Effect of Ag addition on the age hardening and precipitation behavior in an Al-Cu-Li-Mg-Zn-Mn-Zr alloy

Abstract

Effect of Ag addition on the age hardening and precipitation behavior of an Al-Cu-Li-Mg-Zn-Mn-Zr alloy has been investigated by transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) with an energy dispersive X ray spectroscope (EDS), convergent beam electron diffraction (CBED) and hardness. The results showed that the precipitation behavior of the Ag free Al-Cu-Li-Mg-Zn-Mn-Zr alloy was SSS → δ′/GP zone/δ′ + T1 → cubic σ phase + θ′ + T1 + S′/S phase, while in the Ag added Al-Cu-Li-Mg-Zn-Mn-Zr alloy, it was SSS → θ′ + δ′ + T1 → θ′ + δ′ + T1 + S′/S phase. The Ag added alloy exhibited higher strength than the Ag free alloy due to a promoted T1 precipitation, while the hardness loss of the Ag free alloy was smaller than that of the Ag added alloy due to the precipitation of cubic σ phase. Ag was mainly concentrated in T1 precipitates and stimulated the precipitation of T1 rather than providing nucleation sites for cubic σ phase. Hence, the precipitation of cubic σ phase was suppressed by competing with T1 for solutes in the Ag added Al-Cu-Li-Mg-Zn-Mn-Zr alloy.