| Mechanical properties of 7050 aluminum alloy under different heat treatments have been measured by room temperature tensile test. The microstructure was studied by optical metallography (OM), transmission electron microscope (TEM), small angle X-ray scattering (SAXS) and differential thermal scanning calorimetry (DSC) etc. The effect of heat treatment on mechanical properties and microstructure was investigated. The precipitation evolution during isothermal ageing was quantitatively characterized as well. A precipitation hardening model was modified to predict the precipitation hardening effect of 7050 aluminum alloy during the entire artificial ageing process. The results show that the tensile strength and yield strength increase with the increase of ageing time. The incretment is faster when aged at 160℃than that at 120℃. The yield strength reaches a high value when the alloy aged at 120℃for 24 hours, however, it only needed 8 hours for 160℃. We can obtain higher mechanical properties through two step ageing: pre-aged at 120℃for 4 hours followed by aged at 160℃for 3 hours.There are mainlyη′phase with sizes of a few nanometers after aged at 120℃for 24 hours, and a small amount of GP zones can also be observed.ηphase was the primary precipitates in the samples aged at 160℃for 24 hours(T7). The width of precipitation free zone (PFZ) increases with the increase of ageing time.The precipitation kinetics of 7050 aluminum alloy aged at 120℃and 160℃are consistent with the classical precipitation process, including nucleation, dissolution, growth ofη′and Ostwald ripening. The radius of precipitates increased with the increasing of ageing time at certain temperatures. When aged at 120℃the increase of precipitate radius is relatively small, and it was only 5nm even aged for 60 hours. The volume fraction evolution of precipitates increased at early ageing stage, then kept constant after late ageing stage.The contribution of precipitation hardening to the yield strength is modeled separately for the cases of strong and weak obstacles. The comparison results with experimental values indicate that the strong obstacles mode can predict the... |
