The precipitation behavior of an Al–Cu–Mg–Ag alloy under ECAP

The changes in the dispersions of secondary phases during equal channel angular pressing (ECAP) at 250 °C were studied in an Al–Cu–Mg–Ag alloy initially processed in a supersaturated solid solution (QA, quenched alloy), on coherent plates of the Ω-phase (OA, over-aged alloy) and on coarse particles of phases in thermodynamic equilibrium (HA, heterogenized alloy). In the QA, the decomposition of the supersaturated solid solution and the precipitation of nanoscale coherent plates of the Ω-phase begin to occur under soaking before ECAP. During ECAP, a shearing offset leads to the fragmentation of the Ω-phase into nanoscale spherical particles followed by their direct allotropic transformation to θ-phase dispersoids with an equiaxed shape. The S-phase (Al2CuMg) particles also nucleate and rapidly grow. In the OA the precipitation behavior is essentially the same. At ε≥4, a dispersion of incoherent particles with an average size of ~30 nm evolves in the QA and the OA. Therefore, a dispersion of thermodynamically metastable phases with coherent interfaces is replaced by a dispersion of phases with incoherent interfaces during ECAP at over-aged temperatures. These phases can be different from those precipitated during static artificial aging. Insignificant changes in the dispersion of secondary phases take place in the HA during ECAP.