Microstructure and Intermetallic Strengthening in an Equal Channel Angular Pressed AA2219. Part II: Strengthening Model.

In the present work, the strengthening effect of the Fe-rich intermetallic phases in a 2219 aluminum alloy subjected to equal channel angular pressing (ECAP) has been studied. Three different deformation conditions, corresponding to the as-extruded, ECAP route A-1 pass, and ECAP route A-2 passes, were considered. Mechanical characterizations have been performed by microhardness tests and tensile tests. All the contributions to the strengthening due to solid solution affects, dislocation boundaries, fine particles, and intermetallics have been quantitatively determined using transmission electron microscopy and field-emission scanning electron microscopy. The microstructure strengthening terms were combined, and the resulting value was shown to be fully consistent with the experimental yield strength obtained either by tensile tests or by microhardness measurements. These were (i) solution hardening; (ii) dislocation hardening through grain, cell, and very-low angle (misorientation angles within 4°, and typically showing Moiré fringes on TEM) boundaries; (iii) strengthening due to the equilibrium θ (Al2Cu), either through shearable (i.e., anti-phase) or by bowing (i.e., Orowan) mechanism; (iv) GP-I, GP-II (θ″) zones, and semi-coherent θ′ pre-precipitate, generated along tangle dislocations by the combined effect of the severe plastic deformation and the adiabatic heating; and (v) coarser intermetallic particle strengthening. In particular, the effect of the Fe-rich intermetallics on the alloy has been evaluated by calculating all the characteristic terms: the so-called parameter of intermetallic appearance index, according to Shabestari, the strengthening due to the load transfer (Δσ LT), and the strengthening due to the presence of the intermetallic phases (Δσ Intermet). Very good agreement was obtained between the strengthening model developed in this study and the experimentally measured yield stress.