Ledge structure and the mechanism of θ′ precipitate growth in AlCu

A model of precipitate plate growth developed for the PtC interstitial alloy system is extended to substitutional systems and compared with observations on θ′ precipitation from Al–4% Cu. By analogy, simple basic units of ledge growth involving conservative and non-conservative partial dislocation propagation on {001} planes are predicted for θ′ formation directly from solid solution. In contrast to current models for Al–Cu that assume a/n[001] dislocations at ledges, single ledges (one θ′ unit cell high), are necessarily associated with partial dislocations whose shear component is 1/2[100] or 1/2[010] analogous to the 1/6〈112〉 Shockley partials in the nucleation of γ′ plates on {111} planes in AlAg. This is shown to be a direct outcome of a simple lattice correspondence between the f.c.c. matrix and the b.c.t. precipitate. In general, however, double ledges (1.16 nm high) with opposite, cancelling, shears form to minimize shape change. Superledges (2.03 nm high) which are frequently observed, in addition minimize volume change. Accommodation of misfit in the broad faces of the precipitate also arises naturally in the model from epitaxial dislocations formed by combinations of growth partials. When θ′ forms from θ″ the vacancy super-saturation provided by dissolving dislocation loops is thought to initiate the transformation. A commonality of behavior found for PtC, AlCu, and AlAg suggests a wider validity for the proposed model.