The deformation behavior of the {111}Al plates in an Al-Cu-Mg-Ag alloy

The deformation behavior of {111} _Al Ω plates has been analyzed in an Al-Cu-Mg-Ag alloy after creep tests to fracture by atomic-resolution scanning transmission electron microscopy (STEM). A transition from particle shearing to Orowan dislocation loop formation was found to occur at critical plate thicknesses of 3 and 3.5 c _θ-o (i.e., 2.54-2.97 nm) at elevated temperature if the Ω phase is considered to compose of the orthorhombic θ lattice (θ-o). For the plates thinner than the critical value, a full dislocation provides the particle segments to shift relative to each other in the planes orthogonal to the particle habit plane. These discrete particle shifts/shears are proportional to ¼ c _θ-o(1 d _111Al ) normal to the broad plate surfaces or the plate habit plane. A full dislocation with Burgers vector b → = [001] _θ-o can dissociate to a few partials inside the θ-o lattice. For the thick plates, a dislocation loop was found to consist of the two most extent segments with jogs making its line somewhat parallel to the broad plate surfaces. Interposition of these two segments belonging to the same dislocation loop along the broad interfaces, depends on elastic strain fields, e.g. , originating from the ledges formed after particle shearing, broad coherent interface structures and the precipitate/matrix volumetric incompatibility in <111> _Al//<001> _θ-o. The dislocation loops cause particle segmentation and deviation by lattice rotation around <110> _Al axes from the well-known Al/Ω orientation relationship.