Effect of precipitates evolution on mechanical properties of Al 7050 alloy during secondary aging

Abstract The microstructure and mechanical properties were investigated in Al 7050 alloy after aging treatments. Results showed that refining the η ′ precipitates and increasing the proportion of the η ′ precipitates significantly improved its mechanical properties. High density dislocations created by cold deformation promoted the nucleation of the main strengthening η ′ phase during aging. High-density fine precipitates was created, which increases the yield strength. However, the strength of the deformed sample decreased by ∼31.6% after the secondary aging at 157 °C, with more degradation than other samples. Microstructure study presented that some precipitates were composed of two distinctive areas. An in situ transformation from a simple triclinic Mg 2 Zn 3 to the η phase (MgZn 2 ) was observed during the secondary aging. The loss in strength after secondary aging at 157 °C proved that the η ′ precipitates transformed into the η precipitates by absorbing solute atoms at a temperature lower than equilibrium precipitation temperature of η precipitates. The residual high-density dislocations in the cold-deformed samples promoted the diffusion of solute atoms, which accelerated the η ′ → η transformation during the secondary aging. Significant reduction in the volume of η ′ precipitations led to the rapid deterioration of mechanical properties.