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 ̄ 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 ∼1.1 nm and an associated displacement vector along ∼[335] f. The curved ∼(44 3 ̄ ) f side facets contained an array of dislocations spaced ∼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 ∼1.1 nm in all interfaces is the plane spacing of the primary O-lattice planes of Zhang and Purdy.