parallel close-packed planes in the two phases. A detailed model of the interface is in excellent agreement with the observed defect structure and the previously determined crystallographic characteristics. The model shows that the major facet near (335)(f) which appears to be defect free, actually contains a closely spaced array of dislocations in near-screw orientation. The uniform ledge height of similar to 1.1 nm in all interfaces is the plane spacing of the primary O-lattice planes of Zhang and Purdy.