Precipitation structure and strengthening mechanisms in an Al-Cu-Mg-Ag alloy

The evolution of the Omega-phase dispersion during aging in an Al-5.6Cu-0.72Mg-0.5Ag-0.32Mn-0.17Sc-0.12Zr-0.1Ge-0.08Ti-0.02Fe-0.01Si (wt%) alloy was examined at the temperatures, T, of 200 and 250 degrees C with dwelt times, tau of 1.8-86.4 and 0.6-25.2 ks, respectively. The precipitation sequence during aging can be written as SSSS -> Ag-Mg clusters ->Omega-phase+theta'-phase+theta-phase+Ag,Mg,Cu-enriched phases. Only a minor portion of Cu is consumed for the precipitation of the Omega-phase. Approximately 3.3 wt% of Cu is retained in the solid solution after peak-aging. Therefore, the Omega-phase is a transition phase with a high free energy. The Omega-phase plates exhibit superior coarsening resistance. The kinetics of the coarsening is successfully predicted by the equation in a form of C(tt)x tau(0.2), where C-tt is a coefficient depending on aging temperature. Effect of aging on yield stress (YS) was analyzed in terms of additive sum of the contribution arising from the grain boundary strengthening, dislocation strengthening, solid solution strengthening and precipitation strengthening mechanisms. The contribution of the precipitate strengthening can be predicted satisfactorily by Nie and Muddle's model of dislocation shearing of the {111}(alpha) plates. It was shown that the Omega-phase is a most efficient strengthening agent in the Al-Cu-Mg(-Ag) alloys and gives the major contribution to the overall YS. It is attributed to high efficiency of interfacial strengthening and high number density of the Omega-phase plates.