Effect of over-aging on the microstructural evolution in an Al–Cu–Mg–Ag alloy during ECAP at 300 °C

Abstract

This report explores over-aging as it relates to microstructural evolution during equal channel angular pressing (ECAP) at 300°C using a novel Al–Cu–Mg–Ag alloy doped with zirconium and scandium. Material characterization was conducted using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray analysis in which microstructural evolution in a quenched alloy (QA) material was compared to that in an over-aged alloy (OA) material. It was shown that over-aging facilitates grain refinement where, at ɛ∼12, a fraction of the high angle boundaries (HABs) and average misorientation angles are ∼0.72 and 30°, respectively. In contrast, a partially recrystallized structure evolves in the QA sample even after ɛ∼12. Intense plastic straining affects precipitation. In the QA, ECAP leads to the precipitation in Ω-phase with a plate-like shape, initially. Further strain leads to a shearing offset associated with a localized dislocation glide followed by dissolution of Ω-phase and precipitation of the θ-phase (Al2Cu) and S-phase (Al2CuMg), both of which share an equaxed shape. In the OA, the dissolution of the Sc enriched θ-phase consisting of relatively coarse particles is followed by precipitation of nanometer-sized scale dispersoids of a Sc depleted θ-phase on deformation-induced boundaries. In addition, Ag and Mg-rich particles being β′(MgAg) and U(AlMgAg) phases precipitate during ECAP, in both states of the Al–Cu–Mg–Ag alloy explored herein.